Say I do this:

c.ensureIndex({a:1});
c.save({a:{b:1}});

Right now I can match the following using the index key:

c.find({a:{b:1}});

But I can't match the following at the index level; I need to load 'b' from the full document.

c.find({'a.b':1});

Since the 'b' field is inside the index key, we could extract it for matching.

Would need to make sure we test handling of multikey cases correctly, so for example:

c.save({a:[{b:1},{c:2}]});
c.find({'a.b':1,'a.c':2});