Requested changes:

 - add fetch default foreign_key to Field::BelongsTo
 - style fixes and specs

lib/administrate/field/associative.rb

@@ -26,7 +26,7 @@ def primary_key
       end
 
       def foreign_key
-        options.fetch(:foreign_key, nil)
+        options.fetch(:foreign_key, :"#{attribute}_id")
       end
     end
   end

lib/administrate/field/belongs_to.rb

@@ -8,7 +8,7 @@ def self.permitted_attribute(attr)
       end
 
       def permitted_attribute
-        foreign_key || self.class.permitted_attribute(attribute)
+        foreign_key
       end
 
       def associated_resource_options

spec/lib/fields/belongs_to_spec.rb

@@ -25,8 +25,9 @@
         Foo = Class.new
         allow(Foo).to receive(:all).and_return([])
 
-        association = Administrate::Field::BelongsTo.
-          with_options(class_name: "Foo")
+        association = Administrate::Field::BelongsTo.with_options(
+          class_name: "Foo",
+        )
         field = association.new(:customers, [], :show)
         candidates = field.associated_resource_options
 
@@ -71,10 +72,11 @@
     it "determines what foreign key is used on the relationship for the form" do
       association =
         Administrate::Field::BelongsTo.with_options(
-          foreign_key: "customers_uuid",
+          foreign_key: "foo_uuid", class_name: "Foo",
         )
       field = association.new(:customers, [], :show)
-      expect(field.permitted_attribute).to eq("customers_uuid")
+      permitted_attribute = field.permitted_attribute
+      expect(permitted_attribute).to eq("foo_uuid")
     end
   end
 end