Added listing of epics

* exposes epics on an epic board list
* adds an index for epic board position table
* abstracts logic in issues list service so it can be reused by issue
  and epic lists

app/services/boards/base_items_list_service.rb

+# frozen_string_literal: true
+
+module Boards
+  class BaseItemsListService < Boards::BaseService
+    include Gitlab::Utils::StrongMemoize
+    include ActiveRecord::ConnectionAdapters::Quoting
+
+    def execute
+      return items.order_closed_date_desc if list&.closed?
+
+      ordered_items
+    end
+
+    private
+
+    def ordered_items
+      raise NotImplementedError
+    end
+
+    def finder
+      raise NotImplementedError
+    end
+
+    def board
+      raise NotImplementedError
+    end
+
+    def item_model
+      raise NotImplementedError
+    end
+
+    # We memoize the query here since the finder methods we use are quite complex. This does not memoize the result of the query.
+    # rubocop: disable CodeReuse/ActiveRecord
+    def items
+      strong_memoize(:items) do
+        filter(finder.execute).reorder(nil)
+      end
+    end
+    # rubocop: enable CodeReuse/ActiveRecord
+
+    def filter(items)
+      # when grouping board issues by epics (used in board swimlanes)
+      # we need to get all issues in the board
+      # TODO: ignore hidden columns -
+      # https://gitlab.com/gitlab-org/gitlab/-/issues/233870
+      return items if params[:all_lists]
+
+      items = without_board_labels(items) unless list&.movable? || list&.closed?
+      items = with_list_label(items) if list&.label?
+      items
+    end
+
+    def list
+      return unless params.key?(:id)
+
+      strong_memoize(:list) do
+        id = params[:id]
+
+        if board.lists.loaded?
+          board.lists.find { |l| l.id == id }
+        else
+          board.lists.find(id)
+        end
+      end
+    end
+
+    def filter_params
+      set_parent
+      set_state
+      set_attempt_search_optimizations
+
+      params
+    end
+
+    def set_parent
+      if parent.is_a?(Group)
+        params[:group_id] = parent.id
+      else
+        params[:project_id] = parent.id
+      end
+    end
+
+    def set_state
+      return if params[:all_lists]
+
+      params[:state] = list && list.closed? ? 'closed' : 'opened'
+    end
+
+    def set_attempt_search_optimizations
+      return unless params[:search].present?
+
+      if board.group_board?
+        params[:attempt_group_search_optimizations] = true
+      else
+        params[:attempt_project_search_optimizations] = true
+      end
+    end
+
+    # rubocop: disable CodeReuse/ActiveRecord
+    def board_label_ids
+      @board_label_ids ||= board.lists.movable.pluck(:label_id)
+    end
+    # rubocop: enable CodeReuse/ActiveRecord
+
+    # rubocop: disable CodeReuse/ActiveRecord
+    def without_board_labels(items)
+      return items unless board_label_ids.any?
+
+      items.where.not('EXISTS (?)', label_links(board_label_ids).limit(1))
+    end
+    # rubocop: enable CodeReuse/ActiveRecord
+
+    # rubocop: disable CodeReuse/ActiveRecord
+    def label_links(label_ids)
+      LabelLink
+        .where('label_links.target_type = ?', item_model)
+        .where(item_model.arel_table[:id].eq(LabelLink.arel_table[:target_id]).to_sql)
+        .where(label_id: label_ids)
+    end
+    # rubocop: enable CodeReuse/ActiveRecord
+
+    # rubocop: disable CodeReuse/ActiveRecord
+    def with_list_label(items)
+      items.where('EXISTS (?)', label_links(list.label_id).limit(1))
+    end
+    # rubocop: enable CodeReuse/ActiveRecord
+  end
+end

app/services/boards/issues/list_service.rb

@@ -2,26 +2,20 @@
 
 module Boards
   module Issues
-    class ListService < Boards::BaseService
+    class ListService < Boards::BaseItemsListService
       include Gitlab::Utils::StrongMemoize
 
       def self.valid_params
         IssuesFinder.valid_params
       end
 
-      def execute
-        return fetch_issues.order_closed_date_desc if list&.closed?
-
-        fetch_issues.order_by_position_and_priority(with_cte: params[:search].present?)
-      end
-
       # rubocop: disable CodeReuse/ActiveRecord
       def metadata
         issues = Issue.arel_table
         keys = metadata_fields.keys
         # TODO: eliminate need for SQL literal fragment
         columns = Arel.sql(metadata_fields.values_at(*keys).join(', '))
-        results = Issue.where(id: fetch_issues.select(issues[:id])).pluck(columns)
+        results = Issue.where(id: items.select(issues[:id])).pluck(columns)
 
