How to retrieve field types

I know how to get field names defined in document model.
document.attribute_names.each do |attr_name|

What I can not find out is how do I get field types. I could probably read first document and use document.field_name.class, but this doesn't help with fields with nil value.

Can you advice me how to get field types and default value definitions.

Thank you
Damjan Rems