Add custom key_name management for polymorphic association

Just add `foreign_key_type_changed: true` to your association key work (`has_one` or `has_many`)

This closes #1285

Signed-off-by: Brice <brice.bene@gmail.com>

Author: benebrice

lib/jsonapi/resource_serializer.rb

@@ -468,16 +468,20 @@ def relationship_object(source, relationship, include_linkage = false)
 
     # Extracts the foreign key value for a to_one relationship.
     def foreign_key_value(source, relationship)
-      related_resource_id = if source.preloaded_fragments.has_key?(format_key(relationship.name))
-        source.preloaded_fragments[format_key(relationship.name)].values.first.try(:id)
-      elsif source.respond_to?("#{relationship.name}_id")
-        # If you have direct access to the underlying id, you don't have to load the relationship
-        # which can save quite a lot of time when loading a lot of data.
-        # This does not apply to e.g. has_one :through relationships.
-        source.public_send("#{relationship.name}_id")
-      else
-        source.public_send(relationship.name).try(:id)
-      end
+      # If you have changed the key_name, don't even try to look at `"#{relationship.name}_id"`
+      # just load the association and call the custom key_name
+      foreign_key_type_changed = relationship.options[:foreign_key_type_changed] || false
+      related_resource_id =
+        if source.preloaded_fragments.has_key?(format_key(relationship.name))
+          source.preloaded_fragments[format_key(relationship.name)].values.first.try(:id)
+        elsif !foreign_key_type_changed && source.respond_to?("#{relationship.name}_id")
+          # If you have direct access to the underlying id, you don't have to load the relationship
+          # which can save quite a lot of time when loading a lot of data.
+          # This does not apply to e.g. has_one :through relationships.
+          source.public_send("#{relationship.name}_id")
+        else
+          source.public_send(relationship.name).try(:id)
+        end
       return nil unless related_resource_id
       @id_formatter.format(related_resource_id)
     end