         Hash[keys.zip(results.flatten)]
       end
@@ -29,74 +23,28 @@ module Boards
 
       private
 
-      def metadata_fields
-        { size: 'COUNT(*)' }
-      end
-
-      # We memoize the query here since the finder methods we use are quite complex. This does not memoize the result of the query.
-      # rubocop: disable CodeReuse/ActiveRecord
-      def fetch_issues
-        strong_memoize(:fetch_issues) do
-          issues = IssuesFinder.new(current_user, filter_params).execute
-
-          filter(issues).reorder(nil)
+      def ordered_items
+        items.order_by_position_and_priority(with_cte: params[:search].present?)
       end
-      end
-      # rubocop: enable CodeReuse/ActiveRecord
-
-      def filter(issues)
-        # when grouping board issues by epics (used in board swimlanes)
-        # we need to get all issues in the board
-        # TODO: ignore hidden columns -
-        # https://gitlab.com/gitlab-org/gitlab/-/issues/233870
-        return issues if params[:all_lists]
 
-        issues = without_board_labels(issues) unless list&.movable? || list&.closed?
-        issues = with_list_label(issues) if list&.label?
-        issues
+      def finder
+        IssuesFinder.new(current_user, filter_params)
       end
 
       def board
         @board ||= parent.boards.find(params[:board_id])
       end
 
-      def list
-        return unless params.key?(:id)
-
-        strong_memoize(:list) do
-          id = params[:id]
-
-          if board.lists.loaded?
-            board.lists.find { |l| l.id == id }
-          else
-            board.lists.find(id)
-          end
-        end
+      def metadata_fields
+        { size: 'COUNT(*)' }
       end
 
       def filter_params
-        set_parent
-        set_state
         set_scope
         set_non_archived
-        set_attempt_search_optimizations
         set_issue_types
 
-        params
-      end
-
-      def set_parent
-        if parent.is_a?(Group)
-          params[:group_id] = parent.id
-        else
-          params[:project_id] = parent.id
-        end
-      end
-
-      def set_state
-        return if params[:all_lists]
-
-        params[:state] = list && list.closed? ? 'closed' : 'opened'
+        super
       end
 
       def set_scope
@@ -107,49 +55,12 @@ module Boards
         params[:non_archived] = parent.is_a?(Group)
       end
 
-      def set_attempt_search_optimizations
-        return unless params[:search].present?
-
-        if board.group_board?
-          params[:attempt_group_search_optimizations] = true
-        else
-          params[:attempt_project_search_optimizations] = true
-        end
-      end
-
       def set_issue_types
         params[:issue_types] = Issue::TYPES_FOR_LIST
       end
 
-      # rubocop: disable CodeReuse/ActiveRecord
-      def board_label_ids
-        @board_label_ids ||= board.lists.movable.pluck(:label_id)
-      end
-      # rubocop: enable CodeReuse/ActiveRecord
-
-      # rubocop: disable CodeReuse/ActiveRecord
-      def without_board_labels(issues)
-        return issues unless board_label_ids.any?
-
-        issues.where.not('EXISTS (?)', issues_label_links.limit(1))
-      end
-      # rubocop: enable CodeReuse/ActiveRecord
-
-      # rubocop: disable CodeReuse/ActiveRecord
-      def issues_label_links
-        LabelLink.where("label_links.target_type = 'Issue' AND label_links.target_id = issues.id").where(label_id: board_label_ids)
-      end
-      # rubocop: enable CodeReuse/ActiveRecord
-
-      # rubocop: disable CodeReuse/ActiveRecord
-      def with_list_label(issues)
-        issues.where('EXISTS (?)', LabelLink.where("label_links.target_type = 'Issue' AND label_links.target_id = issues.id")
-                                            .where("label_links.label_id = ?", list.label_id).limit(1))
-      end
-      # rubocop: enable CodeReuse/ActiveRecord
-
-      def board_group
-        board.group_board? ? parent : parent.group
+      def item_model
+        Issue
       end
     end
   end

