category is not "positionable"

app/models/category.rb

@@ -1,4 +1,9 @@
+require_dependency "concern/positionable"
+
 class Category < ActiveRecord::Base
+
+  include Concern::Positionable
+
   belongs_to :topic, dependent: :destroy
   if rails4?
     belongs_to :topic_only_relative_url,

db/migrate/20131018050738_add_position_to_categories.rb

@@ -0,0 +1,11 @@
+class AddPositionToCategories < ActiveRecord::Migration
+  def up
+    add_column :categories, :position, :integer
+    execute "UPDATE categories SET position = id"
+    change_column :categories, :position, :integer, null: false
+  end
+
+  def down
+    remove_column :categories, :position
+  end
+end

lib/concern/positionable.rb

@@ -0,0 +1,31 @@
+module Concern
+  module Positionable
+    extend ActiveSupport::Concern
+
+    included do
+      before_save do
+        self.position ||= self.class.count
+      end
+    end
+
+    def move_to(position)
+      self.exec_sql "
+      UPDATE #{self.class.table_name}
+      SET position = :position
+      WHERE id = :id", {id: id, position: position}
+
+      self.exec_sql "
+      UPDATE #{self.class.table_name} t
+      SET position = x.position - 1
+      FROM (
+        SELECT i.id, row_number()
+          OVER(ORDER BY i.position asc,
+                        CASE WHEN i.id = :id THEN 0 ELSE 1 END ASC) AS position
+        FROM #{self.class.table_name} i
+        WHERE i.position IS NOT NULL
+      ) x
+      WHERE x.id = t.id AND t.position <> x.position - 1
+      ", {id: id}
+    end
+  end
+end

spec/components/concern/positionable_spec.rb

@@ -0,0 +1,52 @@
+require "spec_helper"
+require_dependency "concern/positionable"
+
+describe Concern::Positionable do
+
+  def positions
+    TestItem.order('position asc, id asc').pluck(:id)
+  end
+
+  context "move_to" do
+    before do
+      class TestItem < ActiveRecord::Base
+        include Concern::Positionable
+      end
+
+      Topic.exec_sql("create temporary table test_items(id int primary key, position int)")
+    end
+
+    after do
+      Topic.exec_sql("drop table test_items")
+
+      # import is making my life hard, we need to nuke this out of orbit
+      des = ActiveSupport::DescendantsTracker.class_variable_get :@@direct_descendants
+      des[ActiveRecord::Base].delete(TestItem)
+      Object.send(:remove_const, :TestItem)
+    end
+
+    it "can position stuff correctly" do
+      5.times do |i|
+        Topic.exec_sql("insert into test_items(id,position) values(#{i}, #{i})")
+      end
+
+      positions.should == [0,1,2,3,4]
+      TestItem.find(3).move_to(0)
+      positions.should == [3,0,1,2,4]
+      TestItem.pluck(:position).sort.should == [0,1,2,3,4]
+
+
+      # this is somewhat odd, but when there is not positioning
+      # not much we can do
+      TestItem.find(1).move_to(5)
+      positions.should == [3,0,2,4,1]
+
+      TestItem.pluck(:position).sort.should == [0,1,2,3,4]
+
+      item = TestItem.new
+      item.id = 7
+      item.save
+      item.position.should == 5
+    end
+  end
+end