Question

mongo> db.users.findOne({username: ‘foo’});

   {
    "__v" : 266,
    "_id" : ObjectId("50752b5f00a0f5ab64000002"),
    "followers" : [
            ObjectId("505e2f1de888c7d701000001"),
            ObjectId("506146fe72c0280723000001"),
            ObjectId("50752b5f00a0f5ab64000002"), //remove this
            ObjectId("50752b5f00a0f5ab64000002") //remove this
    ],

I want to remove the last two items in followers[] collection.

Answer 1

You can try this (if you are sure they are the last two):

db.users.update({"_id" : ObjectId("50752b5f00a0f5ab64000002")}, {$pop: {followers:1}});
db.users.update({"_id" : ObjectId("50752b5f00a0f5ab64000002")}, {$pop: {followers:1}});

$pop will remove the last element of an array.

However, if you need to remove specific ids from the list regardless of where they are, you can use $pull, or $pullAll if you want to remove several at once.