Create EpicBoardPosition class

- with RelativePositioning
- with scope

changelogs/unreleased/cablett-247867-epics-relative-position.yml

+---
+title: Add Epic Board Position model to store relative positioning of epics on a board
+merge_request: 48120
+author:
+type: added

db/migrate/20201119003042_add_epic_board_positions.rb

+# frozen_string_literal: true
+
+class AddEpicBoardPositions < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    with_lock_retries do
+      create_table :epic_board_positions do |t|
+        t.references :board, foreign_key: { on_delete: :cascade }, null: false, index: false
+        t.references :epic, foreign_key: { on_delete: :cascade }, null: false, index: true
+        t.integer :relative_position
+
+        t.timestamps_with_timezone null: false
+
+        t.index [:board_id, :epic_id], unique: true, name: :index_epic_board_positions_on_board_id_and_epic_id
+      end
+    end
+  end
+
+  def down
+    drop_table :epic_board_positions
+  end
+end

db/schema_migrations/20201119003042

+a3c26d7c88080d5d914d2d4ee0abd80d3d6c51739708d88b42a1c976f9510ee8
\ No newline at end of file

ee/app/models/ee/epic_board_position.rb

+# frozen_string_literal: true
+
+module EE
+  module EpicBoardPosition
+    extend ActiveSupport::Concern
+
+    prepended do
+      include RelativePositioning
+
+      belongs_to :board
+      belongs_to :epic
+
+      validates :board, presence: true
+      validates :epic, presence: true, uniqueness: { scope: :board_id }
+
+      alias_attribute :parent, :board
+
+      scope :order_relative_position, -> do
+        reorder('relative_position ASC', 'id DESC')
+      end
+    end
+
+    class_methods do
+      extend ::Gitlab::Utils::Override
+
+      def relative_positioning_query_base(position)
+        where(board_id: position.board_id)
+      end
+
+      def relative_positioning_parent_column
+        :board_id
+      end
+    end
+  end
+end

ee/spec/factories/epic_board_positions.rb

+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :epic_board_position do
+    epic
+    board
+    relative_position { RelativePositioning::START_POSITION }
+  end
+end

ee/spec/models/epic_board_position_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe EpicBoardPosition do
+  let_it_be(:epic) { create(:epic) }
+  let_it_be(:group) { create(:group) }
+  let_it_be(:board) { create(:board, group: group) }
+  let_it_be(:epic_board_position) { create(:epic_board_position, epic: epic, board: board) }
+
+  describe 'associations' do
+    subject { build(:epic_board_position) }
+
+    it { is_expected.to belong_to(:epic) }
+    it { is_expected.to belong_to(:board) }
+  end
+
+  describe 'validations' do
+    subject { build(:epic_board_position) }
+
+    it { is_expected.to validate_presence_of(:epic) }
+    it { is_expected.to validate_presence_of(:board) }
+
+    specify { expect(subject).to be_valid }
+
+    it 'is valid with nil relative position' do
+      subject.relative_position = nil
+
+      expect(subject).to be_valid
+    end
+
+    it 'disallows a record with same epic and board' do
+      expect(build(:epic_board_position, epic: epic, board: board)).not_to be_valid
+    end
+  end
+
+  describe 'scopes' do
+    describe '.order_relative_position' do
+      let(:first) { epic_board_position }
+      let!(:second) { create(:epic_board_position, board: board, relative_position: RelativePositioning::START_POSITION + 7 ) }
+
+      it 'returns epic_board_positions in order' do
+        expect(described_class.order_relative_position).to eq([first, second])
+      end
+    end
+  end
+
+  context 'relative positioning' do
+    let_it_be(:positioning_group) { create(:group) }
+    let_it_be(:positioning_board) { create(:board, group: positioning_group) }
+
+    it_behaves_like "a class that supports relative positioning" do
+      let(:factory) { :epic_board_position }
+      let(:default_params) { { parent: positioning_board } }
+    end
+  end
+end