changelogs/unreleased/epic_boards_epic_list.yml

+---
+title: Added epic board position database index
+merge_request: 50277
+author:
+type: added

db/migrate/20210104163218_add_epic_board_position_index.rb

+# frozen_string_literal: true
+
+class AddEpicBoardPositionIndex < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_boards_epic_board_positions_on_scoped_relative_position'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :boards_epic_board_positions, [:epic_board_id, :epic_id, :relative_position], name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :boards_epic_board_positions, INDEX_NAME
+  end
+end

db/schema_migrations/20210104163218

+37aa0564d2ade1cab56a669facccbaaf08e4d9856c7a4cc120968d33cff161bd
\ No newline at end of file

db/structure.sql

@@ -20906,6 +20906,8 @@ CREATE UNIQUE INDEX index_boards_epic_board_positions_on_epic_board_id_and_epic_
 
 CREATE INDEX index_boards_epic_board_positions_on_epic_id ON boards_epic_board_positions USING btree (epic_id);
 
+CREATE INDEX index_boards_epic_board_positions_on_scoped_relative_position ON boards_epic_board_positions USING btree (epic_board_id, epic_id, relative_position);
+
 CREATE INDEX index_boards_epic_boards_on_group_id ON boards_epic_boards USING btree (group_id);
 
 CREATE INDEX index_boards_epic_lists_on_epic_board_id ON boards_epic_lists USING btree (epic_board_id);

doc/api/graphql/reference/gitlab_schema.graphql

@@ -8459,6 +8459,11 @@ type EpicBoard {
     """
     first: Int
 
+    """
+    Find an epic board list by ID.
+    """
+    id: BoardsEpicListID
+
     """
     Returns the last _n_ elements from the list.
     """
@@ -9070,6 +9075,31 @@ type EpicIssueEdge {
 Represents an epic board list
 """
 type EpicList {
+  """
+  List epics.
+  """
+  epics(
+    """
+    Returns the elements in the list that come after the specified cursor.
+    """
+    after: String
+
+    """
+    Returns the elements in the list that come before the specified cursor.
+    """
+    before: String
+
+    """
+    Returns the first _n_ elements from the list.
+    """
+    first: Int
+
+    """
+    Returns the last _n_ elements from the list.
+    """
+    last: Int
+  ): EpicConnection
+
   """
   Global ID of the board list.
   """

doc/api/graphql/reference/gitlab_schema.json

@@ -23390,6 +23390,16 @@
               "name": "lists",
               "description": "Epic board lists.",
               "args": [
+                {
+                  "name": "id",
+                  "description": "Find an epic board list by ID.",
+                  "type": {
+                    "kind": "SCALAR",
+                    "name": "BoardsEpicListID",
+                    "ofType": null
+                  },
+                  "defaultValue": null
+                },
                 {
                   "name": "after",
                   "description": "Returns the elements in the list that come after the specified cursor.",
@@ -25184,6 +25194,59 @@
           "name": "EpicList",
           "description": "Represents an epic board list",
           "fields": [
+            {
+              "name": "epics",
+              "description": "List epics.",
+              "args": [
+                {
+                  "name": "after",
+                  "description": "Returns the elements in the list that come after the specified cursor.",
+                  "type": {
+                    "kind": "SCALAR",
+                    "name": "String",
+                    "ofType": null
+                  },
+                  "defaultValue": null
+                },
+                {
+                  "name": "before",
+                  "description": "Returns the elements in the list that come before the specified cursor.",
+                  "type": {
+                    "kind": "SCALAR",
+                    "name": "String",
+                    "ofType": null
+                  },
+                  "defaultValue": null
+                },
+                {
+                  "name": "first",
+                  "description": "Returns the first _n_ elements from the list.",
+                  "type": {
+                    "kind": "SCALAR",
+                    "name": "Int",
+                    "ofType": null
+                  },
+                  "defaultValue": null
+                },
+                {
+                  "name": "last",
+                  "description": "Returns the last _n_ elements from the list.",
+                  "type": {
+                    "kind": "SCALAR",
+                    "name": "Int",
+                    "ofType": null
+                  },
+                  "defaultValue": null
+                }
+              ],
+              "type": {
+                "kind": "OBJECT",
+                "name": "EpicConnection",
+                "ofType": null
+              },
+              "isDeprecated": false,
+              "deprecationReason": null
+            },
             {
               "name": "id",
               "description": "Global ID of the board list.",

doc/api/graphql/reference/index.md

@@ -1481,6 +1481,7 @@ Represents an epic board list.
 
