Don't expose SingularAssociation#replace which is internal API

Originally `SingularAssociation#replace` abstract method is private, and
doesn't intend to be called directly.

activerecord/lib/active_record/associations/belongs_to_association.rb

@@ -16,19 +16,6 @@ module ActiveRecord
         end
       end
 
-      def replace(record)
-        if record
-          raise_on_type_mismatch!(record)
-          update_counters_on_replace(record)
-          set_inverse_instance(record)
-          @updated = true
-        else
-          decrement_counters
-        end
-
-        self.target = record
-      end
-
       def target=(record)
         replace_keys(record)
         super
@@ -56,6 +43,18 @@ module ActiveRecord
       end
 
       private
+        def replace(record)
+          if record
+            raise_on_type_mismatch!(record)
+            update_counters_on_replace(record)
+            set_inverse_instance(record)
+            @updated = true
+          else
+            decrement_counters
+          end
+
+          self.target = record
+        end
 
         def update_counters(by)
           if require_counter_update? && foreign_key_present?

activerecord/lib/active_record/associations/has_one_association.rb

@@ -23,35 +23,6 @@ module ActiveRecord
         end
       end
 
-      def replace(record, save = true)
-        raise_on_type_mismatch!(record) if record
-        load_target
-
-        return target unless target || record
-
-        assigning_another_record = target != record
-        if assigning_another_record || record.has_changes_to_save?
-          save &&= owner.persisted?
-
-          transaction_if(save) do
-            remove_target!(options[:dependent]) if target && !target.destroyed? && assigning_another_record
-
-            if record
-              set_owner_attributes(record)
-              set_inverse_instance(record)
-
-              if save && !record.save
-                nullify_owner_attributes(record)
-                set_owner_attributes(target) if target
-                raise RecordNotSaved, "Failed to save the new associated #{reflection.name}."
-              end
-            end
-          end
-        end
-
-        self.target = record
-      end
-
       def delete(method = options[:dependent])
         if load_target
           case method
@@ -68,6 +39,33 @@ module ActiveRecord
       end
 
       private
+        def replace(record, save = true)
+          raise_on_type_mismatch!(record) if record
+
+          return target unless load_target || record
+
+          assigning_another_record = target != record
+          if assigning_another_record || record.has_changes_to_save?
+            save &&= owner.persisted?
+
+            transaction_if(save) do
+              remove_target!(options[:dependent]) if target && !target.destroyed? && assigning_another_record
+
+              if record
+                set_owner_attributes(record)
+                set_inverse_instance(record)
+
+                if save && !record.save
+                  nullify_owner_attributes(record)
+                  set_owner_attributes(target) if target
+                  raise RecordNotSaved, "Failed to save the new associated #{reflection.name}."
+                end
+              end
+            end
+          end
+
+          self.target = record
+        end
 
         # The reason that the save param for replace is false, if for create (not just build),
         # is because the setting of the foreign keys is actually handled by the scoping when

activerecord/lib/active_record/associations/has_one_through_association.rb

@@ -6,12 +6,12 @@ module ActiveRecord
     class HasOneThroughAssociation < HasOneAssociation #:nodoc:
       include ThroughAssociation
 
-      def replace(record, save = true)
-        create_through_record(record, save)
-        self.target = record
-      end
-
       private
+        def replace(record, save = true)
+          create_through_record(record, save)
+          self.target = record
+        end
+
         def create_through_record(record, save)
           ensure_not_nested