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]}
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.
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" }}
])
[
{
"_id": "1",
"tags": [0]
},
{
"_id": "2",
"tags": [1, 2]
}
]
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.
db.inventory.find( { tags: { $in: [ 'aaa' ] } } ) will give me the first and second doc (from the docs in the question) as a whole. However, I need the indices of tags array that match the query.db.inventory.find( { "tags": { $in: 'aaa' } }, { "tags.$": 1 } ) This gives a list of elements of the array that matches. Not their indices. However, I need their indices.