Database
db={listings:[{"_id":ObjectId("5349b4ddd2781d08c09890f3"),"name":"Hotel Radisson Blu","moods":[ObjectId("507f1f77bcf86cd799439010"),ObjectId("507f1f77bcf86cd799439011")]}],moods:[{"_id":ObjectId("507f1f77bcf86cd799439011"),"name":"Sports"},{"_id":ObjectId("507f1f77bcf86cd799439010"),"name":"Spanish Food"},{"_id":ObjectId("507f1f77bcf86cd799439009"),"name":"Action"}]}
Query
db.moods.aggregate([{"$lookup":{"from":"listings","as":"count",let:{id:"$_id"},pipeline:[{"$match":{"$expr":{"$in":["$$id","$moods"]}}}]}},{$addFields:{count:{$size:"$count"}}}])