 | Field | Type | Description |
 | ----- | ---- | ----------- |
+| `epics` | EpicConnection | List epics. |
 | `id` | BoardsEpicListID! | Global ID of the board list. |
 | `label` | Label | Label of the list. |
 | `listType` | String! | Type of the list. |

ee/app/graphql/resolvers/boards/board_list_epics_resolver.rb

+# frozen_string_literal: true
+
+module Resolvers
+  module Boards
+    class BoardListEpicsResolver < BaseResolver
+      type Types::EpicType.connection_type, null: true
+
+      alias_method :list, :object
+
+      def resolve(**args)
+        filter_params = { board_id: list.epic_board.id, id: list.id }
+        service = ::Boards::Epics::ListService.new(list.epic_board.group, context[:current_user], filter_params)
+
+        offset_pagination(service.execute)
+      end
+    end
+  end
+end

ee/app/graphql/resolvers/boards/epic_lists_resolver.rb

@@ -8,11 +8,9 @@ module Resolvers
 
       type Types::Boards::EpicListType.connection_type, null: true
 
-      when_single do
       argument :id, ::Types::GlobalIDType[::Boards::EpicList],
-                 required: true,
+               required: false,
                description: 'Find an epic board list by ID.'
-      end
 
       alias_method :epic_board, :object
 

ee/app/graphql/types/boards/epic_list_type.rb

@@ -23,6 +23,10 @@ module Types
 
       field :label, Types::LabelType, null: true,
             description: 'Label of the list.'
+
+      field :epics, Types::EpicType.connection_type, null: true,
+            resolver: Resolvers::Boards::BoardListEpicsResolver,
+            description: 'List epics.'
     end
     # rubocop: enable Graphql/AuthorizeTypes
   end

ee/app/models/boards/epic_board.rb

@@ -10,5 +10,9 @@ module Boards
     validates :name, length: { maximum: 255 }
 
     scope :order_by_name_asc, -> { order(arel_table[:name].lower.asc).order(id: :asc) }
+
+    def lists
+      epic_lists
+    end
   end
 end

ee/app/models/boards/epic_list.rb

@@ -2,6 +2,8 @@
 
 module Boards
   class EpicList < ApplicationRecord
+    # TODO: we can move logic shared with with List model to
+    # a module. https://gitlab.com/gitlab-org/gitlab/-/issues/296559
     belongs_to :epic_board, optional: false, inverse_of: :epic_lists
     belongs_to :label, inverse_of: :epic_lists
 
@@ -12,9 +14,18 @@ module Boards
     validates :position, numericality: { only_integer: true, greater_than_or_equal_to: 0 }, if: :label?
 
     scope :ordered, -> { order(:list_type, :position) }
+    scope :movable, -> { where(list_type: list_types.slice(*movable_types).values) }
+
+    def self.movable_types
+      [:label]
+    end
 
     def title
       label? ? label.name : list_type.humanize
     end
+
+    def movable?
+      label?
+    end
   end
 end

ee/app/models/ee/epic.rb

@@ -57,6 +57,7 @@ module EE
       has_many :issues, through: :epic_issues
       has_many :user_mentions, class_name: "EpicUserMention", dependent: :delete_all # rubocop:disable Cop/ActiveRecordDependent
       has_many :boards_epic_user_preferences, class_name: 'Boards::EpicUserPreference', inverse_of: :epic
+      has_many :epic_board_positions, class_name: 'Boards::EpicBoardPosition', inverse_of: :epic_board
 
       validates :group, presence: true
       validate :validate_parent, on: :create
@@ -103,10 +104,18 @@ module EE
         reorder(::Gitlab::Database.nulls_last_order('start_date', 'DESC'), 'id DESC')
       end
 
+      scope :order_closed_date_desc, -> { reorder(closed_at: :desc) }
+
       scope :order_relative_position, -> do
         reorder('relative_position ASC', 'id DESC')
       end
 
