prepare_FOO not working with simple backend

[simod]

Hello list,

I'm trying to implement a search_index that doesn't use faceting if
there is the simple backend instead of elasticsearch. It looks like,
when the simple backend is enabled, he's not compiling the fields that
have prepare_FOO method. (with elasticsearch everything works
perfectly)

I have something like:

type = index.CharField()

prepare_type(self,obj):
return 'type'

Am I missing something or doing something wrong?
And is there a best practice to have haystack working with of without
faceting depending on the backend?

Thanks in advance!