CODEWITHSUNDEEP
mongodbmongodb-queryaggregation-framework
Parvez M Robin
Parvez M Robin

Reputation: 174

In MongoDB aggregation pipeline, how to project indices of embedded array that matched?

In a mongodb aggregation pipeline, I want to $project the indices of an embedded array (a sub-document) that matches a previous $match stage.

Say, I have the following docs.

{_id: '1', tags: ['aaa', 'bbb', 'ccc']},
{_id: '2', tags: ['baa', 'aaa', 'aaa']},
{_id: '3', tags: ['aac', 'cbb', 'aca']},

Now, if I query with {tags: 'aaa'}, I want to output something similar to

{_id: '1', tags: [0]},
{_id: '2', tags: [1, 2]}

Upvotes: 1

Views: 630

Answers (3)

the_mahasagar
the_mahasagar

Reputation: 1201

db.inventory.aggregate([
  { $match : {tags : 'aaa' }},
  { $unwind : { path: "$tags", includeArrayIndex: "arrayIndex"}},
  { $match : {tags : 'aaa' }},
  { $group : {
      _id : '$_id',
      tags : { $push : '$arrayIndex'}
    }
  }
 ])

Output : 

{ "_id" : "2", "tags" : [ NumberLong(1), NumberLong(2) ] }
{ "_id" : "1", "tags" : [ NumberLong(0) ] }

Another way : 

db.inventory.aggregate([
 { $match : {tags : 'aaa' }},
 { $project  : {
    tags: {
      $filter: {
        input: {
          $zip: {
            inputs: [ "$tags", { $range: [0, { $size: "$tags" }] } ]
          }
        },
        as: "tagWithIndex",
        cond: {
          $let: {
            vars: {
              tag : { $arrayElemAt: [ "$$tagWithIndex", 0 ] }
            },
            in: { $eq: [ "$$tag", 'aaa' ] }
          }
        }
      }
    }
 }},
 { $unwind  : '$tags'},
 { $group : {
       _id : '$_id',
       tags  : {
          $push : { $arrayElemAt: [ "$tags", 1]}
       }
   }
 }
])

Output : 

{ "_id" : "2", "tags" : [ 1, 2 ] }
{ "_id" : "1", "tags" : [ 0 ] }

hope this helps.

Upvotes: 1

Ashh
Ashh

Reputation: 46481

You need to $map over the $size of the $tags array to include index of the each element inside the tags array and then you can easily use $filter aggregation to exclude the elements which do contain letter aaa

db.collection.aggregate([
  { "$match": { "tags": "aaa" }},
  { "$project": {
    "tags": {
      "$filter": {
        "input": {
          "$map": {
            "input": { "$range": [0, { "$size": "$tags" }] },
            "in": {
              "string": { "$arrayElemAt": ["$tags", "$$this"] },
              "index": "$$this"
            }
          }
        },
        "cond": { "$eq": ["$$this.string", "aaa"] }
      }
    }
  }},
  { "$project": { "tags": "$tags.index" }}
])

Output

[
  {
    "_id": "1",
    "tags": [0]
  },
  {
    "_id": "2",
    "tags": [1, 2]
  }
]

Upvotes: 1

Talhakaanozkan
Talhakaanozkan

Reputation: 174

If you're searching for an array, you should use $in.

db.inventory.find( { tags: { $in: [ 'aaa' ] } } )

You can also write the same in the match. spelling is the same.

Will help for detail. That's what you're looking for.

Source : https://docs.mongodb.com/manual/reference/operator/query/in/

db.inventory.find( { "tags": { $in: 'aaa' } },
                  { "tags.$": 1 } )

This is probably what you want.

Upvotes: 0

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