There is a `BSON::Registry` which determines how BSON is deserialized. Mongoid has this code:

if allow_decimal128
  BSON::Registry.register(BSON::Decimal128::BSON_TYPE, BSON::Decimal128)
else
  BSON::Registry.register(BSON::Decimal128::BSON_TYPE, BigDecimal)
end 

However, BSON gem does not honor this registry for for Hash, it always deserializes as BSON::Document, even when the registry is set to Hash.