If you have an index on {"my.field.name": 1}, then the document {"my.field.name": "a", "my": {"field": {"name": "b"}}, loc: [70, 40]} will produce an index key with value "b", as expected. However, if you have an index on {"loc": "2dsphere", "my.field.name": 1}, then it will index "a" (along with the encoded geo point). I'm not sure, but this may also hold true for other "special" index types that re-implement the logic to extract "normal" index keys for compound indexes.