MongoDB query sample: using $unwind

This is the sample collection I am using in this example:

db.getCollection('testCol').find({}).pretty()

{
    "_id" : ObjectId("59764121cd6857bf00a34809"),
    "a" : 1,
    "b" : 1
}


{
    "_id" : ObjectId("59764126cd6857bf00a3480a"),
    "a" : 2,
    "b" : 2
}


{
    "_id" : ObjectId("5976412ccd6857bf00a3480b"),
    "a" : 3,
    "b" : 3
}

{
    "_id" : ObjectId("59764294cd6857bf00a3480c"),
    "a" : 4,
    "b" : 4,
    "arr" : [
        {
            "a" : 6,
            "b" : 6
        },
        {
            "a" : 7,
            "b" : 7
        }
    ]
}

This will give me the number of element inside of the array:

db.getCollection('testCol').aggregate([{$match:{$and:[{a:{"$eq": 4}}]}},
{ $unwind : "$arr" },
    { $group: {
        _id: '',
        count: { $sum: 1 }}}
])


This was useful for me when I had to do count validation after loading data from Oracle (or any rdbms) into MongoDB when I am using a custom defined _id which isn't unique. We can push any entries with duplicated custom _id into array.