Fix duplicate query problem

We were loading board issues twice, because we invalidated the
issues scope after loading them by adding another `preload`,
so the query fired again during template rendering.
By eager-loading issues upfront, we can enforce this to always be
just one query.

app/controllers/boards/issues_controller.rb

@@ -24,17 +24,14 @@ module Boards
       push_frontend_feature_flag(:board_search_optimization, board.group)
     end
 
-    # rubocop: disable CodeReuse/ActiveRecord
     def index
       list_service = Boards::Issues::ListService.new(board_parent, current_user, filter_params)
-      issues = list_service.execute
-      issues = issues.page(params[:page]).per(params[:per] || 20).without_count
+      issues = issues_from(list_service)
+
       Issue.move_nulls_to_end(issues) if Gitlab::Database.read_write?
-      issues = issues.preload(associations_to_preload)
 
       render_issues(issues, list_service.metadata)
     end
-    # rubocop: enable CodeReuse/ActiveRecord
 
     def create
       service = Boards::Issues::CreateService.new(board_parent, project, current_user, issue_params)
@@ -67,6 +64,14 @@ module Boards
 
     private
 
+    def issues_from(list_service)
+      issues = list_service.execute
+      issues.page(params[:page]).per(params[:per] || 20)
+            .without_count
+            .preload(associations_to_preload) # rubocop: disable CodeReuse/ActiveRecord
+            .load
+    end
+
     def associations_to_preload
       [
         :milestone,

changelogs/unreleased/mk-fix-issue-board-query-dupe.yml

+---
+title: Fix redundant query execution when loading board issues
+merge_request: 27505
+author:
+type: performance

spec/controllers/boards/issues_controller_spec.rb

@@ -112,6 +112,13 @@ describe Boards::IssuesController do
 
           expect { list_issues(user: user, board: group_board, list: list3) }.not_to exceed_query_limit(control_count + (2 * 8 - 1))
         end
+
+        it 'does not query issues table more than once' do
+          recorder = ActiveRecord::QueryRecorder.new { list_issues(user: user, board: board, list: list1) }
+          query_count = recorder.occurrences.select { |query,| query.start_with?('SELECT issues.*') }.each_value.first
+
+          expect(query_count).to eq(1)
+        end
       end
 
       context 'with invalid list id' do