Currently only a single array is expanded when creating keys for an index. So for example the implementation is incomplete when creating an index on

{ 'a.b': 1 }

for object { a: [

{ b: [ 2 ] }

] }.

for an index on

{ a:1,b:1}

and object

{ a:[ 2 ], b:[ 2] }

we decided not to allow indexing at one point because the # of keys would be exponential in the # of fields