`join_keys` no longer needs a class passed to it

Reflections only use their own information to create a `join_keys`
object.  This means that we can call `join_keys` on a reflection object
and have it be context-free.

activerecord/lib/active_record/associations/association_scope.rb

@@ -61,7 +61,7 @@ def join(table, constraint)
         end
 
         def last_chain_scope(scope, table, reflection, owner, association_klass)
-          join_keys = reflection.join_keys(association_klass)
+          join_keys = reflection.join_keys
           key = join_keys.key
           foreign_key = join_keys.foreign_key
 
@@ -81,7 +81,7 @@ def transform_value(value)
         end
 
         def next_chain_scope(scope, table, reflection, association_klass, foreign_table, next_reflection)
-          join_keys = reflection.join_keys(association_klass)
+          join_keys = reflection.join_keys
           key = join_keys.key
           foreign_key = join_keys.foreign_key
 

activerecord/lib/active_record/associations/join_dependency/join_association.rb

@@ -34,7 +34,7 @@ def join_constraints(foreign_table, foreign_klass, join_type, tables, chain)
             table = tables.shift
             klass = reflection.klass
 
-            join_keys   = reflection.join_keys(klass)
+            join_keys   = reflection.join_keys
             key         = join_keys.key
             foreign_key = join_keys.foreign_key
 

activerecord/lib/active_record/reflection.rb

@@ -172,16 +172,8 @@ def class_name
 
       JoinKeys = Struct.new(:key, :foreign_key) # :nodoc:
 
-      def join_keys(association_klass)
-        JoinKeys.new(join_pk(association_klass), join_fk)
-      end
-
-      def join_pk(klass)
-        foreign_key
-      end
-
-      def join_fk
-        active_record_primary_key
+      def join_keys
+        get_join_keys klass
       end
 
       # Returns a list of scopes that should be applied for this Reflection
@@ -292,6 +284,20 @@ def alias_candidate(name)
       def chain
         collect_join_chain
       end
+
+      def get_join_keys(association_klass)
+        JoinKeys.new(join_pk(association_klass), join_fk)
+      end
+
+      private
+
+        def join_pk(_)
+          foreign_key
+        end
+
+        def join_fk
+          active_record_primary_key
+        end
     end
 
     # Base class for AggregateReflection and AssociationReflection. Objects of
@@ -719,14 +725,6 @@ def association_class
         end
       end
 
-      def join_fk
-        foreign_key
-      end
-
-      def join_pk(klass)
-        polymorphic? ? association_primary_key(klass) : association_primary_key
-      end
-
       def join_id_for(owner) # :nodoc:
         owner[foreign_key]
       end
@@ -736,6 +734,14 @@ def join_id_for(owner) # :nodoc:
         def calculate_constructable(macro, options)
           !polymorphic?
         end
+
+        def join_fk
+          foreign_key
+        end
+
+        def join_pk(klass)
+          polymorphic? ? association_primary_key(klass) : association_primary_key
+        end
     end
 
     class HasAndBelongsToManyReflection < AssociationReflection # :nodoc:
@@ -755,7 +761,7 @@ def collection?
     class ThroughReflection < AbstractReflection #:nodoc:
       attr_reader :delegate_reflection
       delegate :foreign_key, :foreign_type, :association_foreign_key,
-               :active_record_primary_key, :type, to: :source_reflection
+               :active_record_primary_key, :type, :get_join_keys, to: :source_reflection
 
       def initialize(delegate_reflection)
         @delegate_reflection = delegate_reflection
@@ -855,10 +861,6 @@ def has_scope?
           through_reflection.has_scope?
       end
 
-      def join_keys(association_klass)
-        source_reflection.join_keys(association_klass)
-      end
-
       # A through association is nested if there would be more than one join table
       def nested?
         source_reflection.through_reflection? || through_reflection.through_reflection?
@@ -993,6 +995,7 @@ def collect_join_reflections(seed)
       end
 
       private
+
         def actual_source_reflection # FIXME: this is a horrible name
           source_reflection.send(:actual_source_reflection)
         end
@@ -1054,10 +1057,6 @@ def plural_name
         @reflection.plural_name
       end
 
-      def join_keys(association_klass)
-        @reflection.join_keys(association_klass)
-      end
-
       def type
         @reflection.type
       end
@@ -1071,6 +1070,10 @@ def source_type_info
         source_type = @previous_reflection.options[:source_type]
         lambda { |object| where(type => source_type) }
       end
+
+      def get_join_keys(association_klass)
+        @reflection.get_join_keys(association_klass)
+      end
     end
 
     class RuntimeReflection < PolymorphicReflection # :nodoc: