Filter out deleted issues

app/services/related_issues/list_service.rb

@@ -31,11 +31,11 @@ module RelatedIssues
         <<-SQL.strip_heredoc
           SELECT issues.*, related_issues.id as related_issues_id FROM issues
           INNER JOIN related_issues ON related_issues.related_issue_id = issues.id
-          WHERE related_issues.issue_id = #{@issue.id}
+          WHERE related_issues.issue_id = #{@issue.id} AND issues.deleted_at IS NULL
           UNION ALL
           SELECT issues.*, related_issues.id as related_issues_id FROM issues
           INNER JOIN related_issues ON related_issues.issue_id = issues.id
-          WHERE related_issues.related_issue_id = #{@issue.id}
+          WHERE related_issues.related_issue_id = #{@issue.id} AND issues.deleted_at IS NULL
           ORDER BY related_issues_id
         SQL
       )

spec/services/related_issues/list_service_spec.rb

@@ -37,7 +37,7 @@ describe RelatedIssues::ListService, service: true do
 
       it 'verifies number of queries' do
         recorded = ActiveRecord::QueryRecorder.new { subject }
-        expect(recorded.count).to be_within(1).of(39)
+        expect(recorded.count).to be_within(1).of(42)
       end
 
       it 'returns related issues JSON' do
@@ -84,6 +84,47 @@ describe RelatedIssues::ListService, service: true do
       end
     end
 
+    context 'referencing issue with removed relationships' do
+      context 'when referenced a deleted issue' do
+        let(:issue_b) { create :issue, project: project }
+        let!(:related_issue) do
+          create(:related_issue, issue: issue, related_issue: issue_b)
+        end
+
+        it 'ignores issue' do
+          issue_b.destroy!
+
+          is_expected.to eq([])
+        end
+      end
+
+      context 'when referenced an issue with deleted project' do
+        let(:issue_b) { create :issue, project: project }
+        let!(:related_issue) do
+          create(:related_issue, issue: issue, related_issue: issue_b)
+        end
+
+        it 'ignores issue' do
+          project.destroy!
+
+          is_expected.to eq([])
+        end
+      end
+
+      context 'when referenced an issue with deleted namespace' do
+        let(:issue_b) { create :issue, project: project }
+        let!(:related_issue) do
+          create(:related_issue, issue: issue, related_issue: issue_b)
+        end
+
+        it 'ignores issue' do
+          project.namespace.destroy!
+
+          is_expected.to eq([])
+        end
+      end
+    end
+
     context 'user cannot see relations' do
       context 'when user cannot see the referenced issue' do
         let!(:related_issue) do