MongoDB creates array of arrays in $group $push instead of flat array

Question

I am trying to group a set of documents after an $unwind operation. My documents look like this:

{ 
    "_id" : ObjectId("5cdb5b5acadf5100019da2f4"), 
    "allowedLocations" : [
        {
            "type" : "country", 
            "value" : "world", 
            "label" : "World"
        }
    ], 
    "disallowedLocations" : [
        {
            "type" : "country", 
            "value" : "CF", 
            "label" : "Central African Republic"
        }, 
        {
            "type" : "country", 
            "value" : "CN", 
            "label" : "China"
        }
    ], 
}

{ 
    "_id" : ObjectId("5cdb5b5acadf5100019da2f4"), 
    "allowedLocations" : [
        {
            "type" : "country", 
            "value" : "US", 
            "label" : "United States of America"
        }
    ], 
    "disallowedLocations" : [
        {
            "type" : "country", 
            "value" : "CA", 
            "label" : "Canada"
        }, 
        {
            "type" : "country", 
            "value" : "MX", 
            "label" : "Mexico"
        }
    ], 
}

I want to group them by _id and then concatenate the allowedLocations and disallowedLocations arrays into one. The group stage in my pipeline looks like this:

{ 
    "$group" : {
        "_id" : "$_id", 
        "allowedLocations" : {
            "$push" : "$allowedLocations"
        }, 
        "disallowedLocations" : {
            "$push" : "disallowedLocations"
        }
    }
}

The problem is, the result I get is not a document with both arrays concatenated, but an array of arrays, each element of the array being the array of each document:

{ 
    "_id" : ObjectId("5cdb5b5acadf5100019da2f4"), 
    "allowedLocations" : [
        [
            {
                "type" : "country", 
                "value" : "US", 
                "label" : "United States of America"
            }
        ],
        [
            {
                "type" : "country", 
                "value" : "world", 
                "label" : "World"
            }
        ],
    ], 
    "disallowedLocations" : [
        [
            {
                "type" : "country", 
                "value" : "CF", 
                "label" : "Central African Republic"
            }, 
            {
                "type" : "country", 
                "value" : "CN", 
                "label" : "China"
            }
        ],
        [
            {
                "type" : "country", 
                "value" : "CA", 
                "label" : "Canada"
            }, 
            {
                "type" : "country", 
                "value" : "MX", 
                "label" : "Mexico"
            }
        ]
    } 
}

Is there a way to produce a flat array with only objects as elements? I also tried with $concatArrays before the push but that creates more arrays inside the arrays.

Why don't you just $unwind the disallowedLocations and allowedLocations before executing the $group? — Arsen Davtyan
– Arsen Davtyan, Commented Jun 3, 2019 at 22:17

Vikash_Singh · Accepted Answer · 2019-06-04 05:54:24Z

14

Two solutions here. You can either run $unwind on both arrays to get single allowed and disallowed location per document and then run your $group stage:

db.col.aggregate([
    {
        $unwind: "$allowedLocations"
    },
    {
        $unwind: "$disallowedLocations"
    },
    { 
        "$group" : {
            "_id" : "$_id", 
            "allowedLocations" : {
                "$addToSet" : "$allowedLocations"
            }, 
            "disallowedLocations" : {
                "$addToSet" : "$disallowedLocations"
            }
        }
    }
])

or you can run your $group first and then use $reduce to flatten allowedLocations and disallowedLocations:

db.col.aggregate([
    { 
        "$group" : {
            "_id" : "$_id", 
            "allowedLocations" : {
                "$push" : "$allowedLocations"
            }, 
            "disallowedLocations" : {
                "$push" : "$disallowedLocations"
            }
        }
    },
    {
        $project: {
            _id: 1,
            allowedLocations: {
                $reduce: {
                    input: "$allowedLocations",
                    initialValue: [],
                    in: { $concatArrays: [ "$$value", "$$this" ] }
                }
            },
            disallowedLocations: {
                $reduce: {
                    input: "$disallowedLocations",
                    initialValue: [],
                    in: { $concatArrays: [ "$$value", "$$this" ] }
                }
            }
        }
    }
])

edited Jun 4, 2019 at 5:54

Vikash_Singh

1,8962 gold badges16 silver badges27 bronze badges

answered Jun 4, 2019 at 4:25

mickl

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2 Comments

Vikash_Singh Over a year ago

the first solution should have $addToSet instead of $push to avoid duplicate entries in array.

mickl Over a year ago

@VikashSingh that's true, thanks. Nonetheless I would prefer second one ;)

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