How to get common array elements mongodb

Question

First document:

    {
    "_id" : 1,
    "array" : [ 
        {
            "userID" : 1,
            "name" : "name 1"
        }, 
        {
            "userID" : 2,
            "name" : "name 2"
        }, 
        {
            "userID" : 3,
            "name" : "name 3"
        }, 
        {
            "userID" : 6,
            "name" : "name 6"
        }
    ]
}

Second document:

    {
    "_id" : 2,
    "array" : [ 
        {
            "userID" : 1,
            "name" : "name 1"
        }, 
        {
            "userID" : 2,
            "name" : "name 2"
        }, 
        {
            "userID" : 3,
            "name" : "name 3"
        }, 
        {
            "userID" : 4,
            "name" : "name 4"
        }, 
        {
            "userID" : 5,
            "name" : "name 5"
        }
    ]
}

is there any way to get output like this:

   {    
    "array" : [  
        {  
            "userID" : 1,  
            "name" : "name 1" 
        },  
        {  
            "userID" : 2,  
            "name" : "name 2"  
        },   
        {  
            "userID" : 3,  
            "name" : "name 3"  
        }  
    ]  

    }

I want to compare array fields of two documents and retrieve common elements in it. And compare array fields only by common 'userID'. Can we do it by $setIntersection? I need to check only two array fields at a time.

Will there always be 2 documents ? and is this (the 2 documents) an aggregation result ? — Elvis
– Elvis, Commented Jan 21, 2019 at 15:26
it is not an aggregation result. and i will take two documents by document._id — Arun Girivasan
– Arun Girivasan, Commented Jan 21, 2019 at 15:34
by First Document and Secoend Document do you mean, this are different different Objects right? or part of a SIngle Object? — Saikat Chakrabortty
– Saikat Chakrabortty, Commented Jan 21, 2019 at 15:37
you could also look into this answer , stackoverflow.com/questions/30646534/… — Saikat Chakrabortty
– Saikat Chakrabortty, Commented Jan 21, 2019 at 15:40
yes by first document and second document, these are two different objects — Arun Girivasan
– Arun Girivasan, Commented Jan 21, 2019 at 15:40

Saravana · Accepted Answer · 2019-01-21 16:04:57Z

1

you can use $setIntersection

db.t67.aggregate([
    //{$match : {}} add filter criteria to filter documents
    {$group : {_id : null, array : {$push : "$array"}}},
    {$project : {_id :0, array : {$reduce : {input : "$array", initialValue : {$arrayElemAt : ["$array", 0]}, in : {$setIntersection : ["$$this", "$$value"]}}}}}
])

output

> db.t67.aggregate([
...     {$group : {_id : null, array : {$push : "$array"}}},
... {$project : {_id :0, array : {$reduce : {input : "$array", initialValue : {$arrayElemAt : ["$array", 0]}, in : {$setIntersection : ["$$this", "$$value"]}}}}}
... ])
{ "array" : [ { "userID" : 1, "name" : "name 1" }, { "userID" : 2, "name" : "name 2" }, { "userID" : 3, "name" : "name 3" } ] }

or $unwind and $group

db.t67.aggregate([
    // $match stages to filter 2 documents
    {$unwind : "$array"},
    {$group : {_id : "$array", count : {$sum : 1}}},
    {$match : {count : {$gt : 1}}},
    {$project : {count : 0}},
    {$group : {_id : null, array : {$push : "$_id"}}},
    {$project : {_id : 0}}
])

output

{ "_id" : null, "array" : [ { "userID" : 3, "name" : "name 3" }, { "userID" : 2, "name" : "name 2" }, { "userID" : 1, "name" : "name 1" } ] }

edited Jan 21, 2019 at 16:04

answered Jan 21, 2019 at 15:41

Saravana

12.8k2 gold badges42 silver badges60 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Arun Girivasan Over a year ago

thanks for your reply.here we unwind array field of all documents, right? my collection has lot of documents like this. so i want to take only particular documents. not all documents. i can't take particular documents by passing document id from this query.

Saravana Over a year ago

you need to add $match stage to filter the documents, in the answer I have commented the $match stage where you need to specify the filter criteria

Collectives™ on Stack Overflow

How to get common array elements mongodb

1 Answer 1

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related