Suppose you have the following modules:

-- Schema/Foo.hs
module Schema.Foo where

mkPersist $(persistFileWith "foo.persistentmodels")

-- foo.persistentmodels
Foo
    name Text
    Primary name

-- Schema/Bar.hs

module Schema.Bar where

import Schema.Foo

mkPersist $(persistFileWith "bar.persistentmodels")

-- bar.persistentmodels
Bar
    foo Text
    Foreign Foo fk_bar_to_foo foo

This code will currently fail, because the HaskellName "Foo" is not present in the EmbedEntityMap, because it was not parsed from the same block of text.

I think there needs to be some way to lookup the entity without requiring it be present in the map. Attempting to call entityDef (Nothing :: Maybe $(Foo)) instead of relying on what's in the Map might work.