Let a model's own position writer compose with ours

CHANGELOG.md

@@ -1,6 +1,7 @@
 ## [Unreleased]
 
 - Fix the healer not locking scopes without a `belongs_to` association (or with a nil one).
+- A position writer defined in the model now composes with positioning's via `super`.
 
 ## [0.4.7] - 2025-08-09
 

lib/positioning.rb

@@ -46,10 +46,14 @@ def positioned(on: [], column: :position)
           define_method(:"prior_#{column}") { Mechanisms.new(self, column).prior }
           define_method(:"subsequent_#{column}") { Mechanisms.new(self, column).subsequent }
 
-          redefine_method(:"#{column}=") do |position|
-            send :"#{column}_will_change!"
-            super(position)
-          end
+          # Defined in a module so that a writer defined in the model itself can call super to reach it,
+          # rather than replacing it (or being replaced by it).
+          include(Module.new do
+            define_method(:"#{column}=") do |position|
+              send :"#{column}_will_change!"
+              super(position)
+            end
+          end)
 
           before_create { Mechanisms.new(self, column).create_position }
           before_update { Mechanisms.new(self, column).update_position }

test/models/custom_writer_item.rb

@@ -0,0 +1,9 @@
+class CustomWriterItem < ActiveRecord::Base
+  belongs_to :list
+
+  positioned on: :list
+
+  def position=(position)
+    super((position == "top") ? :first : position)
+  end
+end

test/support/active_record.rb

@@ -95,6 +95,14 @@ def configure_connection
 
     add_index :products, :position, unique: true
 
+    create_table :custom_writer_items, force: true do |t|
+      t.string :name
+      t.integer :position, null: false
+      t.references :list, null: false
+    end
+
+    add_index :custom_writer_items, [:list_id, :position], unique: true
+
     create_table :categorised_items, force: true do |t|
       t.string :name
       t.integer :position, null: false

test/test_helper.rb

@@ -19,5 +19,6 @@
 require_relative "models/author/student"
 require_relative "models/author/teacher"
 require_relative "models/blog"
+require_relative "models/custom_writer_item"
 require_relative "models/post"
 require_relative "models/entity"

test/test_positioning.rb

@@ -652,6 +652,22 @@ def test_that_the_default_list_scope_works
       Positioning::Mechanisms.new(second_item, :position).send(:positioning_scope).order(:position)
   end
 
+  def test_that_a_custom_position_writer_composes_with_ours
+    first_list = List.create name: "First List"
+    second_list = List.create name: "Second List"
+    first_item = CustomWriterItem.create list: first_list, name: "First Item"
+    second_item = CustomWriterItem.create list: first_list, name: "Second Item"
+    third_item = CustomWriterItem.create list: second_list, name: "Third Item"
+    fourth_item = CustomWriterItem.create list: second_list, name: "Fourth Item"
+
+    second_item.update position: "top"
+    assert_equal [1, 2], [second_item.reload, first_item.reload].map(&:position)
+
+    # Assigning the same position it already has still counts as an explicit position in the new scope
+    second_item.update list: second_list, position: 1
+    assert_equal [1, 2, 3], [second_item.reload, third_item.reload, fourth_item.reload].map(&:position)
+  end
+
   def test_that_destroyed_via_positioning_scope_does_not_call_contract
     list = List.create name: "First List"
     list.items.create name: "First Item"