+      scope :order_relative_position_on_board, ->(board_id) do
+        left_joins(:epic_board_positions)
+          .where(boards_epic_board_positions: { epic_board_id: [nil, board_id] })
+          .reorder(::Gitlab::Database.nulls_last_order('boards_epic_board_positions.relative_position', 'ASC'), 'epics.id DESC')
+      end
+
       scope :with_api_entity_associations, -> { preload(:author, :labels, group: :route) }
       scope :start_date_inherited, -> { where(start_date_is_fixed: [nil, false]) }
       scope :due_date_inherited, -> { where(due_date_is_fixed: [nil, false]) }

ee/app/services/boards/epics/list_service.rb

+# frozen_string_literal: true
+
+module Boards
+  module Epics
+    class ListService < Boards::BaseItemsListService
+      private
+
+      def finder
+        EpicsFinder.new(current_user, filter_params.merge(group_id: parent.id))
+      end
+
+      def board
+        @board ||= parent.epic_boards.find(params[:board_id])
+      end
+
+      def ordered_items
+        items.order_relative_position_on_board(board.id)
+      end
+
+      def item_model
+        ::Epic
+      end
+    end
+  end
+end

ee/app/services/ee/boards/issues/list_service.rb

@@ -28,9 +28,9 @@ module EE
           end
         end
 
-        override :issues_label_links
+        override :label_links
         # rubocop: disable CodeReuse/ActiveRecord
-        def issues_label_links
+        def label_links(label_ids)
           if has_valid_milestone?
             super.where("issues.milestone_id = ?", board.milestone_id)
           else

ee/spec/factories/boards/epic_lists.rb

@@ -3,7 +3,7 @@
 FactoryBot.define do
   factory :epic_list, class: 'Boards::EpicList' do
     epic_board
-    label
+    association :label, factory: :group_label
     list_type { :label }
     sequence(:position)
   end

ee/spec/graphql/resolvers/boards/board_list_epics_resolver_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Resolvers::Boards::BoardListEpicsResolver do
+  include GraphqlHelpers
+
+  let_it_be(:user) { create(:user) }
+  let_it_be(:group) { create(:group) }
+  let_it_be(:development) { create(:group_label, group: group, name: 'Development') }
+  let_it_be(:testing) { create(:group_label, group: group, name: 'Testing') }
+
+  let_it_be(:board) { create(:epic_board, group: group) }
+  let_it_be(:list1) { create(:epic_list, epic_board: board, label: development, position: 0) }
+  let_it_be(:list2) { create(:epic_list, epic_board: board, label: testing, position: 0) }
+  let_it_be(:list1_epic1) { create(:labeled_epic, group: group, labels: [development]) }
+  let_it_be(:list1_epic2) { create(:labeled_epic, group: group, labels: [development]) }
+  let_it_be(:list2_epic1) { create(:labeled_epic, group: group, labels: [testing]) }
+
+  let_it_be(:epic_pos1) { create(:epic_board_position, epic: list1_epic1, epic_board: board, relative_position: 20) }
+  let_it_be(:epic_pos2) { create(:epic_board_position, epic: list1_epic2, epic_board: board, relative_position: 10) }
+  let_it_be(:epic_pos3) { create(:epic_board_position, epic: list1_epic1, relative_position: 30) }
+
+  specify do
+    expect(described_class).to have_nullable_graphql_type(Types::EpicType.connection_type)
+  end
+
+  describe '#resolve' do
+    let(:args) { {} }
+
+    subject(:result) { resolve(described_class, ctx: { current_user: user }, obj: list1, args: args) }
+
+    before do
+      stub_licensed_features(epics: true)
+      group.add_reporter(user)
+    end
+
+    it 'returns epics on the board list ordered by position on the board' do
+      expect(result.items).to eq([list1_epic2, list1_epic1])
+    end
+  end
+end

ee/spec/graphql/types/boards/epic_list_type_spec.rb

@@ -6,7 +6,7 @@ RSpec.describe GitlabSchema.types['EpicList'] do
   specify { expect(described_class.graphql_name).to eq('EpicList') }
 
   it 'has specific fields' do
-    expected_fields = %w[id title list_type position label]
+    expected_fields = %w[id title list_type position label epics]
 
