MongoDB filter array fields within each document of a collection

Question

I have a MongoDB collection whose structure is something like this:

/* 1 */
{
    "_id" : ObjectId("5c09a454dd42360001fd2515"),
    "conversations" : {
        "0" : {
            "message" : "Message 1",
              .
              .
              .
        },
        "1" : {
            "message" : "Message 1"
               .
               .
               .
        },
        "2" : {
            "message" : "Message 5"
              .
              .
              .
        },
        "3" : {
            "message" : "Message 1"
              .
              .
              .
        },
        "4" : {
            "message" : "Message 2"
              .
              .
              .
        },
        "5" : {
            "message" : "Message 3"

        },
        "6" : {
            "message" : "compliance"

        },
        "7" : {
            "message" : "Google"

        }
}

/* 2 */
{
    "_id" : ObjectId("5c09a673c2a98f00012f4efb"),

    "conversations" : {
        "0" : {
            "message" : "Message 11"

        },
        "1" : {
            "message" : "Google",

        },
        "2" : {
            "message" : "Message 7"

        }
}

/* 3 */
{
    "_id" : ObjectId("5c09f570173f7900015a82b2"),

    "conversations" : {
        "0" : {
            "message" : "Message 4"
        },

}

.
.
.

In above sample data, I have a set of documents inside the collection and within each document there is an array of dictionary field named conversations. Within this field I have list of dictionary fields(0,1,2...). What I want to do is filter all the documents where within each conversationsfield, the message value is Message 4.

I know the way to filter to a specific field within each document is db.getCollection('collection_1').find({conversation:""}), but I don't know how do I apply it to list of dictionary fields like for my case. Any help?

mickl · Accepted Answer · 2019-03-22 20:50:06Z

1

You need $objectToArray operator to turn an object with unknown keys into an array of k and v fields. Then inside of $expr you can apply $filter on that array to check if any message is equal to "Message 4"

db.collection.aggregate([
    {
        $match: {
            $expr: {
                $ne: [
                    {
                        $size: {
                            $filter: {
                                input: { $objectToArray: "$conversations" },
                                as: "conv",
                                cond: { $eq: [ "$$conv.v.message", "Message 4" ] }
                            }
                        }
                    },
                    0
                ]
            }
        }
    }
])

Example

answered Mar 22, 2019 at 20:50

mickl

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4 Comments

user2966197 Over a year ago

Is the above answer very specific to the sample data I have? What if my documents have additional fields alongside conversations?

mickl Over a year ago

@user2966197 still you need to use $objectToArray if your keys are dynamically generated and it should still be working fine since conv.v will contain more than one field "message". You can try it out here: mongoplayground.net/p/tYJBT_ygKE5

user2966197 Over a year ago

Ok. And where in the query do I specify the collection name? My Collection name is "collection_1" and I don't see any place in above query to specify it? Will it be like db.conversation_1.aggregate()?

mickl Over a year ago

@user2966197 yes, just like you do it for .find()

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