I'm wanting to create an aggregation step to match documents where the value of a field in a document exists within an array in the same document.
In a very worked example (note this is very simplified; this will be fitting into a larger existing pipeline), given documents:
{
"_id":{"$oid":"61a9085af9733d0274c41990"},
"myArray":[
{"$oid":"61a9085af9733d0274c41991"},
{"$oid":"61a9085af9733d0274c41992"},
{"$oid":"61a9085af9733d0274c41993"}
],
"myField":{"$oid":"61a9085af9733d0274c41991"} // < In 'myArray' collection
}
and
{
"_id":{"$oid":"61a9085af9733d0274c41990"},
"myArray":[
{"$oid":"61a9085af9733d0274c41991"},
{"$oid":"61a9085af9733d0274c41992"},
{"$oid":"61a9085af9733d0274c41993"}
],
"myField":{"$oid":"61a9085af9733d0274c41994"} // < Not in 'myArray' collection
}
I want to match the first one because the value of myField exists in the collection, but not the second document.
It feels like this should be a really simple $elemMatch operation with an $eq operator, but I can't make it work and every example I've found uses literals. What I've got currently is below, and I've tried with various combinations of quotes and dollar signs round myField.
[{
$match: {
myArray: {
$elemMatch: {
$eq: '$this.myField'
}
}
}
}]
Am I doing something very obviously wrong? Is it not possible to use the value of a field in the same document with an $eq?
Hoping that someone can come along and point out where I'm being stupid :)
Thanks
You can simply do a $in in an aggregation pipeline.
db.collection.aggregate([
{
"$match": {
$expr: {
"$in": [
"$myField",
"$myArray"
]
}
}
}
])
Here is the Mongo playground for your reference.
Related
I want to match documents in my pipeline based on whether the field to match is contained within an array that is within my documents.
Example document to match:
{
'wishlist': ['123','456','789'],
'productId': '123'
}
Example match aggregation:
{
$match: {
'productId': {$in: '$wishlist'}
}
}
This isn't working - error is '$in needs an array' - but '$wishlist' is an array? so clearly the stage isn't picking up the path reference.
How would I get something like this to work?
Thanks!
If you want to match the internal field of the document, you can use $expr expression operator, and I see that field has an array value then you have to use $in operator's aggregation syntax,
{
$match: {
$expr: {
$in: ["$productId", '$wishlist']
}
}
}
In MongoDB how can I do a search that is filtered by a predicate applied on the last element of an embedded array?
I know if I wanted to do it on the first one I could do this:
db.inventory.find( { 'instock.0.qty': { $lte: 20 } } )
As specified on the documentation.
How do I write an analog query that looks at the last element, when I don't know the exact size of the embedded array?
we can use $arrayElemAt and pass -1 to it as a second argument to get the last element in the array
something like this
db.collection.find({
$expr: {
$gt: [
{
$arrayElemAt: ["$instock.qty", -1]
},
10
]
}
})
you can test it here Mongo Playground
hope it helps
I'm sorry but I'm little confuse with a query , Kindly help. suppose we've one document that contains
{
"_id":100,
"name":"Demarcus Audette",
"scores":[
{
"score":47.42608580155614,
"type":"exam"
},
{
"score":44.83416623719906,
"type":"quiz"
},
{
"score":39.01726616178844,
"type":"homework"
},
"score":89.01726616178844,
"type":"homework"
}
]
}
And I want to write a query that should return only rows which contains homework in that , that means the out put should be like below
{
"_id":100,
"name":"Demarcus Audette",
"scores":[
{
"score":39.01726616178844,
"type":"homework"
},
"score":89.01726616178844,
"type":"homework"
}
]
}
Kindly suggest. Thanks in Advance
Use the $elemMatch operator.
db.collection.find({ "scores": { $elemMatch: { "type": "homework" } } } );
EDIT
What you are asking is not possible. You will need the above query and filter out the rest in whatever language you are programming. You can also use an aggregate function using $unwind and $match.
db.collection.aggregate(
{$unwind: "$messages"},
{$match: {"scores.type": "homework"}}
);
$unwind flattens your array and $match is your actually query which will return matching documents. Please note that $unwind will create a different document for each element in your array. This means you will get two results when you filter on 'homework' according to your example.
This is my object:
{ "_id" : ObjectId("53fdcb6796cb9b9aa86f05b9"), "list" : [ "a", "b" ], "complist" : [ { "a" : "a", "b" : "b" }, { "a" : "c", "b" : "d" } ] }
And this is what I want to accomplish: check if "list" contains a certain element and get only the field "a" from the objects on "complist" while reading the document regardless of any of these values. I'm building a forum system, this is the query that will return the details of a forum. I need to read the forum information while knowing if the user is in the forum's white list.
