Related
Hi I am newbie to dynamoDB. Below is the schema of the dynamo table
{
"user_id":1, // partition key
"dob":"1991-09-12", // sort key
"movies_watched":{
"1":{
"movie_name":"twilight",
"movie_released_year":"1990",
"movie_genre":"action"
},
"2":{
"movie_name":"harry potter",
"movie_released_year":"1996",
"movie_genre":"action"
},
"3":{
"movie_name":"lalaland",
"movie_released_year":"1998",
"movie_genre":"action"
},
"4":{
"movie_name":"serendipity",
"movie_released_year":"1999",
"movie_genre":"action"
}
}
..... 6 more attributes
}
I want to insert a new item if the item(that user id with dob) did not exist, otherwise add the movies to existing movies_watched map by checking if the movie is not already available the movies_watched map .
Currently, I am trying to use update(params) method.
Below is my approach:
function getInsertQuery (item) {
const exp = {
UpdateExpression: 'set',
ExpressionAttributeNames: {},
ExpressionAttributeValues: {}
}
Object.entries(item).forEach(([key, item]) => {
if (key !== 'user_id' && key !== 'dob' && key !== 'movies_watched') {
exp.UpdateExpression += ` #${key} = :${key},`
exp.ExpressionAttributeNames[`#${key}`] = key
exp.ExpressionAttributeValues[`:${key}`] = item
}
})
let i = 0
Object.entries(item. movies_watched).forEach(([key, item]) => {
exp.UpdateExpression += ` movies_watched.#uniqueID${i} = :uniqueID${i},`
exp.ExpressionAttributeNames[`#uniqueID${i}`] = key
exp.ExpressionAttributeValues[`:uniqueID${i}`] = item
i++
})
exp.UpdateExpression = exp.UpdateExpression.slice(0, -1)
return exp
}
The above method just creates update expression with expression names and values for all top level attributes as well as nested attributes (with document path).
It works well if the item is already available by updating movies_watched map. But throws exception if the item is not available and while inserting. Below is exception:
The document path provided in the update expression is invalid for update
However, I am still not sure how to check for duplicate movies in movies_watched map
Could someone guide me in right direction, any help is highly appreciated!
Thanks in advance
There is no way to do this, given your model, without reading an item from DDB before an update (at that point the process is trivial). If you don't want to impose this additional read capacity on your table for update, then you would need to re-design your data model:
You can change movies_watched to be a Set and hold references to movies. Caveat is that Set can contain only Numbers or Strings, thus you would have movie id or name or keep the data but as JSON Strings in your Set and then parse it back into JSON on read. With SET you can perform ADD operation on the movies_watched attribute. https://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Expressions.UpdateExpressions.html#Expressions.UpdateExpressions.ADD
You can go with single table design approach and have these movies watched as separate items with (PK:userId and SK:movie_id). To get a user you would perform a query and specify only PK=userId -> you will get a collection where one item is your user record and others are movies_watched. If you are new to DynamoDB and are learning the ropes, then I would suggest go with this approach. https://www.alexdebrie.com/posts/dynamodb-single-table/
const tours = await Tour.find()
when i use console.log(typeof tours) it shows object in console log.
but when i use console.log(tours) it shows an array of objects.
so i'm bit confused about what it actually returns back?
In JavaScript the value of typeof when used on an array is "object". The correct way to check if a variable is an array is Array.isArray() - it will return true or false depending if the argument passed is an array.
I have an graphql/apollo-server/graphql-yoga endpoint. This endpoint exposes data returned from a database (or a REST endpoint or some other service).
I know my data source is returning the correct data -- if I log the result of the call to the data source inside my resolver, I can see the data being returned. However, my GraphQL field(s) always resolve to null.
If I make the field non-null, I see the following error inside the errors array in the response:
Cannot return null for non-nullable field
Why is GraphQL not returning the data?
There's two common reasons your field or fields are resolving to null: 1) returning data in the wrong shape inside your resolver; and 2) not using Promises correctly.
Note: if you're seeing the following error:
Cannot return null for non-nullable field
the underlying issue is that your field is returning null. You can still follow the steps outlined below to try to resolve this error.
The following examples will refer to this simple schema:
type Query {
post(id: ID): Post
posts: [Post]
}
type Post {
id: ID
title: String
body: String
}
Returning data in the wrong shape
Our schema, along with the requested query, defines the "shape" of the data object in the response returned by our endpoint. By shape, we mean what properties objects have, and whether those properties' values' are scalar values, other objects, or arrays of objects or scalars.
In the same way a schema defines the shape of the total response, the type of an individual field defines the shape of that field's value. The shape of the data we return in our resolver must likewise match this expected shape. When it doesn't, we frequently end up with unexpected nulls in our response.
Before we dive into specific examples, though, it's important to grasp how GraphQL resolves fields.
