Database
[{"_id":ObjectId("606c774ff34c295ec7ac5eb7"),"child_table":[{"_id":ObjectId("606c7768f34c291171ac5ec2"),"string3":"y"},{"_id":ObjectId("606c7768f34c291171ac5ef6"),"string3":"u"}]}]
Query
db.collection.update({_id:ObjectId("606c774ff34c295ec7ac5eb7")},{$pop:{child_table:1}})