MongoDB - Returning only matching Array from Object

Question

i have the following document in a collection:

{
    "_id" : 101,
    "students" : [ 
        {
            "name" : "john",            
            "age" : 10,
            "city":'CA'
        },
        {
            "name" : "danial",            
            "age" : 15,
            "city":'KA'
        }
    ]
}

{
    "_id" : 102,
    "students" : [ 
        {
            "name" : "adam",            
            "age" : 20,
            "city":'NY'
        },
        {
            "name" : "johnson",            
            "age" : 12,
            "city":'CA'
        }
    ]
}

And i fire the following query:

db.data.find({'students.city':'CA'})

This returns me "students" objects from both the documents, as one instance matches the filter ("city"; 'CA') in both.

However, i desire to only get the matching array in the result. That is, i want the following result:

{
    "_id" : 101,
    "students" : [ 
        {
            "name" : "john",            
            "age" : 10,
            "city":'CA'
        }
    ]
}

{
    "_id" : 102,
    "students" : [        
        {
            "name" : "johnson",            
            "age" : 12,
            "city":'CA'
        }
    ]
}

Please help.

Community · Accepted Answer · 2017-03-17 08:26:47Z

3

You need to use $elemMatch in your projection:

> db.data.find({'students.city':'CA'},{ students:{ $elemMatch:{'city':'CA'} }})
{ "_id" : 101, "students" : [ { "name" : "john", "age" : 10, "city" : "CA" } ] }
{ "_id" : 102, "students" : [ { "name" : "johnson", "age" : 12, "city" : "CA" } ] }

Btw: I'd strongly suggest reading the Open letter to students with homework problems.

edited Mar 17, 2017 at 8:26

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answered Feb 15, 2016 at 12:01

Markus W Mahlberg

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Comments

Community · Accepted Answer · 2017-05-23 12:00:24Z

0

I think you should use an aggregation operation.

Here you have the documentation: https://docs.mongodb.org/manual/aggregation/

And a similar question: Retrieve only the queried element in an object array in MongoDB collection

edited May 23, 2017 at 12:00

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answered Feb 15, 2016 at 11:55

Telmo Ivo

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4 Comments

Markus W Mahlberg Over a year ago

Nope, that's plainly wrong. See my answer.

Telmo Ivo Over a year ago

yep. you are right. after confirming again. it's better your way. Correct me if I'm wrong: I said to use aggregate because I thought you cannot return multiple elements of an array matching your criteria using a find() operation. but since it's different documents and only 1 matching element per document.your way it's the right one.

Markus W Mahlberg Over a year ago

I haven't checked, but I think it could be possible to use an "$or" in the projection...

Gopal Over a year ago

@MarkusWMahlberg i tried to use $or inside the $elemMatch to return multiple elements, but it was not working.

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