With a find I can use the query
db.itens.find({},{list:{$elemMatch:{$in:["a"]}}})
to get only the first element that matches a certain value. This way I can just check if the returned array is not empty and I know if "list" contains the value I'm looking for. I can't do it on the query because I want the document regardless of it containing the value I'm looking for in the "list" value. I need the document AND know if "list" has a certain value.
With an aggregate I can use the query
db.itens.aggregate({$project:{"complist.a":1}})
to read only the field "a" of the objects contained in complist. This is going to get the forum's threads basic information, I don't want all the information of the threads, just a couple of things.
But when I try to use the query
db.itens.aggregate({$project:{"complist.b":1,list:{$elemMatch:{$in:["a"]}}}})
to try and do both, it throws me an error saying the operator $elemMatch is not valid.
Am I doing something wrong here with the $elemMatch in aggregate? Is there a better way to accomplish this?
Quite on old question but literally none of the proposed answers are good.
TLDR:
You can't use $elemMatch in a $project stage. but you can achieve the same result using other aggregation operators like $filter.
db.itens.aggregate([
{
$project: {
compList: {
$filter: {
input: "$complist",
as: "item",
cond: {$eq: ["$$item.a", 1]}
}
}
}
}
])
And if you want just the first item from the array that matches the condition similarly to what $elemMatch does you can incorporate $arrayElemAt
In Depth Explanation:
First let's understand $elemMatch:
$elemMatch is a query expressions while also this projection version of it exists this refers to a query projection and not $project aggregation stage.
So what? what does this have to do with anything? well a $project stage has certain input structure it can have while the one we want to use is:
<field>: <expression>
What is a valid expression?
Expressions can include field paths, literals, system variables, expression objects, and expression operators. Expressions can be nested.
So we want to use an expression operator, but as you can see from the doc's $elemMatch is not part of it. hence it's not a valid expression to be used in an aggregation $project stage.
For some reason $elemMatch doesn't work in aggregations. You need to use the new $filter operator in Mongo 3.2. See https://docs.mongodb.org/manual/reference/operator/aggregation/filter/
The answer to this question maybe help.
db.collection_name.aggregate({
"$match": {
"complist": {
"$elemMatch": {
"a": "a"
}
}
}
});
Actually, the simplest solution is to just $unwind your array, then $match the appropriate documents. You can wind-up the appropriate documents again using $group and $push.
Although the question is old, here is my contribution for November 2017.
I had similar problem and doing two consecutive match operations worked for me. The code below is a subset of my whole code and I changed elements names, so it's not tested. Anyway this should point you in the right direction.
db.collection.aggregate([
{
"$match": {
"_id": "ID1"
}
},
{
"$unwind": "$sub_collection"
},
{
"$match": {
"sub_collection.field_I_want_to_match": "value"
}
}
])
For aggregations simply use $expr:
db.items.aggregate([
{
"$match": {
"$expr": {"$in": ["a", "$list"]}
}
},
])
Well, it happens you can use "array.field" on a find's projection block.
db.itens.find({},{"complist.b":1,list:{$elemMatch:{$in:["a"]}}})
did what I needed.
In MongoDB, is there any easy way to check Order of element in Array? For example I have a document like this:
{
_id: 1,
tags: ["mongodb", "rethinkdb", "couchbase", "others"]
}
I would like to check in tags field if mongodb come before rethinkdb or not(lets see in array element, mongodb=0, rethinkdb=1 index, so mongodb come first and our case match.)?
but if there is another document (like below) where rethinkdb comes before mongodb,It case does not match.
{
_id: 2,
tags: ["rethinkdb", "mongodb", "couchbase"]
}
Here mongodb(1) comes after rethinkdb(0) so our case does not match.
Your question is not really as clear as you think it is, and thus why there are several ways to answer it:
If you are looking just to find out if a document has "mongodb" as the first element of the array then you just issue a query like this:
db.collection.find({ "tags.0": "mongodb" })
And that will return only the documents that match the given value at the specified index position using "dot notation".
If you actually expect to match if an array is in an "expected order" then you can get some help from the aggregation pipeline and set operators that are available and other features in MongoDB 2.6:
db.collection.aggregate([
{ "$project": {
"$_id": "$$ROOT",
"matched": { "$setEquals": [
"$tags",
["mongodb", "rethinkdb", "couchbase", "others"]
]}
}},
{ "$match": { "matched": true }}
])
Or if your want is to make sure that the "mongodb" value comes before the "rethinkdb" value, then you will need to evaluate in JavaScript with mapReduce, or something equally not nice like the $where operator:
db.collection.find({
"$where": function() {
return this.tags.indexOf("mongodb") < this.tags.indexOf("rethinkdb");
}
})