Currently the syntax for the $pull operand is confusing and conflicts with normal behaviour of $elemMatch.

Given document :

{"somearray" : [

{ "a" : 1, "b" : 1, "c" : 1 }

,

{ "a" : 1, "b" : 1 }

,

{ "a" : 1 }

] }

The current and expected behaviour of using $elemMatch or omitting it for an update to pull out an element is as follows :

UPDATE: db.test.update({}, {$pull : {somearray : {$elemMatch : {a:1, b:1}}}})
EXPECTED BEHAVIOUR : partial match on first item, so pull that
CURRENT BEHAVIOUR : has no effect

UPDATE: db.test.update({}, {$pull : {somearray :

{a:1, b:1}

}})
EXPECTED BEHAVIOUR : exact match on second item, so pull that
CURRENT BEHAVIOUR : removes item 1 and 2, so it's partial matching which is the usage pattern for the $elemMatch case

The end result is both behaviour that's less than intuitive and presents the problem i'm currently facing with our migration to mongo : How do I remove just the second item with exact/full document matching?

If this isn't a bug the $elemMatch example should be removed from the docs and the docs itself should elaborate on $pull only supporting partial matching. If it is a bug however it does present the problem if it being impossible to fix without breaking backward compatibility.