     expect(described_class).to include_graphql_fields(*expected_fields)
   end

ee/spec/models/epic_spec.rb

@@ -19,7 +19,8 @@ RSpec.describe Epic do
     it { is_expected.to have_many(:epic_issues) }
     it { is_expected.to have_many(:children) }
     it { is_expected.to have_many(:user_mentions).class_name('EpicUserMention') }
-    it { is_expected.to have_many(:boards_epic_user_preferences).class_name('Boards::EpicUserPreference') }
+    it { is_expected.to have_many(:boards_epic_user_preferences).class_name('Boards::EpicUserPreference').inverse_of(:epic) }
+    it { is_expected.to have_many(:epic_board_positions).class_name('Boards::EpicBoardPosition').inverse_of(:epic_board) }
   end
 
   describe 'scopes' do
@@ -45,6 +46,21 @@ RSpec.describe Epic do
         expect(described_class.not_confidential_or_in_groups(group)).to match_array([confidential_epic, public_epic])
       end
     end
+
+    describe '.order_relative_position_on_board' do
+      let_it_be(:board) { create(:epic_board) }
+      let_it_be(:epic1) { create(:epic) }
+      let_it_be(:epic2) { create(:epic) }
+      let_it_be(:epic3) { create(:epic) }
+
+      it 'returns epics ordered by position on the board, null last' do
+        create(:epic_board_position, epic: epic2, epic_board: board, relative_position: 10)
+        create(:epic_board_position, epic: epic1, epic_board: board, relative_position: 20)
+        create(:epic_board_position, epic: epic3, epic_board: board, relative_position: 20)
+
+        expect(described_class.order_relative_position_on_board(board.id)).to eq([epic2, epic3, epic1, public_epic, confidential_epic])
+      end
+    end
   end
 
   describe 'validations' do

ee/spec/requests/api/graphql/boards/epic_board_list_epics_query_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe 'get list of epics for an epic  board list' do
+  include GraphqlHelpers
+
+  let_it_be(:current_user) { create(:user) }
+  let_it_be(:group) { create(:group, :private) }
+  let_it_be(:development) { create(:group_label, group: group, name: 'Development') }
+  let_it_be(:board) { create(:epic_board, group: group) }
+  let_it_be(:list) { create(:epic_list, epic_board: board, label: development) }
+
+  let_it_be(:epic1) { create(:labeled_epic, group: group, labels: [development]) }
+  let_it_be(:epic2) { create(:labeled_epic, group: group, labels: [development]) }
+  let_it_be(:epic3) { create(:labeled_epic, group: group, labels: [development]) }
+  let_it_be(:epic4) { create(:labeled_epic, group: group) }
+
+  let_it_be(:epic_pos1) { create(:epic_board_position, epic: epic1, epic_board: board, relative_position: 20) }
+  let_it_be(:epic_pos2) { create(:epic_board_position, epic: epic2, epic_board: board, relative_position: 10) }
+
+  def pagination_query(params = {})
+    graphql_query_for(:group, { full_path: group.full_path },
+      <<~BOARDS
+        epicBoard(id: "#{board.to_global_id}") {
+          lists(id: "#{list.to_global_id}") {
+            nodes {
+              #{query_nodes(:epics, all_graphql_fields_for('epics'.classify), include_pagination_info: true, args: params)}
+            }
+          }
+        }
+      BOARDS
+    )
+  end
+
+  before do
+    stub_licensed_features(epics: true)
+    group.add_developer(current_user)
+  end
+
+  describe 'sorting and pagination' do
+    let(:data_path) { [:group, :epicBoard, :lists, :nodes, 0, :epics] }
+    let(:expected_results) { [epic2.to_global_id.to_s, epic1.to_global_id.to_s, epic3.to_global_id.to_s] }
+
+    def pagination_results_data(nodes)
+      nodes.map { |list| list['id'] }
+    end
+
+    it_behaves_like 'sorted paginated query' do
+      # currently we don't support custom sorting for epic lists,
+      # nil value will be ignored by ::Graphql::Arguments
+      let(:sort_param) { nil }
+      let(:first_param) { 2 }
+    end
+  end
+end

