Enable Merge Train checkbox based on Merge Pipelines checkbox

The Merge Train checkbox is enabled only when the Merge Pipelines
checkbox is enabled and checked. The initial state is set by querying
`ciCdSettings` via GraphQL.

app/views/projects/_merge_request_merge_options_settings.html.haml

 - form = local_assigns.fetch(:form)
 
-.form-group
+.form-group#project-merge-options{ data: { project_full_path: @project.full_path } }
   %b= s_('ProjectSettings|Merge options')
   %p.text-secondary= s_('ProjectSettings|Additional merge request capabilities that influence how and when merges will be performed')
   = render_if_exists 'projects/merge_pipelines_settings', form: form

ee/app/assets/javascripts/pages/projects/edit/index.js

@@ -3,6 +3,7 @@
 import '~/pages/projects/edit';
 import mountApprovals from 'ee/approvals/mount_project_settings';
 import initProjectAdjournedDeleteButton from 'ee/projects/project_adjourned_delete_button';
+import initMergeOptionSettings from 'ee/pages/projects/edit/merge_options';
 import UsersSelect from '~/users_select';
 import UserCallout from '~/user_callout';
 import groupsSelect from '~/groups_select';
@@ -15,3 +16,4 @@ new UserCallout({ className: 'js-mr-approval-callout' });
 mountApprovals(document.getElementById('js-mr-approvals-settings'));
 
 initProjectAdjournedDeleteButton();
+initMergeOptionSettings();

ee/app/assets/javascripts/pages/projects/edit/merge_options.js

+import gql from 'graphql-tag';
+import createDefaultClient from '~/lib/graphql';
+
+const containerEl = document.querySelector('#project-merge-options');
+const mergePipelinesCheckbox = document.querySelector('.js-merge-options-merge-pipelines');
+const mergeTrainsCheckbox = document.querySelector('.js-merge-options-merge-trains');
+
+const getCiCdSettingsQuery = (projectFullPath) =>
+  gql`
+    query {
+      project(fullPath:"${projectFullPath}") {
+        id,
+        ciCdSettings {
+          mergePipelinesEnabled,
+          mergeTrainsEnabled,
+        }
+      }
+    }
+  `;
+
+const disableMergeTrains = () => {
+  mergeTrainsCheckbox.disabled = true;
+  mergeTrainsCheckbox.checked = false;
+};
+
+export default function () {
+  const { projectFullPath } = containerEl.dataset;
+  const defaultClient = createDefaultClient();
+
+  defaultClient
+    .query({
+      query: getCiCdSettingsQuery(projectFullPath),
+    })
+    .then((result) => {
+      const { mergePipelinesEnabled, mergeTrainsEnabled } = result?.data?.project?.ciCdSettings;
+      mergePipelinesCheckbox.checked = mergePipelinesEnabled;
+      mergeTrainsCheckbox.checked = mergeTrainsEnabled;
+
+      if (!mergePipelinesEnabled) {
+        disableMergeTrains();
+      }
+    })
+    .catch(() => {
+      if (!mergePipelinesCheckbox.checked) {
+        disableMergeTrains();
+      }
+    });
+
+  mergePipelinesCheckbox.addEventListener('change', () => {
+    mergeTrainsCheckbox.disabled = !mergePipelinesCheckbox.checked;
+
+    if (!mergePipelinesCheckbox.checked) {
+      mergeTrainsCheckbox.checked = false;
+    }
+  });
+}

ee/app/views/projects/_merge_trains_settings.html.haml

 - return unless merge_trains_available?
 
 .form-check
-  = form.check_box :merge_trains_enabled, class: 'form-check-input', data: { qa_selector: 'merge_trains_checkbox' }
-  = form.label :merge_trains_enabled, class: 'form-check-label' do
-    = s_('ProjectSettings|Enable merge trains.')
-    .text-secondary.mb-2
-      = s_('ProjectSettings|When approved for merge, merge requests are queued and pipelines validate the combined results of the source and target branches before merge.')
-      = link_to sprite_icon('question-o'),
-        help_page_path('ci/merge_request_pipelines/pipelines_for_merged_results/merge_trains/index.md',
-        anchor: 'merge-trains'),
-        target: '_blank',
-        rel: 'noopener noreferer'
+  .gl-pl-6
+    = form.check_box :merge_trains_enabled, class: 'form-check-input js-merge-options-merge-trains gl-pl-6', data: { qa_selector: 'merge_trains_checkbox' }
+    = form.label :merge_trains_enabled, class: 'form-check-label' do
+      = s_('ProjectSettings|Enable merge trains.')
+      .text-secondary.mb-2
+        = s_('ProjectSettings|When approved for merge, merge requests are queued and pipelines validate the combined results of the source and target branches before merge.')
+        = link_to sprite_icon('question-o'),
+          help_page_path('ci/merge_request_pipelines/pipelines_for_merged_results/merge_trains/index.md',
+          anchor: 'merge-trains'),
+          target: '_blank',
+          rel: 'noopener noreferer'

ee/changelogs/unreleased/set-merge-option-states.yml

+---
+title: Enable Merge Train checkbox based on Merge Pipelines checkbox
+merge_request: 51233
+author:
+type: changed

