Can i filter one collection that has _id in other collection and have result on in mongodb

Question

I have the collection "A" like this

{
"_id" : ObjectId("5c4c1f2a5173562961468b30"),
"name" : "first"
},
{
"_id" : ObjectId("57c162f267045ec439ba2485"),
"name" : "second"
}

Like above i have multiple doc in a collection,

Now i have colection "B" like below which has _id of Collections a's doc

{
"_id" : ObjectId("57d14ad36495a197593ab7ab"),
"a_id" : ObjectId("5c4c1f2a5173562961468b30"),
"name" : "ab"
},
{
"_id" : ObjectId("57d941503dd051cc04205e1e"),
"a_id" : ObjectId("5c4c1f2a5173562961468b30"),
"name" : "cd "
}

Now i need a query that will give me result collection "A" based on how many documents are associated with "B"

If "A" has 2 documents in "B" I need those 2 "B" documents

Could you provide an example what are the query conditions and what is the ouput？ — yellowB, Mar 09 '19 at 09:01
i want output all doc of collection a which has more than 2 associate doc in collection b — Foram Sojitra, Mar 09 '19 at 09:10
For more specific details of output you need to add needed output sample too. Thanks. — Ashok, Mar 09 '19 at 09:28

Ashok · Answer 1 · 2019-03-09T12:08:21.557

0

This simple aggregation make your result

 db.A.aggregate([
  {
    "$lookup": {
      "from": "B",
      "localField": "_id",
      "foreignField": "a_id",
      "as": "docuentInB"
    }
  },
  {
    $match: {
      $expr: { $gte: [{ $size: "$docuentInB" }, 2 ] }
    }
  },
  {
    $project: {
      docuentInB: 0
    }
  }
])

edited Mar 09 '19 at 12:08

answered Mar 09 '19 at 09:28

Ashok

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I don't want data of b what i need is just data from collection a which has more than 2 doc in b – Foram Sojitra Mar 09 '19 at 10:54
1

@ForamSojitra Please add $gt or $gte (greater then equal now present) as per requirement in stage 2nd. – Ashok Mar 09 '19 at 12:09

Can i filter one collection that has _id in other collection and have result on in mongodb

1 Answers1