ee/spec/services/boards/epics/list_service_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Boards::Epics::ListService do
+  describe '#execute' do
+    let_it_be(:user) { create(:user) }
+    let_it_be(:group) { create(:group) }
+    let_it_be(:board) { create(:epic_board, group: group) }
+
+    let_it_be(:development) { create(:group_label, group: group, name: 'Development') }
+    let_it_be(:testing) { create(:group_label, group: group, name: 'Testing') }
+
+    let_it_be(:backlog) { create(:epic_list, epic_board: board, list_type: :backlog) }
+    let_it_be(:list1) { create(:epic_list, epic_board: board, label: development, position: 0) }
+    let_it_be(:list2) { create(:epic_list, epic_board: board, label: testing, position: 1) }
+    let_it_be(:closed) { create(:epic_list, epic_board: board, list_type: :closed) }
+
+    let_it_be(:backlog_epic1) { create(:epic, group: group) }
+    let_it_be(:list1_epic1) { create(:labeled_epic, group: group, labels: [development]) }
+    let_it_be(:list1_epic2) { create(:labeled_epic, group: group, labels: [development]) }
+    let_it_be(:list1_epic3) { create(:labeled_epic, group: group, labels: [development]) }
+    let_it_be(:list2_epic1) { create(:labeled_epic, group: group, labels: [testing]) }
+
+    let_it_be(:closed_epic1) { create(:labeled_epic, :closed, group: group, labels: [development], closed_at: 1.day.ago) }
+    let_it_be(:closed_epic2) { create(:labeled_epic, :closed, group: group, labels: [testing], closed_at: 2.days.ago) }
+    let_it_be(:closed_epic3) { create(:epic, :closed, group: group, closed_at: 1.week.ago) }
+
+    before do
+      stub_licensed_features(epics: true)
+      group.add_developer(user)
+    end
+
+    it_behaves_like 'items list service' do
+      let(:parent) { group }
+      let(:backlog_items) { [backlog_epic1] }
+      let(:list1_items) { [list1_epic1, list1_epic2, list1_epic3] }
+      let(:closed_items) { [closed_epic1, closed_epic2, closed_epic3] }
+      let(:all_items) { backlog_items + list1_items + closed_items + [list2_epic1] }
+      let(:list_factory) { :epic_list }
+      let(:new_list) { create(:epic_list, epic_board: board) }
+    end
+
+    it 'returns epics sorted by position on the board' do
+      create(:epic_board_position, epic: list1_epic1, epic_board: board, relative_position: 20)
+      create(:epic_board_position, epic: list1_epic2, epic_board: board, relative_position: 10)
+      create(:epic_board_position, epic: list1_epic1, relative_position: 30)
+
+      epics = described_class.new(group, user, { board_id: board.id, id: list1.id }).execute
+
+      expect(epics).to eq([list1_epic2, list1_epic1, list1_epic3])
+    end
+  end
+end

spec/lib/gitlab/import_export/all_models.yml

@@ -723,6 +723,7 @@ epic:
 - user_mentions
 - note_authors
 - boards_epic_user_preferences
+- epic_board_positions
 epic_issue:
 - epic
 - issue

spec/support/shared_examples/services/boards/issues_list_service_shared_examples.rb

@@ -19,78 +19,12 @@ RSpec.shared_examples 'issues list service' do
     end
   end
 
