How to define parameters/arguments to a list resolver? - graphql-js

Question

I have this schema and the corresponding resolvers:
const schema = buildSchema(
`
type Query {
posts(id: Int): [Post]
}
type Post {
id: Int!,
title: String,
date: String
}`
);
const resolvers = {
posts(root, { id }, context, info) {
console.log(id); // Undefined
return [
{
id: 0,
date: '21/04/2018',
title: 'Post 1'
},
{
id: 1,
date: '07/10/2018',
title: 'Post 2'
}
];
},
Post(postObj) {
return {
id: postObj.id,
title: postObj.title,
date: postObj.date
}
}
}
The problem is that when I query for posts with an specified id, like this:
query {
posts(id: 0) {
title
}
}
... I get an error that says I haven't defined such argument (id).
I defined the id argument according to the GraphQL Docs. Any suggestions of what may be causing this error and how to solve it?

Answer 1

When you use buildSchema, you effectively prevent yourself from being able to define custom resolvers for a given field in your schema. Instead, the default resolver will always be used. The default resolver simply takes the "parent" or "root" object for a given field, looks up the property on that parent object with the same name as the field and returns its value.
You can "get away" with this for simpler schemas by passing in a root object along with your schema. This root object then becomes the parent object referenced by the default resolver, but only for top level fields (like each field you define for your Query type). So in this case, when GraphQL resolves your query, the default resolver sees a posts property on the parent object and returns that. Because posts is actually a function, it calls the function first and then returns the value, but the arguments it calls it with are not the same arguments that a resolver is called with.
Resolvers receive four parameters -- 1) the "root" or "parent" value, 2) arguments, 3) context and 4) an "info" object containing additional data about the request. Any function called by the default resolver will only get the last 3 parameters (so no "root" value).
In other words, you should change your root object to look more like this:
const root = {
posts({ id }, context, info) {
return [
{
id: 0,
date: '21/04/2018',
title: 'Post 1'
},
{
id: 1,
date: '07/10/2018',
title: 'Post 2'
}
];
},
}
However, does it this way will only let you handle top-level fields like queries. You will not be able to customize resolver behavior for fields on other types, like Post. To do that, you should use graphql-tools' makeExecutableSchema.