The $bit update modifier doesn't work when field does not exist. In this case it also causes other update modifiers to be ignored. To be consistent with other modifieds (like $inc) it should probably treat a nonexistent field as an implied zero.

To reproduce:

> // $bit or should work if field doesn't exist:
> db.test.insert(

{x:1}

)
> db.test.find()

{ "_id" : ObjectId("4dc29da73691b2a95024293b"), "x" : 1 }

> db.test.update({}, { $bit : { or :

{ y : 2 }

}}) // y does not exist
> db.test.find()

{ "_id" : ObjectId("4dc29da73691b2a95024293b"), "x" : 1 }

>
> // $bit with field that doesn't exist causes other modifiers to be ignored
> db.test.update({}, { $set :

{ x : 2 }

, $bit : { or :

{ y : 2 }

}}) // y does not exist
> db.test.find()

{ "_id" : ObjectId("4dc29da73691b2a95024293b"), "x" : 1 }

>
> // $bit with field that doesn't exist causes other modifiers to be ignored
> db.test.update({}, { $set :

{ x : 2 }

, $bit : { and :

{ y : 2 }

}}) // y does not exist, $set of x is ignored
> db.test.find()

{ "_id" : ObjectId("4dc29da73691b2a95024293b"), "x" : 1 }

>