0

I have two collections in the following format -

collection1:

{
    "_id": "col1id1",
    "name": "col1doc1",
    "properties": [ "<_id1>", "<_id2>", "<_id3>"]
}

collection2:

{
    "_id": "<_id1>",
    "name": "doc1",
    "boolean_field": false
},
{
    "_id": "<_id2>",
    "name": "doc2",
    "boolean_field": true
},
{
    "_id": "<_id3>",
    "name": "doc3",
    "boolean_field" : false
}

The desired output is:

{
    "_id": "col1id1",
    "name": "col1doc1",
    "property_names": ["doc1", "doc3"]
}

The field properties of document in collection1 has three IDs of documents in collection2 but the output after join operation should contain only those which have the boolean_field value as false.

How can I perform this filter with a join operation in MongoDB?

1

In MongoDB 3.6+ you can add a pipeline to your $lookup stage to include additional join conditions.

A solution was less obvious than I expected since your source lookup field is an array, but the following should return the expected outcome:

db.collection1.aggregate([
    { $match: {
        _id: "col1id1"
    }},

    // Filtered lookup based on agg pipeline
    { $lookup: {
        from: "collection2",
        // let references fields in the original collection
        let: { property: "$properties" },
        pipeline: [
            { $match:
                // use $expr to compare fields using let variables
                { $expr:
                    { $and: [
                        { $in: [ "$_id", "$$property" ] },
                        { $eq: [ "$boolean_field", false ] }
                    ]}
                }
            },
            { $project: { "_id": 0, "name": 1 }}
        ],
        as: "property_names"
    }},

    // Project desired output fields 
    { $project: {
        "name" : "$name",
        "property_names": {
            $map: {
                input: "$property_names", 
                in: "$$this.name"
            }
    }
    }
}
]).pretty()

Sample output:

{
    "_id" : "col1id1",
    "name" : "col1doc1",
    "property_names" : [
        "doc1",
        "doc3"
    ]
}

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.