ee/spec/features/projects/settings/disable_merge_trains_setting_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe 'Disable Merge Trains Setting', :js do
+  let_it_be_with_reload(:project) { create(:project) }
+  let_it_be(:user) { create(:user) }
+
+  before do
+    stub_licensed_features(merge_pipelines: true, merge_trains: true)
+
+    project.add_maintainer(user)
+    sign_in(user)
+  end
+
+  shared_examples 'loads correct checkbox state' do
+    it 'merge pipelines checkbox is always enabled' do
+      expect(find('#project_merge_pipelines_enabled')).not_to be_disabled
+    end
+
+    it 'merge trains checkbox is enabled only when merge_pipelines_enabled is true' do
+      expect(find('#project_merge_trains_enabled').disabled?).not_to eq(project.merge_pipelines_enabled)
+    end
+  end
+
+  context 'when visiting the project settings page' do
+    using RSpec::Parameterized::TableSyntax
+
+    where(:merge_pipelines_setting, :merge_trains_setting) do
+      true  | true
+      true  | false
+      false | true
+      false | false
+    end
+
+    with_them do
+      before do
+        project.update!(merge_pipelines_enabled: merge_pipelines_setting, merge_trains_enabled: merge_trains_setting)
+        visit edit_project_path(project)
+        wait_for_requests
+      end
+
+      include_examples 'loads correct checkbox state'
+    end
+  end
+
+  context 'when merge pipelines is enabled' do
+    before do
+      project.update!(merge_pipelines_enabled: true)
+      visit edit_project_path(project)
+      wait_for_requests
+    end
+
+    include_examples 'loads correct checkbox state'
+
+    it "checking merge trains checkbox doesn't affect merge pipelines checkbox" do
+      check('Enable merge trains.')
+
+      expect(find('#project_merge_trains_enabled')).to be_checked
+      expect(find('#project_merge_pipelines_enabled')).not_to be_disabled
+      expect(find('#project_merge_pipelines_enabled')).to be_checked
+    end
+
+    it 'unchecking merge pipelines checkbox disables merge trains checkbox' do
+      uncheck('Enable merged results pipelines.')
+
+      expect(find('#project_merge_pipelines_enabled')).not_to be_checked
+      expect(find('#project_merge_trains_enabled')).to be_disabled
+    end
+
+    it 'unchecking merge pipelines checkbox unchecks merge trains checkbox if it was previously checked' do
+      check('Enable merge trains.')
+      uncheck('Enable merged results pipelines.')
+
+      expect(find('#project_merge_pipelines_enabled')).not_to be_checked
+      expect(find('#project_merge_trains_enabled')).to be_disabled
+      expect(find('#project_merge_trains_enabled')).not_to be_checked
+    end
+  end
+
+  context 'when merge pipelines is disabled' do
+    before do
+      project.update!(merge_pipelines_enabled: false)
+      visit edit_project_path(project)
+      wait_for_requests
+    end
+
+    include_examples 'loads correct checkbox state'
+
+    it 'checking merge pipelines checkbox enables merge trains checkbox' do
+      check('Enable merged results pipelines.')
+
+      expect(find('#project_merge_pipelines_enabled')).to be_checked
+      expect(find('#project_merge_trains_enabled')).not_to be_disabled
+    end
+
+    it 'checking merge pipelines checkbox should leave merge trains checkbox unchecked' do
+      check('Enable merged results pipelines.')
+
+      expect(find('#project_merge_pipelines_enabled')).to be_checked
+      expect(find('#project_merge_trains_enabled')).not_to be_checked
+    end
+  end
+
+  context 'when both merge pipelines and merge trains are enabled' do
+    before do
+      project.update!(merge_pipelines_enabled: true, merge_trains_enabled: true)
+      visit edit_project_path(project)
+      wait_for_requests
+    end
+
+    include_examples 'loads correct checkbox state'
+
+    it 'unchecking merge pipelines checkbox disables and unchecks merge trains checkbox' do
+      uncheck('Enable merged results pipelines.')
+
+      expect(find('#project_merge_pipelines_enabled')).not_to be_checked
+      expect(find('#project_merge_trains_enabled')).to be_disabled
+      expect(find('#project_merge_trains_enabled')).not_to be_checked
+    end
+
+    it "unchecking merge trains checkbox doesn't affect merge pipelines checkbox" do
+      uncheck('Enable merge trains.')
+
+      expect(find('#project_merge_trains_enabled')).not_to be_checked
+      expect(find('#project_merge_pipelines_enabled')).not_to be_disabled
+      expect(find('#project_merge_pipelines_enabled')).to be_checked
+    end
+  end
+end

ee/spec/features/projects/settings/user_manages_merge_trains_spec.rb

@@ -9,12 +9,14 @@ RSpec.describe 'User manages merge trains option', :js do
   before do
     stub_licensed_features(merge_pipelines: true, merge_trains: true)
 
+    project.update!(merge_pipelines_enabled: true)
     project.add_maintainer(user)
     sign_in(user)
   end
 
   it 'sees unchecked merge trains checkbox' do
     visit edit_project_path(project)
+    wait_for_requests
 
     expect(page.find('#project_merge_trains_enabled')).not_to be_checked
   end
@@ -22,6 +24,7 @@ RSpec.describe 'User manages merge trains option', :js do
   context 'when user enabled the checkbox' do
     before do
       visit edit_project_path(project)
+      wait_for_requests
 
       check('Enable merge trains.')
     end