The name of the key on the associated record is abstracted as `reflection.join_primary_key`

activerecord/lib/active_record/associations/preloader/association.rb

@@ -22,16 +22,16 @@ def run(preloader)
           end
         end
 
-        # The name of the key on the associated records
-        def association_key_name
-          raise NotImplementedError
-        end
-
         private
           def options
             reflection.options
           end
 
+          # The name of the key on the associated records
+          def association_key_name
+            reflection.join_primary_key(klass)
+          end
+
           # The name of the key on the model which declares the association
           def owner_key_name
             reflection.join_foreign_key

activerecord/lib/active_record/associations/preloader/belongs_to.rb

@@ -4,9 +4,6 @@ module ActiveRecord
   module Associations
     class Preloader
       class BelongsTo < SingularAssociation #:nodoc:
-        def association_key_name
-          options[:primary_key] || klass && klass.primary_key
-        end
       end
     end
   end

activerecord/lib/active_record/associations/preloader/has_many.rb

@@ -4,9 +4,6 @@ module ActiveRecord
   module Associations
     class Preloader
       class HasMany < CollectionAssociation #:nodoc:
-        def association_key_name
-          reflection.foreign_key
-        end
       end
     end
   end

activerecord/lib/active_record/associations/preloader/has_one.rb

@@ -4,9 +4,6 @@ module ActiveRecord
   module Associations
     class Preloader
       class HasOne < SingularAssociation #:nodoc:
-        def association_key_name
-          reflection.foreign_key
-        end
       end
     end
   end

activerecord/lib/active_record/reflection.rb

@@ -292,13 +292,17 @@ def chain
       end
 
       def get_join_keys(association_klass)
-        JoinKeys.new(join_pk(association_klass), join_foreign_key)
+        JoinKeys.new(join_primary_key(association_klass), join_foreign_key)
       end
 
       def build_scope(table, predicate_builder = predicate_builder(table))
         Relation.create(klass, table, predicate_builder)
       end
 
+      def join_primary_key(_)
+        foreign_key
+      end
+
       def join_foreign_key
         active_record_primary_key
       end
@@ -313,10 +317,6 @@ def predicate_builder(table)
           PredicateBuilder.new(TableMetadata.new(klass, table))
         end
 
-        def join_pk(_)
-          foreign_key
-        end
-
         def primary_key(klass)
           klass.primary_key || raise(UnknownPrimaryKey.new(klass))
         end
@@ -736,6 +736,10 @@ def association_class
         end
       end
 
+      def join_primary_key(klass)
+        polymorphic? ? association_primary_key(klass) : association_primary_key
+      end
+
       def join_foreign_key
         foreign_key
       end
@@ -745,10 +749,6 @@ def join_foreign_key
         def calculate_constructable(macro, options)
           !polymorphic?
         end
-
-        def join_pk(klass)
-          polymorphic? ? association_primary_key(klass) : association_primary_key
-        end
     end
 
     class HasAndBelongsToManyReflection < AssociationReflection # :nodoc: