I need to get the length of a string value in MongoDB using aggregation functions.
it works in
db.collection_name.find({"$where":"this.app_name.length===12"})
but when implanted to
db.collection_name.aggregate({$match:
{"$where":"this.app_name.length===12"}
},
{
$group :
{
_id : 1,
app_downloads : {$sum: "$app_downloads"}
}
}
);
I got this result:
failed: exception: $where is not allowed inside of a $match aggregation expression
The question is: is it possible to use $where in aggregation functions?
or is there any way of getting the length of a string value in aggregation function?
Thanks in advance
Eric
MongoDB doesn't support $where in aggregation pipeline and hope this will never happen, because JavaScript slows things down. Never the less, you still have options:
1) Мaintain additional field(e.g. app_name_len) than will store app_name length and query it, when needed.
2) You can try extremely slow MapReduce framework, where you allowed to write aggregations with JavaScript.
Today I had the same problem.
Mongodb doesn't support this.app_name.length, but you can do this condition with $regex - this is not very quick, but it still works.
{"app_name": { $regex: /^.{12}$/ }}
A simple way to achieve the behaviour expected of OP would be chaining up $expr with $strLenCP
db.collection.find({
$expr: {
$eq: [
12,
{
$strLenCP: "$app_name"
}
]
}
})
Mongo Playground
Related
Following the MongoDB documentation, you can use the $in operator with a regular expression like wise db.inventory.find( { tags: { $in: [ /^be/, /^st/ ] } } ). Is there a way to achieve the same result using mongoengine?
For example pass {"tags__in": ["/^be/", "/^st/"]} to my query?
I don't think this is supported by MongoEngine in normal query construct (i.e I doubt that Doc.objects(tags__in=["/^be/", "/^st/"] will work)) but there is support for __raw__ query (https://docs.mongoengine.org/guide/querying.html#raw-queries)
so the following should work
Inventory.objects(__raw__={"tags": { "$in":["/^be/", "/^st/"]}})
I have been reading up on some mongodb documentation and ran across some confusing terminology, namely how to differentiate when a symbol will be used as an aggregate function or as an operator.
For example, the $size function either calculates the number of items in an array, or checks if the number of elements in an array is equal to a number, is there any way to know what the function will do at what time? Through trial and error I discovered that the $size will throw an error unless a number is passed to it in the $match step, but is there some rule/guideline so I can know what it will do beforehand?
db.collection.aggregate([
{
$project: {
key: 1,
number: {
$size: "$key"
}
}
},
{
$match: {
key: {
$size: 1
}
}
}
])
For querying data in MongoDB you can use the find method or the aggregate method. There are operators which you can use with these methods.
These query operators are used with the find method. Some of these can also used with the $match stage of the aggregate method (details later in the post).
These aggregation pipeline operators are used within the aggregate's stages. Some of these can also be used with the find method (details later in the post).
You will notice that, there are common operator names; for example, $eq, $gte, $or, $type, $size, etc. But, their usage and / or functionality can be different. The $eq operator has same function but different usage syntax and the $typeoperator has different functionality (and usage syntax).
And, some of these operators can be used with both the methods.
Some Usage Scenarios:
Lets consider a users collection and some queries:
{ "_id" : 1, "age" : 21, "firstname" : "John" }
{ "_id" : 2, "age" : 18, "firstname" : "John" }
{ "_id" : 3, "age" : "39", "firstname" : "Johnson" }
The query:
db.users.find( { firstname: { $eq: "John"}, age: { $gt: 20 } } )
This query's filter is same as { firstname: "John"}, age: { $gt: 20 } }. This uses the query operators $eq and $gt. The same query can be written in the aggregate method's $match stage:
db.users.aggregate([
{ $match: { firstname: "John", age: { $gt: 20 } } },
])
The operators used in this case are the same query operators. The query comparison operators can be used with the aggregate method's $match and $lookup stages.
Another Scenario:
db.users.aggregate([
{ $project: { ageGreaterThan20: { $gt: [ "$age", 20 ] } } },
])
This is the usage of aggregation comparison operator $gt. Note this is used within the aggregation query, but within the $project stage.
