Each concrete classes have responsibility to return `association_class`

activerecord/lib/active_record/reflection.rb

@@ -483,28 +483,7 @@ def belongs_to?; false; end
       # Returns +true+ if +self+ is a +has_one+ reflection.
       def has_one?; false; end
 
-      def association_class
-        case macro
-        when :belongs_to
-          if polymorphic?
-            Associations::BelongsToPolymorphicAssociation
-          else
-            Associations::BelongsToAssociation
-          end
-        when :has_many
-          if options[:through]
-            Associations::HasManyThroughAssociation
-          else
-            Associations::HasManyAssociation
-          end
-        when :has_one
-          if options[:through]
-            Associations::HasOneThroughAssociation
-          else
-            Associations::HasOneAssociation
-          end
-        end
-      end
+      def association_class; raise NotImplementedError; end
 
       def polymorphic?
         options[:polymorphic]
@@ -629,6 +608,14 @@ def initialize(name, scope, options, active_record)
       def macro; :has_many; end
 
       def collection?; true; end
+
+      def association_class
+        if options[:through]
+          Associations::HasManyThroughAssociation
+        else
+          Associations::HasManyAssociation
+        end
+      end
     end
 
     class HasOneReflection < AssociationReflection # :nodoc:
@@ -639,6 +626,14 @@ def initialize(name, scope, options, active_record)
       def macro; :has_one; end
 
       def has_one?; true; end
+
+      def association_class
+        if options[:through]
+          Associations::HasOneThroughAssociation
+        else
+          Associations::HasOneAssociation
+        end
+      end
     end
 
     class BelongsToReflection < AssociationReflection # :nodoc:
@@ -650,6 +645,14 @@ def macro; :belongs_to; end
 
       def belongs_to?; true; end
 
+      def association_class
+        if polymorphic?
+          Associations::BelongsToPolymorphicAssociation
+        else
+          Associations::BelongsToAssociation
+        end
+      end
+
       def join_keys(association_klass)
         key = polymorphic? ? association_primary_key(association_klass) : association_primary_key
         JoinKeys.new(key, foreign_key)