Understanding default resolver behavior
While you certainly can write a resolver for every field in your schema, it's often not necessary because GraphQL.js uses a default resolver when you don't provide one.
At a high level, what the default resolver does is simple: it looks at the value the parent field resolved to and if that value is a JavaScript object, it looks for a property on that Object with the same name as the field being resolved. If it finds that property, it resolves to the value of that property. Otherwise, it resolves to null.
Let's say in our resolver for the post field, we return the value { title: 'My First Post', bod: 'Hello World!' }. If we don't write resolvers for any of the fields on the Post type, we can still request the post:
query {
post {
id
title
body
}
}
and our response will be
{
"data": {
"post" {
"id": null,
"title": "My First Post",
"body": null,
}
}
}
The title field was resolved even though we didn't provide a resolver for it because the default resolver did the heavy lifting -- it saw there was a property named title on the Object the parent field (in this case post) resolved to and so it just resolved to that property's value. The id field resolved to null because the object we returned in our post resolver did not have an id property. The body field also resolved to null because of a typo -- we have a property called bod instead of body!
Pro tip: If bod is not a typo but what an API or database actually returns, we can always write a resolver for the body field to match our schema. For example: (parent) => parent.bod
One important thing to keep in mind is that in JavaScript, almost everything is an Object. So if the post field resolves to a String or a Number, the default resolver for each of the fields on the Post type will still try to find an appropriately named property on the parent object, inevitably fail and return null. If a field has an object type but you return something other than object in its resolver (like a String or an Array), you will not see any error about the type mismatch but the child fields for that field will inevitably resolve to null.
Common Scenario #1: Wrapped Responses
If we're writing the resolver for the post query, we might fetch our code from some other endpoint, like this:
function post (root, args) {
// axios
return axios.get(`http://SOME_URL/posts/${args.id}`)
.then(res => res.data);
// fetch
return fetch(`http://SOME_URL/posts/${args.id}`)
.then(res => res.json());
// request-promise-native
return request({
uri: `http://SOME_URL/posts/${args.id}`,
json: true
});
}
The post field has the type Post, so our resolver should return an object with properties like id, title and body. If this is what our API returns, we're all set. However, it's common for the response to actually be an object which contains additional metadata. So the object we actually get back from the endpoint might look something like this:
{
"status": 200,
"result": {
"id": 1,
"title": "My First Post",
"body": "Hello world!"
},
}
In this case, we can't just return the response as-is and expect the default resolver to work correctly, since the object we're returning doesn't have the id , title and body properties we need. Our resolver isn't needs to do something like:
function post (root, args) {
// axios
return axios.get(`http://SOME_URL/posts/${args.id}`)
.then(res => res.data.result);
// fetch
return fetch(`http://SOME_URL/posts/${args.id}`)
.then(res => res.json())
.then(data => data.result);
// request-promise-native
return request({
uri: `http://SOME_URL/posts/${args.id}`,
json: true
})
.then(res => res.result);
}
Note: The above example fetches data from another endpoint; however, this sort of wrapped response is also very common when using a database driver directly (as opposed to using an ORM)! For example, if you're using node-postgres, you'll get a Result object that includes properties like rows, fields, rowCount and command. You'll need to extract the appropriate data from this response before returning it inside your resolver.
Common Scenario #2: Array Instead of Object
What if we fetch a post from the database, our resolver might look something like this:
function post(root, args, context) {
return context.Post.find({ where: { id: args.id } })
}
where Post is some model we're injecting through the context. If we're using sequelize, we might call findAll. mongoose and typeorm have find. What these methods have in common is that while they allow us to specify a WHERE condition, the Promises they return still resolve to an array instead of a single object. While there's probably only one post in your database with a particular ID, it's still wrapped in an array when you call one of these methods. Because an Array is still an Object, GraphQL will not resolve the post field as null. But it will resolve all of the child fields as null because it won't be able to find the appropriately named properties on the array.
You can easily fix this scenario by just grabbing the first item in the array and returning that in your resolver:
function post(root, args, context) {
return context.Post.find({ where: { id: args.id } })
.then(posts => posts[0])
}
If you're fetching data from another API, this is frequently the only option. On the other hand, if you're using an ORM, there's often a different method that you can use (like findOne) that will explicitly return only a single row from the DB (or null if it doesn't exist).
function post(root, args, context) {
return context.Post.findOne({ where: { id: args.id } })
}
A special note on INSERT and UPDATE calls: We often expect methods that insert or update a row or model instance to return the inserted or updated row. Often they do, but some methods don't. For example, sequelize's upsert method resolves to a boolean, or tuple of the the upserted record and a boolean (if the returning option is set to true). mongoose's findOneAndUpdate resolves to an object with a value property that contains the modified row. Consult your ORM's documentation and parse the result appropriately before returning it inside your resolver.
Common Scenario #3: Object Instead of Array
In our schema, the posts field's type is a List of Posts, which means its resolver needs to return an Array of objects (or a Promise that resolves to one). We might fetch the posts like this:
function posts (root, args) {
return fetch('http://SOME_URL/posts')
.then(res => res.json())
}
However, the actual response from our API might be an object that wraps the the array of posts:
{
"count": 10,
"next": "http://SOME_URL/posts/?page=2",
"previous": null,
"results": [
{
"id": 1,
"title": "My First Post",
"body" "Hello World!"
},
...
]
}
We can't return this object in our resolver because GraphQL is expecting an Array. If we do, the field will resolve to null and we'll see an error included in our response like:
Expected Iterable, but did not find one for field Query.posts.
Unlike the two scenarios above, in this case GraphQL is able to explicitly check the type of the value we return in our resolver and will throw if it's not an Iterable like an Array.
Like we discussed in the first scenario, in order to fix this error, we have to transform the response into the appropriate shape, for example:
function posts (root, args) {
return fetch('http://SOME_URL/posts')
.then(res => res.json())
.then(data => data.results)
}
Not Using Promises Correctly
GraphQL.js makes use of the Promise API under the hood. As such, a resolver can return some value (like { id: 1, title: 'Hello!' }) or it can return a Promise that will resolve to that value. For fields that have a List type, you may also return an array of Promises. If a Promise rejects, that field will return null and the appropriate error will be added to the errors array in the response. If a field has an Object type, the value the Promise resolves to is what will be passed down as the parent value to the resolvers of any child fields.
A Promise is an "object represents the eventual completion (or failure) of an asynchronous operation, and its resulting value." The next few scenarios outline some common pitfalls encountered when dealing with Promises inside resolvers. However, if you're not familiar with Promises and the newer async/await syntax, it's highly recommended you spend some time reading up on the fundamentals.
Note: the next few examples refer to a getPost function. The implementation details of this function are not important -- it's just a function that returns a Promise, which will resolve to a post object.
Common Scenario #4: Not Returning a Value
A working resolver for the post field might looks like this:
function post(root, args) {
return getPost(args.id)
}
getPosts returns a Promise and we're returning that Promise. Whatever that Promise resolves to will become the value our field resolves to. Looking good!
But what happens if we do this:
function post(root, args) {
getPost(args.id)
}
We're still creating a Promise that will resolve to a post. However, we're not returning the Promise, so GraphQL is not aware of it and it will not wait for it to resolve. In JavaScript functions without an explicit return statement implicitly return undefined. So our function creates a Promise and then immediately returns undefined, causing GraphQL to return null for the field.
If the Promise returned by getPost rejects, we won't see any error listed in our response either -- because we didn't return the Promise, the underlying code doesn't care about whether it resolves or rejects. In fact, if the Promise rejects, you'll see an
UnhandledPromiseRejectionWarning in your server console.
Fixing this issue is simple -- just add the return.
Common Scenario #5: Not chaining Promises correctly
You decide to log the result of your call to getPost, so you change your resolver to look something like this:
function post(root, args) {
return getPost(args.id)
.then(post => {
console.log(post)
})
}
When you run your query, you see the result logged in your console, but GraphQL resolves the field to null. Why?
When we call then on a Promise, we're effectively taking the value the Promise resolved to and returning a new Promise. You can think of it kind of like Array.map except for Promises. then can return a value, or another Promise. In either case, what's returned inside of then is "chained" onto the original Promise. Multiple Promises can be chained together like this by using multiple thens. Each Promise in the chain is resolved in sequence, and the final value is what's effectively resolved as the value of the original Promise.
In our example above, we returned nothing inside of the then, so the Promise resolved to undefined, which GraphQL converted to a null. To fix this, we have to return the posts:
function post(root, args) {
return getPost(args.id)
.then(post => {
console.log(post)
return post // <----
})
}
If you have multiple Promises you need to resolve inside your resolver, you have to chain them correctly by using then and returning the correct value. For example, if we need to call two other asynchronous functions (getFoo and getBar) before we can call getPost, we can do:
function post(root, args) {
return getFoo()
.then(foo => {
// Do something with foo
return getBar() // return next Promise in the chain
})
.then(bar => {
// Do something with bar
return getPost(args.id) // return next Promise in the chain
})
Pro tip: If you're struggling with correctly chaining Promises, you may find async/await syntax to be cleaner and easier to work with.
Common Scenario #6
Before Promises, the standard way to handle asynchronous code was to use callbacks, or functions that would be called once the asynchronous work was completed. We might, for example, call mongoose's findOne method like this:
function post(root, args) {
return Post.findOne({ where: { id: args.id } }, function (err, post) {
return post
})
The problem here is two-fold. One, a value that's returned inside a callback isn't used for anything (i.e. it's not passed to the underlying code in any way). Two, when we use a callback, Post.findOne doesn't return a Promise; it just returns undefined. In this example, our callback will be called, and if we log the value of post we'll see whatever was returned from the database. However, because we didn't use a Promise, GraphQL doesn't wait for this callback to complete -- it takes the return value (undefined) and uses that.
