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 arr...