Change ElasticDeleteProjectWorker to delete by project_id

ee/app/workers/elastic_delete_project_worker.rb

@@ -25,16 +25,18 @@ class ElasticDeleteProjectWorker
           bool: {
             should: [
               {
-                has_parent: {
-                  parent_type: 'project',
-                  query: {
-                    term: { id: project_id }
-                  }
+                term: {
+                  _id: es_id
                 }
               },
               {
                 term: {
-                  _id: es_id
+                  project_id: project_id
+                }
+              },
+              {
+                term: {
+                  target_project_id: project_id # handle merge_request which aliases project_id to target_project_id
                 }
               }
             ]

ee/spec/workers/elastic_delete_project_worker_spec.rb

@@ -14,7 +14,7 @@ RSpec.describe ElasticDeleteProjectWorker, :elastic do
     stub_ee_application_setting(elasticsearch_indexing: true)
   end
 
-  it 'deletes a project with all nested objects' do
+  it 'deletes a project with all nested objects', :aggregate_failures do
     project = create :project, :repository
     issue = create :issue, project: project
     milestone = create :milestone, project: project
@@ -38,5 +38,12 @@ RSpec.describe ElasticDeleteProjectWorker, :elastic do
     expect(Milestone.elastic_search('*', **search_options).total_count).to be(0)
     expect(Note.elastic_search('*', **search_options).total_count).to be(0)
     expect(MergeRequest.elastic_search('*', **search_options).total_count).to be(0)
+
+    # verify that entire index is empty
+    # searches use joins on the parent record (project)
+    # and the previous queries will not find data left in the index
+    helper = Gitlab::Elastic::Helper.default
+
+    expect(helper.documents_count).to be(0)
   end
 end