-  it 'avoids N+1' do
-    params = { board_id: board.id }
-    control = ActiveRecord::QueryRecorder.new { described_class.new(parent, user, params).execute }
-
-    create(:list, board: board)
-
-    expect { described_class.new(parent, user, params).execute }.not_to exceed_query_limit(control)
-  end
-
-  context 'issues are ordered by priority' do
-    it 'returns opened issues when list_id is missing' do
-      params = { board_id: board.id }
-
-      issues = described_class.new(parent, user, params).execute
-
-      expect(issues).to eq [opened_issue2, reopened_issue1, opened_issue1]
-    end
-
-    it 'returns opened issues when listing issues from Backlog' do
-      params = { board_id: board.id, id: backlog.id }
-
-      issues = described_class.new(parent, user, params).execute
-
-      expect(issues).to eq [opened_issue2, reopened_issue1, opened_issue1]
-    end
-
-    it 'returns opened issues that have label list applied when listing issues from a label list' do
-      params = { board_id: board.id, id: list1.id }
-
-      issues = described_class.new(parent, user, params).execute
-
-      expect(issues).to eq [list1_issue3, list1_issue1, list1_issue2]
-    end
-  end
-
-  context 'issues are ordered by date of closing' do
-    it 'returns closed issues when listing issues from Closed' do
-      params = { board_id: board.id, id: closed.id }
-
-      issues = described_class.new(parent, user, params).execute
-
-      expect(issues).to eq [closed_issue1, closed_issue2, closed_issue3, closed_issue4, closed_issue5]
-    end
-  end
-
-  context 'with list that does not belong to the board' do
-    it 'raises an error' do
-      list = create(:list)
-      service = described_class.new(parent, user, board_id: board.id, id: list.id)
-
-      expect { service.execute }.to raise_error(ActiveRecord::RecordNotFound)
-    end
-  end
-
-  context 'with invalid list id' do
-    it 'raises an error' do
-      service = described_class.new(parent, user, board_id: board.id, id: nil)
-
-      expect { service.execute }.to raise_error(ActiveRecord::RecordNotFound)
-    end
-  end
-
-  context 'when :all_lists is used' do
-    it 'returns issues from all lists' do
-      params = { board_id: board.id, all_lists: true }
-
-      issues = described_class.new(parent, user, params).execute
-
-      expected = [opened_issue2, reopened_issue1, opened_issue1, list1_issue1,
-                  list1_issue2, list1_issue3, list2_issue1, closed_issue1,
-                  closed_issue2, closed_issue3, closed_issue4, closed_issue5]
-      expect(issues).to match_array(expected)
-    end
+  it_behaves_like 'items list service' do
+    let(:backlog_items) { [opened_issue2, reopened_issue1, opened_issue1] }
+    let(:list1_items) { [list1_issue3, list1_issue1, list1_issue2] }
+    let(:closed_items) { [closed_issue1, closed_issue2, closed_issue3, closed_issue4, closed_issue5] }
+    let(:all_items) { backlog_items + list1_items + closed_items + [list2_issue1] }
+    let(:list_factory) { :list }
+    let(:new_list) { create(:list, board: board) }
   end
 end

spec/support/shared_examples/services/boards/items_list_service_shared_examples.rb

+# frozen_string_literal: true
+
+RSpec.shared_examples 'items list service' do
+  it 'avoids N+1' do
+    params = { board_id: board.id }
+    control = ActiveRecord::QueryRecorder.new { described_class.new(parent, user, params).execute }
+
+    new_list
+
+    expect { described_class.new(parent, user, params).execute }.not_to exceed_query_limit(control)
+  end
+
+  it 'returns opened items when list_id is missing' do
+    params = { board_id: board.id }
+
+    items = described_class.new(parent, user, params).execute
+
+    expect(items).to match_array(backlog_items)
+  end
+
+  it 'returns opened items when listing items from Backlog' do
+    params = { board_id: board.id, id: backlog.id }
+
+    items = described_class.new(parent, user, params).execute
+
+    expect(items).to match_array(backlog_items)
+  end
+
+  it 'returns opened items that have label list applied when listing items from a label list' do
+    params = { board_id: board.id, id: list1.id }
+
+    items = described_class.new(parent, user, params).execute
+
+    expect(items).to match_array(list1_items)
+  end
+
+  it 'returns closed items when listing items from Closed sorted by closed_at in descending order' do
+    params = { board_id: board.id, id: closed.id }
+
+    items = described_class.new(parent, user, params).execute
+
+    expect(items).to eq(closed_items)
+  end
+
+  it 'raises an error if the list does not belong to the board' do
+    list = create(list_factory) # rubocop:disable Rails/SaveBang
+    service = described_class.new(parent, user, board_id: board.id, id: list.id)
+
+    expect { service.execute }.to raise_error(ActiveRecord::RecordNotFound)
+  end
+
+  it 'raises an error if list id is invalid' do
+    service = described_class.new(parent, user, board_id: board.id, id: nil)
+
+    expect { service.execute }.to raise_error(ActiveRecord::RecordNotFound)
+  end
+
+  it 'returns items from all lists if :all_list is used' do
+    params = { board_id: board.id, all_lists: true }
+
+    items = described_class.new(parent, user, params).execute
+
+    expect(items).to match_array(all_items)
+  end
+end