Allow categories with null position, which means sort them based on activity. Mix absolutely positioned (position is not null) categories with null position categories.

spec/components/category_list_spec.rb

@@ -4,11 +4,11 @@ require 'category_list'
 describe CategoryList do
 
   let(:user) { Fabricate(:user) }
+  let(:admin) { Fabricate(:admin) }
   let(:category_list) { CategoryList.new(Guardian.new user) }
 
   context "security" do
     it "properly hide secure categories" do
-      admin = Fabricate(:admin)
       user = Fabricate(:user)
 
       cat = Fabricate(:category)
@@ -73,4 +73,35 @@ describe CategoryList do
 
   end
 
+  describe 'category order' do
+    let(:category_ids) { CategoryList.new(Guardian.new(admin)).categories.map(&:id) - [SiteSetting.uncategorized_category_id] }
+
+    before do
+      uncategorized = Category.find(SiteSetting.uncategorized_category_id)
+      uncategorized.position = 100
+      uncategorized.save
+    end
+
+    it "returns topics in specified order" do
+      cat1, cat2 = Fabricate(:category, position: 1), Fabricate(:category, position: 0)
+      category_ids.should == [cat2.id, cat1.id]
+    end
+
+    it "returns default order categories" do
+      cat1, cat2 = Fabricate(:category, position: nil), Fabricate(:category, position: nil)
+      category_ids.should include(cat1.id)
+      category_ids.should include(cat2.id)
+    end
+
+    it "mixes default order categories with absolute position categories" do
+      cat1, cat2, cat3 = Fabricate(:category, position: 0), Fabricate(:category, position: 2), Fabricate(:category, position: nil)
+      category_ids.should == [cat1.id, cat3.id, cat2.id]
+    end
+
+    it "handles duplicate position values" do
+      cat1, cat2, cat3, cat4 = Fabricate(:category, position: 0), Fabricate(:category, position: 0), Fabricate(:category, position: nil), Fabricate(:category, position: 0)
+      category_ids.should == [cat1.id, cat2.id, cat4.id, cat3.id]
+    end
+  end
+
 end

spec/components/concern/positionable_spec.rb

@@ -37,8 +37,7 @@ describe Concern::Positionable do
       TestItem.find(3).move_to(1)
       positions.should == [0,3,1,2,4]
 
-      # this is somewhat odd, but when there is not positioning
-      # not much we can do
+      # this is somewhat odd, but when there is no such position, not much we can do
       TestItem.find(1).move_to(5)
       positions.should == [0,3,2,4,1]
 
@@ -47,7 +46,34 @@ describe Concern::Positionable do
       item = TestItem.new
       item.id = 7
       item.save
-      item.position.should == 5
+      item.position.should be_nil
+    end
+
+    it "can set records to have null position" do
+      5.times do |i|
+        Topic.exec_sql("insert into test_items(id,position) values(#{i}, #{i})")
+      end
+
+      TestItem.find(2).use_default_position
+      TestItem.find(2).position.should be_nil
+
+      TestItem.find(1).move_to(4)
+      TestItem.order('id ASC').pluck(:position).should == [0,4,nil,2,3]
+    end
+
+    it "can maintain null positions when moving things around" do
+      5.times do |i|
+        Topic.exec_sql("insert into test_items(id,position) values(#{i}, null)")
+      end
+
+      TestItem.find(2).move_to(0)
+      TestItem.order('id asc').pluck(:position).should == [nil,nil,0,nil,nil]
+      TestItem.find(0).move_to(4)
+      TestItem.order('id asc').pluck(:position).should == [4,nil,0,nil,nil]
+      TestItem.find(2).move_to(1)
+      TestItem.order('id asc').pluck(:position).should == [4,nil,1,nil,nil]
+      TestItem.find(0).move_to(1)
+      TestItem.order('id asc').pluck(:position).should == [1,nil,2,nil,nil]
     end
   end
 end