Most more popular libraries, including mongoose support Promises out of the box. Those that don't frequently have complimentary "wrapper" libraries that add this functionality. When working with GraphQL resolvers, you should avoid using methods that utilize a callback, and instead use ones that return Promises.
Pro tip: Libraries that support both callbacks and Promises frequently overload their functions in such a way that if a callback is not provided, the function will return a Promise. Check the library's documentation for details.
If you absolutely have to use a callback, you can also wrap the callback in a Promise:
function post(root, args) {
return new Promise((resolve, reject) => {
Post.findOne({ where: { id: args.id } }, function (err, post) {
if (err) {
reject(err)
} else {
resolve(post)
}
})
})
I had the same issue on Nest.js.
If you like to solve the issue. You can add {nullable: true} option to your #Query decorator.
Here's an example.
#Resolver(of => Team)
export class TeamResolver {
constructor(
private readonly teamService: TeamService,
private readonly memberService: MemberService,
) {}
#Query(returns => Team, { name: 'team', nullable: true })
#UseGuards(GqlAuthGuard)
async get(#Args('id') id: string) {
return this.teamService.findOne(id);
}
}
Then, you can return null object for query.
Coming from Flutter here.
I couldn't find any flutter related solution to this so since my search always brought me here, lemme just add it here.
The exact error was:
Failure performing sync query to AppSync:
[GraphQLResponse.Error{message='Cannot return null for non-nullable
type: 'AWSTimestamp' within parent
So, in my schema (on the AppSync console) I had this:
type TypeName {
id: ID!
...
_version: Int!
_deleted: Boolean
_lastChangedAt: AWSTimestamp!
createdAt: AWSDateTime!
updatedAt: AWSDateTime!
}
I got the error from the field _lastChangedAt as AWSTimestamp couldn't be null.
All I had to do was remove the null-check (!) from the field and it was resolved.
Now, I don't know the implications of this in the long run but I'll update this answer if necessary.
EDIT: The implication of this as I have found out is anything I do, amplify.push that change is reversed. Just go back to your appsync console and change it again while you test. So this isn't a sustainable solution but chatter I've picked up online suggests improvements are coming to amplify flutter very soon.
#Thomas Hennes got it spot on for me
The title field was resolved even though we didn't provide a resolver for it because the default resolver did the heavy lifting -- it saw there was a property named title on the Object the parent field (in this case post) resolved to and so it just resolved to that property's value. The id field resolved to null because the object we returned in our post resolver did not have an id property. The body field also resolved to null because of a typo -- we have a property called bod instead of body!
Pro tip: If bod is not a typo but what an API or database actually returns, we can always write a resolver for the body field to match our schema. For example: (parent) => parent.bod
One important thing to keep in mind is that in JavaScript, almost everything is an Object. So if the post field resolves to a String or a Number, the default resolver for each of the fields on the Post type will still try to find an appropriately named property on the parent object, inevitably fail and return null. If a field has an object type but you return something other than object in its resolver (like a String or an Array), you will not see any error about the type mismatch but the child fields for that field will inevitably resolve to null.
In case anyone has used apollo-server-express and getting null value.
// This will return values, as you expect.
const typeDefs = require('./schema');
const resolvers = require('./resolver');
const server = new ApolloServer({typeDefs,resolvers});
// This will return null, since ApolloServer constructor is not using correct properties.
const withDifferentVarNameSchema = require('./schema');
const withDifferentVarNameResolver= require('./resolver');
const server = new ApolloServer({withDifferentVarNameSchema,withDifferentVarNameResolver});
Note: While creating an instance of Apolloserver pass the typeDefs and resolvers var name only.
If none of the above helped, and you have a global interceptor that envelopes all the responses for example inside a "data" field, you must disable this for graphql other wise graphql resolvers convert to null.
This is what I did to the interceptor on my case:
intercept(
context: ExecutionContext,
next: CallHandler,
): Observable<Response<T>> {
if (context['contextType'] === 'graphql') return next.handle();
return next
.handle()
.pipe(map(data => {
return {
data: isObject(data) ? this.transformResponse(data) : data
};
}));
}
I have an graphql/apollo-server/graphql-yoga endpoint. This endpoint exposes data returned from a database (or a REST endpoint or some other service).
I know my data source is returning the correct data -- if I log the result of the call to the data source inside my resolver, I can see the data being returned. However, my GraphQL field(s) always resolve to null.
If I make the field non-null, I see the following error inside the errors array in the response:
Cannot return null for non-nullable field
Why is GraphQL not returning the data?
