Use 'preload' in export to CSV

If issues are filtered by multiple labels, then GROUP BY statement
is used to filter only issues with these labels. This is problem in
combination with '.includes' which may use JOIN statement to load
associated resources in single query instead of a separate query.

Then the generated SQL query fails because associated resources
are included in 'SELECT ...' statements but not in the GROUP BY
statement.

Usage of '.preload' instead of '.includes' ensures that Active Record
uses a separate query instead of JOIN.

Closes #4376

changelogs/unreleased-ee/3841-fix-export-to-csv.yml

+---
+title: Fix export to CSV if a filter with multiple labels is used
+merge_request:
+author:
+type: fixed

ee/app/services/issues/export_csv_service.rb

@@ -21,7 +21,7 @@ module Issues
 
     def csv_builder
       @csv_builder ||=
-        CsvBuilder.new(@issues.includes(:author, :assignees, :timelogs), header_to_value_hash)
+        CsvBuilder.new(@issues.preload(:author, :assignees, :timelogs), header_to_value_hash)
     end
 
     private

spec/services/issues/export_csv_service_spec.rb

@@ -127,6 +127,20 @@ describe Issues::ExportCsvService do
       expect(csv[0]['Time Spent']).to eq '560'
       expect(csv[1]['Time Spent']).to eq '0'
     end
+
+    context 'with issues filtered by labels and project' do
+      let(:subject) do
+        described_class.new(
+          IssuesFinder.new(user,
+                           project_id: project.id,
+                           label_name: %w(Idea Feature)).execute)
+      end
+
+      it 'returns only filtered objects' do
+        expect(csv.count).to eq(1)
+        expect(csv[0]['Issue ID']).to eq issue.iid.to_s
+      end
+    end
   end
 
   context 'with minimal details' do