Database
db={"Collection_A":[{"ref_id":ObjectId("5e9561edf8beb57100dded8f"),"features":[{"_id":ObjectId("5e9561edf8beb57100dded91"),"k":"foo","v":"bar"},{"_id":ObjectId("5e9561edf8beb57100dded92"),"k":"foo2","v":"bar2"}]},{"ref_id":ObjectId("5e9561edf8beb57100dded8f"),"features":[{"_id":ObjectId("5e9561edf8beb57100dded91"),"k":"foo22","v":"bar33"},{"_id":ObjectId("5e9561edf8beb57100dded92"),"k":"foo212","v":"bar211"}]},{"ref_id":ObjectId("5e9561edf8beb57100dded8f"),"features":[{"_id":ObjectId("5e9561edf8beb57100dded91"),"k":"foo","v":"bar33"},{"_id":ObjectId("5e9561edf8beb57100dded92"),"k":"foo212","v":"bar211"}]}],"Collection_B":[{"ref_id":ObjectId("5e9561edf8beb57100dded8f")}]}
Query
db.Collection_A.aggregate([{"$match":{"features":{"$elemMatch":{"k":"foo","v":"bar"}}}}])