There's two common reasons your field or fields are resolving to null: 1) returning data in the wrong shape inside your resolver; and 2) not using Promises correctly.
Note: if you're seeing the following error:
Cannot return null for non-nullable field
the underlying issue is that your field is returning null. You can still follow the steps outlined below to try to resolve this error.
The following examples will refer to this simple schema:
type Query {
post(id: ID): Post
posts: [Post]
}
type Post {
id: ID
title: String
body: String
}
Returning data in the wrong shape
Our schema, along with the requested query, defines the "shape" of the data object in the response returned by our endpoint. By shape, we mean what properties objects have, and whether those properties' values' are scalar values, other objects, or arrays of objects or scalars.
In the same way a schema defines the shape of the total response, the type of an individual field defines the shape of that field's value. The shape of the data we return in our resolver must likewise match this expected shape. When it doesn't, we frequently end up with unexpected nulls in our response.
Before we dive into specific examples, though, it's important to grasp how GraphQL resolves fields.
Understanding default resolver behavior
While you certainly can write a resolver for every field in your schema, it's often not necessary because GraphQL.js uses a default resolver when you don't provide one.
At a high level, what the default resolver does is simple: it looks at the value the parent field resolved to and if that value is a JavaScript object, it looks for a property on that Object with the same name as the field being resolved. If it finds that property, it resolves to the value of that property. Otherwise, it resolves to null.
Let's say in our resolver for the post field, we return the value { title: 'My First Post', bod: 'Hello World!' }. If we don't write resolvers for any of the fields on the Post type, we can still request the post:
query {
post {
id
title
body
}
}
and our response will be
{
"data": {
"post" {
"id": null,
"title": "My First Post",
"body": null,
}
}
}
The title field was resolved even though we didn't provide a resolver for it because the default resolver did the heavy lifting -- it saw there was a property named title on the Object the parent field (in this case post) resolved to and so it just resolved to that property's value. The id field resolved to null because the object we returned in our post resolver did not have an id property. The body field also resolved to null because of a typo -- we have a property called bod instead of body!
Pro tip: If bod is not a typo but what an API or database actually returns, we can always write a resolver for the body field to match our schema. For example: (parent) => parent.bod
One important thing to keep in mind is that in JavaScript, almost everything is an Object. So if the post field resolves to a String or a Number, the default resolver for each of the fields on the Post type will still try to find an appropriately named property on the parent object, inevitably fail and return null. If a field has an object type but you return something other than object in its resolver (like a String or an Array), you will not see any error about the type mismatch but the child fields for that field will inevitably resolve to null.
Common Scenario #1: Wrapped Responses
If we're writing the resolver for the post query, we might fetch our code from some other endpoint, like this:
function post (root, args) {
// axios
return axios.get(`http://SOME_URL/posts/${args.id}`)
.then(res => res.data);
// fetch
return fetch(`http://SOME_URL/posts/${args.id}`)
.then(res => res.json());
// request-promise-native
return request({
uri: `http://SOME_URL/posts/${args.id}`,
json: true
});
}
The post field has the type Post, so our resolver should return an object with properties like id, title and body. If this is what our API returns, we're all set. However, it's common for the response to actually be an object which contains additional metadata. So the object we actually get back from the endpoint might look something like this:
{
"status": 200,
"result": {
"id": 1,
"title": "My First Post",
"body": "Hello world!"
},
}
In this case, we can't just return the response as-is and expect the default resolver to work correctly, since the object we're returning doesn't have the id , title and body properties we need. Our resolver isn't needs to do something like:
function post (root, args) {
// axios
return axios.get(`http://SOME_URL/posts/${args.id}`)
.then(res => res.data.result);
// fetch
return fetch(`http://SOME_URL/posts/${args.id}`)
.then(res => res.json())
.then(data => data.result);
// request-promise-native
return request({
uri: `http://SOME_URL/posts/${args.id}`,
json: true
})
.then(res => res.result);
}
Note: The above example fetches data from another endpoint; however, this sort of wrapped response is also very common when using a database driver directly (as opposed to using an ORM)! For example, if you're using node-postgres, you'll get a Result object that includes properties like rows, fields, rowCount and command. You'll need to extract the appropriate data from this response before returning it inside your resolver.
Common Scenario #2: Array Instead of Object
What if we fetch a post from the database, our resolver might look something like this:
function post(root, args, context) {
return context.Post.find({ where: { id: args.id } })
}
where Post is some model we're injecting through the context. If we're using sequelize, we might call findAll. mongoose and typeorm have find. What these methods have in common is that while they allow us to specify a WHERE condition, the Promises they return still resolve to an array instead of a single object. While there's probably only one post in your database with a particular ID, it's still wrapped in an array when you call one of these methods. Because an Array is still an Object, GraphQL will not resolve the post field as null. But it will resolve all of the child fields as null because it won't be able to find the appropriately named properties on the array.
