Count the number of documents in an embedded array for a collection

embedded_document_count.rb

SomeCollection.collection.aggregate([
                                           {
                                               "$match": { # This ensures that we only calculate a count where the attribute exists
                                                   "some_attribute": {
                                                       "$exists" => true
                                                   }
                                               }
                                           },
                                           {
                                               "$project": {
                                                   count: {
                                                       "$size": "$some_attribute"
                                                   }
                                               }
                                           }
                                       ]).first