Add relative positioning on design model

- Add a `relative_position` column.
- Include the `RelativePositioning` concern and its shared examples.

app/models/design_management/design.rb

@@ -9,6 +9,7 @@ module DesignManagement
     include Referable
     include Mentionable
     include WhereComposite
+    include RelativePositioning
 
     belongs_to :project, inverse_of: :designs
     belongs_to :issue
@@ -87,6 +88,14 @@ module DesignManagement
     # A design is current if the most recent event is not a deletion
     scope :current, -> { visible_at_version(nil) }
 
+    def self.relative_positioning_query_base(design)
+      on_issue(design.issue_id)
+    end
+
+    def self.relative_positioning_parent_column
+      :issue_id
+    end
+
     def status
       if new_design?
         :new

changelogs/unreleased/221167-model-changes.yml

+---
+title: Add relative positioning on designs
+merge_request: 37835
+author:
+type: changed

db/migrate/20200722091435_add_relative_position_to_design_management_designs.rb

+# frozen_string_literal: true
+
+class AddRelativePositionToDesignManagementDesigns < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+
+  def change
+    add_column :design_management_designs, :relative_position, :integer
+  end
+end

db/schema_migrations/20200722091435

+d4e389b1469b968b703432de9ece6c362e45ec4ba3ad20d1f6c6418253969379
\ No newline at end of file

db/structure.sql

@@ -11203,7 +11203,8 @@ CREATE TABLE public.design_management_designs (
     id bigint NOT NULL,
     project_id integer NOT NULL,
     issue_id integer,
-    filename character varying NOT NULL
+    filename character varying NOT NULL,
+    relative_position integer
 );
 
 CREATE SEQUENCE public.design_management_designs_id_seq

spec/lib/gitlab/import_export/safe_model_attributes.yml

@@ -767,6 +767,7 @@ DesignManagement::Design:
 - id
 - project_id
 - filename
+- relative_position
 DesignManagement::Action:
 - id
 - event

spec/models/design_management/design_spec.rb

@@ -11,6 +11,11 @@ RSpec.describe DesignManagement::Design do
   let_it_be(:design3) { create(:design, :with_versions, issue: issue, versions_count: 1) }
   let_it_be(:deleted_design) { create(:design, :with_versions, deleted: true) }
 
+  it_behaves_like 'a class that supports relative positioning' do
+    let(:factory) { :design }
+    let(:default_params) { { issue: issue } }
+  end
+
   describe 'relations' do
     it { is_expected.to belong_to(:project) }
     it { is_expected.to belong_to(:issue) }