How to return Objects with empty sub Array document if sub Array documents don't satisfy the match condition?

Question

Mongoose is not returning Objects if sub Array documents don't satisfy the match condition in mongoose.

MongoDB Query:

db.fruits.aggregate(        
    { $match: { "isActive": true, fruitId: "kipld" } },
    { $unwind: '$types' },
    { $match: { "types.isActive": true } },
    { $group: {
            _id: '$_id', 
            name: {$first: '$name'}, 
            price: { $first: '$price' },
            types : { $push : '$types' }
        }
    },
    { $project: {
            '_id': 1, 
            'name':1, 
            'price': 1, 
            'types': 1,
        }
    }
)

MongoDB Collection:

{
    _id: "abcdefg",
    fruitId: "kipld",
    isActive: true,
    types :[
        {
            name: "Kashmir Apple",
            price: 90,
            isActive: false
        },
        {
            name: "American Apple",
            price: 120,
            isActive: false
        }
    ]
}

Expected result will be like:

{
    _id: "abcdefg",
    fruitId: "kipld",
    isActive: true,
    types :[]
}

But when I'm running the above query I'm not getting anything.

Your query says { $match: { "types.isActive": true } }, but the input document has "tyoes.isActive" values as false. So, no documents get selected. — prasad_
– prasad_, Commented Oct 14, 2019 at 7:16
can I do that to return EMPTY array if there are no true records present? — Rajath
– Rajath, Commented Oct 14, 2019 at 7:31
@Rajath You can. But, not with that query pipeline you are using. — prasad_
– prasad_, Commented Oct 14, 2019 at 7:41

prasad_ · Accepted Answer · 2019-10-14 10:28:03Z

You can get the desired result using the $filter aggregation operator. This just filters all the array elements and returns the array with the matching filter condition. Note the filter is applied to each document's array. So, in your case the $unwind and $group stages are not needed to get the result.

Sample fruits collection:

{
        "_id" : "abcdefg",
        "fruitId" : "kipld",
        "isActive" : true,
        "types" : [
                {
                        "name" : "Kashmir Apple",
                        "price" : 90,
                        "isActive" : false
                },
                {
                        "name" : "American Apple",
                        "price" : 120,
                        "isActive" : false
                }
        ]
}
{
        "_id" : "uvwxyz",
        "fruitId" : "kipld",
        "isActive" : true,
        "types" : [
                {
                        "name" : "Kinnaur Apple",
                        "price" : 80,
                        "isActive" : false
                },
                {
                        "name" : "Shimla Apple",
                        "price" : 70,
                        "isActive" : true
                }
        ]
}

Aggregation Query:

db.fruits.aggregate([
  { $match: { "isActive": true, fruitId: "kipld" } },
  { $project: { _id: 1, fruitId: 1, isActive: 1, 
                result_types: { $filter: {
                                   input: "$types",
                                      as: "type",
                                    cond: { $eq: [ "$$type.isActive", true ] }
                                 } 
                             } } 
   }
])

The Output:

Note the "result_types" empty array where the types.isActive values are false for all elements of "types" array. Only the elements with a value of true are in the result output.

{
        "_id" : "abcdefg",
        "fruitId" : "kipld",
        "isActive" : true,
        "result_types" : [ ]
}
{
        "_id" : "uvwxyz",
        "fruitId" : "kipld",
        "isActive" : true,
        "result_types" : [
                {
                        "name" : "Shimla Apple",
                        "price" : 70,
                        "isActive" : true
                }
        ]
}

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