I have a collection where every document in the collection has an array named foo
that contains a set of embedded documents. Is there currently a trivial way in the MongoDB shell to count how many instances are within foo
? something like:
db.mycollection.foos.count()
or db.mycollection.foos.size()
?
Each document in the array needs to have a unique foo_id
and I want to do a quick count to make sure that the right amount of elements are inside of an array for a random document in the collection.
In MongoDB 2.6, the Aggregation Framework has a new array $size
operator you can use:
> db.mycollection.insert({'foo':[1,2,3,4]})
> db.mycollection.insert({'foo':[5,6,7]})
> db.mycollection.aggregate([{$project: { count: { $size:"$foo" }}}])
{ "_id" : ObjectId("5314b5c360477752b449eedf"), "count" : 4 }
{ "_id" : ObjectId("5314b5c860477752b449eee0"), "count" : 3 }