Use of $COND and $EQ with a array of objects

Question

I'm hoping that someone might be able to answer whether what I'm trying to accomplish below can be done with the MongoDB Aggregation Framework.

I have a user data structure that resembles the following with close to 1 million documents.

{
    "firstName" : "John",
    "lastName" : "Doe",
    "state" : "NJ",
    "email" : "[email protected]"
    "source" : [ 
        {
            "type" : "SOURCE-A",
            "data" : {
                "info" : "abc",
                "info2" : "xyz"
            }
        }, 
        {
            "type" : "SOURCE-B",
            "data" : {
                "info3" : "abc"
            }
        }
    ]
}

For the purposes of feeding data to another system, I need to generate a flat file structure with limited information from the previous dataset. The columns need to represent:

firstname, lastname, email, is_source-a, is_source-b

The part that I'm having difficulty with is the conditional code that attempts to populate "is_source-a" and "is_source-b". I have tried to use the following aggregation query, but can't figure out how to get it working since the $EQ operator used along with $COND doesn't seem to evaluate data inside of an array (always false).

db.collection.aggregate([
    {
        $project : {
            _id : 0,
            firstName : 1,
            lastName: 1,
            "is_source-a" : {
                $cond : [
                    { $eq: [ "$source.type", "source-a" ] },
                    1,
                    0
                ]
            },
            "is_source-b" : {
                $cond : [
                    { $eq: [ "$source.type", "source-b" ] },
                    1,
                    0
                ]
            }
        }
    }
]);

I could $UNWIND the array first, but then I wind up with a multiple records for each user document and don't understand how to consolidate them back.

Is there something that I'm missing with how to use $EQ (or some other operator) along with $COND when dealing with arrays of objects?

Answer 1

You're definitely on the right track, and using $unwind can get you there if you follow it up with a $group to put things back together:

db.collection.aggregate([
    {$unwind: '$source'},
    {$project: {
        _id: 1,
        firstName: 1,
        lastName: 1,
        email: 1,
        'is_source-a': {$eq: ['$source.type', 'SOURCE-A']},
        'is_source-b': {$eq: ['$source.type', 'SOURCE-B']}
    }},
    // group the docs that were duplicated in the $unwind back together by _id,
    // taking the values for most fields from the $first occurrence of the _id,
    // but the $max of the is_source fields so that if its true in any of the
    // docs for that _id it will be true in the output for that _id.
    {$group: {
        _id: '$_id',
        firstName: {$first: '$firstName'},
        lastName: {$first: '$lastName'},
        email: {$first: '$email'},
        'is_source-a': {$max: '$is_source-a'},
        'is_source-b': {$max: '$is_source-b'}
    }},
    // project again to remove _id
    {$project: {
        _id: 0,
        firstName: 1,
        lastName: 1,
        email: 1,
        'is_source-a': '$is_source-a',
        'is_source-b': '$is_source-b'
    }}
])