As you had used the query operators in the aggregation query, you can also use the aggregation operators within the find method. But, this must be used with the "special" $expr operator. For example:
db.users.find( { $expr: { $gt: [ "$age", 20 ] } } )
The advantage of the $expr is that - there are number of aggregation operators which can be used within the find queries. For example, using the $strLenCP:
db.users.find( { $expr: { $gt: [ { $strLenCP: "$firstname" }, 4 ] } } )
You can also use the $expr within the aggregation, within the $match or $lookup stages:
db.users.aggregate([
{ $match: { $expr: { $gt: [ "$age", 20 ] } } },
])
Finally:
I have been reading up on some mongodb documentation and ran across
some confusing terminology, namely how to differentiate when a symbol
will be used as an aggregate function or as an operator. ... but is
there some rule/guideline so I can know what it will do beforehand?
Reading helps, practicing helps better and experience helps the best. You use a specific operator in a specific scenario or use case. To achieve some functionality you use an appropriate method and operator(s).
Reference: Operators
As per my understanding, I'm pointing out few things :
Difference between aggregation and operator
You'll have certain basic functions for crud operations like .find() or .insert() or .delete() or .update() on MongoDB (There are few others like .count(), .distinct() but those are primary)
Versus
aggregation is a whole framework heavily used for complex reads, only two stages in aggregation is capable of writes $out and $merge.
Operators :
There are different types of operators :
Query and Projection Operators : These operators are crucial and are used to filter docs and to transform fields of docs in the response, Usually used in filter and project part of .find(filter, project) or .update(filter, update, project/options) etc.. These are also used in $match stage of aggregation pipeline ($match is similar to filter in .find()). Ex. :- $and, $or, $in, $or & more.
Update Operators : By name, these operators help to update documents in the collection. Usually used in update part of .update() or .findOneAndUpdate() etc.. Ex. :- $set, $unset, $inc & more.
Aggregation :
When it comes to aggregation they call it aggregation framework, definitely for a reason as you can do a lot of things with data using aggregation.
Aggregation has aggregation pipeline , which has syntax .aggregate([]). pipeline is an array with stages. Each stage in aggregation does certain operation on data flowing through them. Ex. :- $match, $project, $group etc..
As we know each document is independent on it's own, most of aggregation stages operate independently on each doc flowing through them.
Aggregation pipeline Operators :
These operators are generally used to achieve what you're looking for, certain operators can't be used in conjunction with other operators or in a stage.
Let's say in $match stage you would mostly use Query operators but not Aggregation operators as aggregation operators can't directly be used in $match in contrast with $project or $addFields stages.
Example with Stages & Operators :
Let's say you got to make Smoothie :
Out of a bunch of groceries you would filter needed fruits(docs in fruits collection) by matching with what you wanted to blend using $match stage ($and to match fruits/veggies, $lt to filter only fruits that hasn't expired and $size to limit no.of fruits needed).
You would peel off skin or chop into pieces to keep just keep useful parts of fruits(docs) using $project.
You would group all of the fruits into a blender using $group & add ingredients like cream & sugar using $addFields - You're too cautious about sugar quantity so you'll use operators like $size to check size and $multiply no.of nutrients based on conditions ($cond) you would $divide both sugar/nutrients to count nutrition value.
You'll iterate on adding ice pieces again and again using $map, $filter to remove un crushed ice pieces.
uhh, you always forget to add veggies (A different collection needs to be merged) based on fruits that already got blended you use $lookup to lookup for matching veggies(docs) & either blend in again with group or just put it on top.
Finally either you drink it from blender (just return the docs no more stages) or take into glass using $out or $merge stage (Remember as I said, only two stages that can write to a collection).
Off course every stage is Optional - You can eat fruits as is instead of blending (get all docs and all their fields) but you know what you wouldn't do that (for many reasons like performance, unnecessary data flowing through network) unless it's pre-prepared product (like a small configuration collection which has limited data with few docs & every doc is need & can be easily retrieved in one DB call).
Note :
Usually you can't use aggregation operators in filter part i.e; .find(filter) or in '$match' stage unless you use $expr.
Starting MongoDB v4.2 you can run aggregation pipeline in update-with-an-aggregation-pipeline where you can take advantage of aggregation stages/operators in update part.
While you search MongoDB's documentation often you would find the same operator in multiple places, Let's say if you search for $size you would find multiple references one is Query operator or as Projection operator or as aggregation operator - So depends on need/where you wanted to use you can refer their documentation for usage cause though name seems to be similar or does almost same but functionality may differ & syntax also differs.
