Currently $elemMatch only supports array values, so queries such as {foo: {$elemMatch:

{a:1, b:2}

}} will match {foo:[

{a:1, b:2, c:3}

]}, but not {foo: {a:1, b:2, c:3}}. I know this can be easily done by query

{'foo.a':1, 'foo.b':2}

, but this behavior is inconsistent with other query operations; e.g.

{a:1}

will match both

{a:[1]}

and

{a:1}

.

I'm currently writing a DSL for mongo, and I find it hard to explain this inconsistency to users. But I don't know if changing this will affect query optimizer.