You can easily fix this scenario by just grabbing the first item in the array and returning that in your resolver:
function post(root, args, context) {
return context.Post.find({ where: { id: args.id } })
.then(posts => posts[0])
}
If you're fetching data from another API, this is frequently the only option. On the other hand, if you're using an ORM, there's often a different method that you can use (like findOne) that will explicitly return only a single row from the DB (or null if it doesn't exist).
function post(root, args, context) {
return context.Post.findOne({ where: { id: args.id } })
}
A special note on INSERT and UPDATE calls: We often expect methods that insert or update a row or model instance to return the inserted or updated row. Often they do, but some methods don't. For example, sequelize's upsert method resolves to a boolean, or tuple of the the upserted record and a boolean (if the returning option is set to true). mongoose's findOneAndUpdate resolves to an object with a value property that contains the modified row. Consult your ORM's documentation and parse the result appropriately before returning it inside your resolver.
Common Scenario #3: Object Instead of Array
In our schema, the posts field's type is a List of Posts, which means its resolver needs to return an Array of objects (or a Promise that resolves to one). We might fetch the posts like this:
function posts (root, args) {
return fetch('http://SOME_URL/posts')
.then(res => res.json())
}
However, the actual response from our API might be an object that wraps the the array of posts:
{
"count": 10,
"next": "http://SOME_URL/posts/?page=2",
"previous": null,
"results": [
{
"id": 1,
"title": "My First Post",
"body" "Hello World!"
},
...
]
}
We can't return this object in our resolver because GraphQL is expecting an Array. If we do, the field will resolve to null and we'll see an error included in our response like:
Expected Iterable, but did not find one for field Query.posts.
Unlike the two scenarios above, in this case GraphQL is able to explicitly check the type of the value we return in our resolver and will throw if it's not an Iterable like an Array.
Like we discussed in the first scenario, in order to fix this error, we have to transform the response into the appropriate shape, for example:
function posts (root, args) {
return fetch('http://SOME_URL/posts')
.then(res => res.json())
.then(data => data.results)
}
Not Using Promises Correctly
GraphQL.js makes use of the Promise API under the hood. As such, a resolver can return some value (like { id: 1, title: 'Hello!' }) or it can return a Promise that will resolve to that value. For fields that have a List type, you may also return an array of Promises. If a Promise rejects, that field will return null and the appropriate error will be added to the errors array in the response. If a field has an Object type, the value the Promise resolves to is what will be passed down as the parent value to the resolvers of any child fields.
A Promise is an "object represents the eventual completion (or failure) of an asynchronous operation, and its resulting value." The next few scenarios outline some common pitfalls encountered when dealing with Promises inside resolvers. However, if you're not familiar with Promises and the newer async/await syntax, it's highly recommended you spend some time reading up on the fundamentals.
Note: the next few examples refer to a getPost function. The implementation details of this function are not important -- it's just a function that returns a Promise, which will resolve to a post object.
Common Scenario #4: Not Returning a Value
A working resolver for the post field might looks like this:
function post(root, args) {
return getPost(args.id)
}
getPosts returns a Promise and we're returning that Promise. Whatever that Promise resolves to will become the value our field resolves to. Looking good!
But what happens if we do this:
function post(root, args) {
getPost(args.id)
}
We're still creating a Promise that will resolve to a post. However, we're not returning the Promise, so GraphQL is not aware of it and it will not wait for it to resolve. In JavaScript functions without an explicit return statement implicitly return undefined. So our function creates a Promise and then immediately returns undefined, causing GraphQL to return null for the field.
If the Promise returned by getPost rejects, we won't see any error listed in our response either -- because we didn't return the Promise, the underlying code doesn't care about whether it resolves or rejects. In fact, if the Promise rejects, you'll see an
UnhandledPromiseRejectionWarning in your server console.
Fixing this issue is simple -- just add the return.
Common Scenario #5: Not chaining Promises correctly
You decide to log the result of your call to getPost, so you change your resolver to look something like this:
function post(root, args) {
return getPost(args.id)
.then(post => {
console.log(post)
})
}
When you run your query, you see the result logged in your console, but GraphQL resolves the field to null. Why?
When we call then on a Promise, we're effectively taking the value the Promise resolved to and returning a new Promise. You can think of it kind of like Array.map except for Promises. then can return a value, or another Promise. In either case, what's returned inside of then is "chained" onto the original Promise. Multiple Promises can be chained together like this by using multiple thens. Each Promise in the chain is resolved in sequence, and the final value is what's effectively resolved as the value of the original Promise.