You can always MongoDB for free at MongoDB University.
I am attempting to update information on one of my fields where two different conditions are met. I don't know if this is the right idea on how to do it, but guidance would be appreciated. I am met with the error of: "unknown top level operator: $eq"
db.shop.update({"$and":[{"item.name":"Milk"}, {"$eq":{"item.price":2.80}}]}, {"$set": {"item.expiry":"21/10/19"}});
You can use $and as
db.shop.update(
{"$and":[{"$eq":["item.name","Milk"]}, {"$eq":["item.price",2.80]}]},
{"$set": {"item.expiry":"21/10/19"}}
)
but it is equivalent to below and will be faster as expression takes time to compute
db.shop.update(
{"item.name":"Milk","item.price":2.80},
{"$set": {"item.expiry":"21/10/19"}}
)
Remove $eq and $and and just use the following query.
db
.shop
.update({
"item.name": "Milk",
"item.price": 2.80
}, {
"$set": {
"item.expiry": "21/10/19"
}
});
I'm trying to get the last 20 records of user collection with mongoose:
User.find({'owner': req.params.id}).
sort(date:'-1').
limit(20).
exec(.....)
This works well, show the last 20 items.
But the items inside the array are sorted from the most recent to the oldest, Is there any way to reverse this with mongoose?
Thanks
You can certainly do this with an aggregation, such as this:
db.user.aggregate[(
{ $match : {"owner" : req.params.id}},
{ $sort : {"date" : -1}},
{ $limit : 20},
{ $sort : {"date" : 1}}
])
Notes on this aggregation:
The first three parts do the same job as the Find in your question
The fourth part applies a further sort, which re-orders the returned 20 records from oldest to most recent
I have written it in native MongoDB aggregation syntax; you will need to adjust the code to generate the same aggregation from Mongoose.
Update: I think this is not possible with a find() with cursor methods, because you would need two different sort() operations. But, MongoDB does not treat them as a sequence of independent operations; the docs give an example of methods written in one order — sort().limit() — being equivalent to the opposite order — limit().sort(), showing that the order cannot be relied upon as meaningful.
Find total and select only latest 20 , may be this is not effective way you found , but this will solve your problem.
User.count({'owner': req.params.id},function(err,count){
if(count){
var skipItem=count-20;
User.find({'owner': req.params.id}).
.skip(skipItem)
.limit(20)
.sort(date:'1').
exec(.....)
}
});
db.users.aggregate([
{ $match: {
'owner': req.params.id
}},
{ $unwind: '[arrayFieldName]' },
{ $sort: {
'[arrayFieldName]': -1/1,
'date':-1
}}
])
I noticed a new $eq operator released with MongoDB 3.0 and I don't understand the purpose of it. For instance these two queries are exactly the same:
db.users.find({age:21})
and
db.users.find({age:{$eq:21}})
Does anyone know why this was necessary?
The problem was that you'd have to handle equality differently from comparison when you had some kind of query builder, so it's
{ a : { $gt : 3 } }
{ a : { $lt : 3 } }
but
{ a : 3 }
for equality, which looks completely different. The same applies for composition of $not, as JohnnyHK already pointed out. Also, comparing with $eq saves you from having to $-escape user provided strings. Therefore, people asked for alternatives that are syntactically closer and it was implemented. The Jira ticket contains a longer discussion which mentions all these points.
The clearer syntax of an $eq operator might also make sense in the aggregation framework to compare two fields, should such a feature be implemented.
Also, the feature has apparently been around since 2.5, was added to the documentation relatively late.
One specific application I can see for $eq is with cases like the $not operator which requires that its value is an operator-expression.
This allows you to construct a query like:
db.zips.find({state: {$not: {$eq: 'NY'}}})
Before, the closest you could get to this semantically was:
db.zips.find({state: {$not: {$regex: /^NY$/}}})
I realize there are other ways to represent the functionality of that query, but if you need to use the $not operator there for other reasons, this would now allow it.
In filter part of an aggregation query if you need to check if some field is equal to a value you can not use assign syntax:
db.sales.aggregate([
{
$project: {
items: {
$filter: {
input: "$items",
as: "item",
cond: { $eq: [ "$$item.price", 100 ] }
}
}
}
}
])