Change EpicsFinder to get epics related to a given issue list

* Lookup epics that are related to a given issue list.
* Update Board/Issues/ListService to fetch all issues related to a board

app/services/boards/issues/list_service.rb

@@ -45,6 +45,9 @@ module Boards
       # rubocop: enable CodeReuse/ActiveRecord
 
       def filter(issues)
+        # sometimes we want to just fetch all issues that a board can display
+        return issues if params[:all]
+
         issues = without_board_labels(issues) unless list&.movable? || list&.closed?
         issues = with_list_label(issues) if list&.label?
         issues

ee/app/finders/epics_finder.rb

@@ -30,7 +30,6 @@ class EpicsFinder < IssuableFinder
 
   def self.scalar_params
     @scalar_params ||= %i[
-      board
       parent_id
       author_id
       author_username
@@ -44,7 +43,7 @@ class EpicsFinder < IssuableFinder
   end
 
   def self.array_params
-    @array_params ||= { label_name: [] }
+    @array_params ||= { issues: [], label_name: [] }
   end
 
   def self.valid_iid_query?(query)
@@ -109,7 +108,7 @@ class EpicsFinder < IssuableFinder
   end
 
   def filter_items(items)
-    items = in_board(items)
+    items = in_issues(items)
     items = by_created_at(items)
     items = by_updated_at(items)
     items = by_author(items)
@@ -156,14 +155,13 @@ class EpicsFinder < IssuableFinder
     items.iid_starts_with(query)
   end
 
-  def in_board(items)
-    board = params[:board]
-    return items unless board.present?
+  def in_issues(items)
+    issues = params[:issues]
+    # ActiveRecord::Relation.empty? would run a SELECT 1 AS one FROM "issues" LIMIT $1
+    # ActiveRecord::Relation.blank? or ActiveRecord::Relation.present? would run SELECT "issues".* FROM "issues"
+    return items if issues.nil? || issues.empty?
 
-    list_service = Boards::Issues::ListService.new(board.resource_parent, current_user, { board_id: board.id })
-    issues = list_service.execute.except(:order).pluck_primary_key
-    epics_in_boards = EpicIssue.where(issue: issues)
-    items.id_in(epics_in_boards.select("epic_id as id"))
+    items.in_issues(issues.select(:id))
   end
 
   def related_groups

ee/app/graphql/ee/types/board_type.rb

@@ -21,7 +21,7 @@ module EE
         field :weight, type: GraphQL::INT_TYPE, null: true,
               description: 'Weight of the board.'
 
-        field :epic_groups, ::Types::EpicType.connection_type, null: true,
+        field :epics, ::Types::EpicType.connection_type, null: true,
               description: 'Epics associated with board issues.',
               resolver: ::Resolvers::BoardGroupings::EpicsResolver
       end

ee/app/graphql/resolvers/board_groupings/epics_resolver.rb

@@ -4,20 +4,32 @@ module Resolvers
   module BoardGroupings
     class EpicsResolver < BaseResolver
 
-     type Types::EpicType, null: true
+      type Types::EpicType, null: true
 
       def resolve(**args)
-        board = object.respond_to?(:sync) ? object.sync : object
+        @board = object.respond_to?(:sync) ? object.sync : object
 
-        return [] unless resolver_object.present?
-        return [] unless epic_feature_enabled?
+        return Epic.none unless board.present?
+        return Epic.none unless epic_feature_enabled?
 
-        EpicsFinder.new(context[:current_user], args.merge(board: board)).execute
+        list_service = Boards::Issues::ListService.new(board.resource_parent, current_user, { all: true, board_id: board.id })
+
+        # get bare issues by removing ordering, grouping and extra selected fields to get just the issues filtered by board scope.
+        issues = list_service.execute.except(:order).except(:group).except(:select).distinct
+
+        # Depending on which level the board is, user can see epics related to issues from various groups in the hierarchy,
+        # so we need to look-up epics from all groups in the hierarchy.
+        board_params = { group_id: group.id, include_ancestor_groups: true, include_descendant_groups: true, issues: issues }
+        EpicsFinder.new(context[:current_user], args.merge(board_params)).execute.limit(10)
       end
 
       private
 
-      attr_reader :resolver_object
+      attr_accessor :board
+
+      def group
+        board.project_board? ? board.project.group : board.group
+      end
 
       def epic_feature_enabled?
         group.feature_available?(:epics)