In our example above, we returned nothing inside of the then, so the Promise resolved to undefined, which GraphQL converted to a null. To fix this, we have to return the posts:
function post(root, args) {
return getPost(args.id)
.then(post => {
console.log(post)
return post // <----
})
}
If you have multiple Promises you need to resolve inside your resolver, you have to chain them correctly by using then and returning the correct value. For example, if we need to call two other asynchronous functions (getFoo and getBar) before we can call getPost, we can do:
function post(root, args) {
return getFoo()
.then(foo => {
// Do something with foo
return getBar() // return next Promise in the chain
})
.then(bar => {
// Do something with bar
return getPost(args.id) // return next Promise in the chain
})
Pro tip: If you're struggling with correctly chaining Promises, you may find async/await syntax to be cleaner and easier to work with.
Common Scenario #6
Before Promises, the standard way to handle asynchronous code was to use callbacks, or functions that would be called once the asynchronous work was completed. We might, for example, call mongoose's findOne method like this:
function post(root, args) {
return Post.findOne({ where: { id: args.id } }, function (err, post) {
return post
})
The problem here is two-fold. One, a value that's returned inside a callback isn't used for anything (i.e. it's not passed to the underlying code in any way). Two, when we use a callback, Post.findOne doesn't return a Promise; it just returns undefined. In this example, our callback will be called, and if we log the value of post we'll see whatever was returned from the database. However, because we didn't use a Promise, GraphQL doesn't wait for this callback to complete -- it takes the return value (undefined) and uses that.
Most more popular libraries, including mongoose support Promises out of the box. Those that don't frequently have complimentary "wrapper" libraries that add this functionality. When working with GraphQL resolvers, you should avoid using methods that utilize a callback, and instead use ones that return Promises.
Pro tip: Libraries that support both callbacks and Promises frequently overload their functions in such a way that if a callback is not provided, the function will return a Promise. Check the library's documentation for details.
If you absolutely have to use a callback, you can also wrap the callback in a Promise:
function post(root, args) {
return new Promise((resolve, reject) => {
Post.findOne({ where: { id: args.id } }, function (err, post) {
if (err) {
reject(err)
} else {
resolve(post)
}
})
})
I had the same issue on Nest.js.
If you like to solve the issue. You can add {nullable: true} option to your #Query decorator.
Here's an example.
#Resolver(of => Team)
export class TeamResolver {
constructor(
private readonly teamService: TeamService,
private readonly memberService: MemberService,
) {}
#Query(returns => Team, { name: 'team', nullable: true })
#UseGuards(GqlAuthGuard)
async get(#Args('id') id: string) {
return this.teamService.findOne(id);
}
}
Then, you can return null object for query.
Coming from Flutter here.
I couldn't find any flutter related solution to this so since my search always brought me here, lemme just add it here.
The exact error was:
Failure performing sync query to AppSync:
[GraphQLResponse.Error{message='Cannot return null for non-nullable
type: 'AWSTimestamp' within parent
So, in my schema (on the AppSync console) I had this:
type TypeName {
id: ID!
...
_version: Int!
_deleted: Boolean
_lastChangedAt: AWSTimestamp!
createdAt: AWSDateTime!
updatedAt: AWSDateTime!
}
I got the error from the field _lastChangedAt as AWSTimestamp couldn't be null.
All I had to do was remove the null-check (!) from the field and it was resolved.
Now, I don't know the implications of this in the long run but I'll update this answer if necessary.
EDIT: The implication of this as I have found out is anything I do, amplify.push that change is reversed. Just go back to your appsync console and change it again while you test. So this isn't a sustainable solution but chatter I've picked up online suggests improvements are coming to amplify flutter very soon.
#Thomas Hennes got it spot on for me
The title field was resolved even though we didn't provide a resolver for it because the default resolver did the heavy lifting -- it saw there was a property named title on the Object the parent field (in this case post) resolved to and so it just resolved to that property's value. The id field resolved to null because the object we returned in our post resolver did not have an id property. The body field also resolved to null because of a typo -- we have a property called bod instead of body!
Pro tip: If bod is not a typo but what an API or database actually returns, we can always write a resolver for the body field to match our schema. For example: (parent) => parent.bod
One important thing to keep in mind is that in JavaScript, almost everything is an Object. So if the post field resolves to a String or a Number, the default resolver for each of the fields on the Post type will still try to find an appropriately named property on the parent object, inevitably fail and return null. If a field has an object type but you return something other than object in its resolver (like a String or an Array), you will not see any error about the type mismatch but the child fields for that field will inevitably resolve to null.
In case anyone has used apollo-server-express and getting null value.
// This will return values, as you expect.
const typeDefs = require('./schema');
const resolvers = require('./resolver');
const server = new ApolloServer({typeDefs,resolvers});
// This will return null, since ApolloServer constructor is not using correct properties.
const withDifferentVarNameSchema = require('./schema');
const withDifferentVarNameResolver= require('./resolver');
const server = new ApolloServer({withDifferentVarNameSchema,withDifferentVarNameResolver});
Note: While creating an instance of Apolloserver pass the typeDefs and resolvers var name only.
