> c.save( {a:1} )
> c.save( {a:2} )
> c.save( {a:3} )
> c.ensureIndex( {a:1} )
> c.find().hint( {a:1} ).batchSize( 2 )._addSpecial( "$returnKey", true )
{ "a" : 1 }
{ "a" : 2 }
{ "_id" : ObjectId("4f40a6bb916db8388b2ba240"), "a" : 3 }
>