If none of the above helped, and you have a global interceptor that envelopes all the responses for example inside a "data" field, you must disable this for graphql other wise graphql resolvers convert to null.
This is what I did to the interceptor on my case:
intercept(
context: ExecutionContext,
next: CallHandler,
): Observable<Response<T>> {
if (context['contextType'] === 'graphql') return next.handle();
return next
.handle()
.pipe(map(data => {
return {
data: isObject(data) ? this.transformResponse(data) : data
};
}));
}
I'm trying to execute a query from java against a Map/Reduce view I have created on the CouchDB.
My map function looks like the following:
function(doc) {
if(doc.type == 'SPECIFIC_DOC_TYPE_NAME' && doc.userID){
for(var g in doc.groupList){
emit([doc.userID,doc.groupList[g].name],1);
}
}
}
and Reduce function:
function (key, values, rereduce) {
return sum(values);
}
The view seems to be working when executed from the Futon interface (without keys specified though).
What I'm trying to do is to count number of some doc types belonging to a single group. I want to query that view using 'userID' and name of the group as a keys.
I'm using Ektorp library for managing CouchDB data, if I execute this query without keys it returns the scalar value, otherwise it just prints an error saying that for reduce query group=true must be specified.
I have tried the following:
ViewQuery query = createQuery("some_doc_name");
List<String> keys = new ArrayList<String>();
keys.add(grupaName);
keys.add(uzytkownikID);
query.group(true);
query.groupLevel(2);
query.dbPath(db.path());
query.designDocId(stdDesignDocumentId);
query.keys(keys);
ViewResult r = db.queryView(query);
return r.getRows().get(0).getValueAsInt();
above example works without 'keys' specified.
I have other queries working with ComplexKey like eg:
ComplexKey key = ComplexKey.of(userID);
return queryView("list_by_userID",key);
but this returns only a list of type T (List) - using CouchDbRepositorySupport of course - and cannot be used with reduce type queries (from what I know).
Is there any way to execute the query with reduce function specified and a complex key with 2 or more values using Ektorp library? Any examples highly appreciated.
Ok, I've found the solution using trial and error approach:
public int getNumberOfDocsAssigned(String userID, String groupName) {
ViewQuery query = createQuery("list_by_userID")
.group(true)
.dbPath(db.path())
.designDocId(stdDesignDocumentId)
.key(new String[]{userID,groupName});
ViewResult r = db.queryView(query);
return r.getRows().get(0).getValueAsInt();
}
So, the point is to send the complex key (not keys) actually as a single (but complex) key containing the String array, for some reason method '.keys(...)' didn't work for me (it takes a Collection as an argument). (for explanation on difference between .key() and .keys() see Hendy's answer)
This method counts all documents assigned to the specific user (specified by 'userID') and specific group (specified by 'groupName').
Hope that helps anybody executing map/reduce queries for retrieving scalar values from CouchDB using Ektorp query.
Addition to Kris's answer:
Note that ViewQuery.keys() is used when you want to query for documents matching a set of keys, not for finding document(s) with a complex key.
Like Kris's answer, the following samples will get document(s) matching the specified key (not "keys")
viewQuery.key("hello"); // simple key
viewQuery.key(documentSlug); // simple key
viewQuery.key(new String[] { userID, groupName }); // complex key, using array
viewQuery.key(ComplexKey.of(userID, groupName)); // complex key, using ComplexKey
The following samples, on the other hand, will get document(s) matching the specified keys, where each key may be either a simple key or a complex key:
// simple key: in essence, same as using .key()
viewQuery.keys(ImmutableSet.of("hello"));
viewQuery.keys(ImmutableSet.of(documentSlug1));
// simple keys
viewQuery.keys(ImmutableSet.of("hello", "world"));
viewQuery.keys(ImmutableSet.of(documentSlug1, documentSlug2));
// complex key: in essence, same as using .key()
viewQuery.keys(ImmutableSet.of(
new String[] { "hello", "world" } ));
viewQuery.keys(ImmutableSet.of(
new String[] { userID1, groupName1 } ));
// complex keys
viewQuery.keys(ImmutableSet.of(
new String[] { "hello", "world" },
new String[] { "Mary", "Jane" } ));
viewQuery.keys(ImmutableSet.of(
new String[] { userID1, groupName1 },
new String[] { userID2, groupName2 } ));
// a simple key and a complex key. while technically possible,
// I don't think anybody actually does this
viewQuery.keys(ImmutableSet.of(
"hello",
new String[] { "Mary", "Jane" } ));
Note: ImmutableSet.of() is from guava library.
new Object[] { ... } seems to have same behavior as ComplexKey.of( ... )
Also, there are startKey() and endKey() for querying using partial key.
To send an empty object {}, use ComplexKey.emptyObject(). (only useful for partial key querying)