Reference pages_deployments in project_pages_deployments

It will be used to access pages_deployment in the API and
updated when new pages deployment is created.

Old pages deployment will removed ~15 minutes after to
allow pages daemon to serve files from old deployment
until it refreshes the cache.

changelogs/unreleased/245304-reference-pages_deployments-in-pages_metadata.yml

+---
+title: Reference pages_deployments in pages_metadata
+merge_request: 42834
+author:
+type: added

db/migrate/20200921130028_add_pages_deployment_id_to_pages_metadata.rb

+# frozen_string_literal: true
+
+class AddPagesDeploymentIdToPagesMetadata < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+
+  def change
+    add_column :project_pages_metadata, :pages_deployment_id, :bigint
+  end
+end

db/migrate/20200921131313_add_foreign_key_to_pages_deployment_id_in_project_pages_metadata.rb

+# frozen_string_literal: true
+
+class AddForeignKeyToPagesDeploymentIdInProjectPagesMetadata < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_project_pages_metadata_on_pages_deployment_id'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index(:project_pages_metadata, :pages_deployment_id, name: INDEX_NAME)
+    add_concurrent_foreign_key :project_pages_metadata, :pages_deployments, column: :pages_deployment_id, on_delete: :nullify
+  end
+
+  def down
+    remove_foreign_key_if_exists :project_pages_metadata, column: :pages_deployment_id
+    remove_concurrent_index_by_name(:project_pages_metadata, INDEX_NAME)
+  end
+end

db/schema_migrations/20200921130028

+60835078e0a0bd191e9b1f0316f894c5223d6849277992b5034ed4ff9a798fe4
\ No newline at end of file

db/schema_migrations/20200921131313

+e4f9e918c86705409555cde065f30ba0c0c405dfd1918f47a169a5dc5c244a8d
\ No newline at end of file

db/structure.sql

@@ -14712,7 +14712,8 @@ ALTER SEQUENCE project_mirror_data_id_seq OWNED BY project_mirror_data.id;
 CREATE TABLE project_pages_metadata (
     project_id bigint NOT NULL,
     deployed boolean DEFAULT false NOT NULL,
-    artifacts_archive_id bigint
+    artifacts_archive_id bigint,
+    pages_deployment_id bigint
 );
 
 CREATE TABLE project_repositories (
@@ -20819,6 +20820,8 @@ CREATE INDEX index_project_mirror_data_on_status ON project_mirror_data USING bt
 
 CREATE INDEX index_project_pages_metadata_on_artifacts_archive_id ON project_pages_metadata USING btree (artifacts_archive_id);
 
+CREATE INDEX index_project_pages_metadata_on_pages_deployment_id ON project_pages_metadata USING btree (pages_deployment_id);
+
 CREATE UNIQUE INDEX index_project_pages_metadata_on_project_id ON project_pages_metadata USING btree (project_id);
 
 CREATE INDEX index_project_pages_metadata_on_project_id_and_deployed_is_true ON project_pages_metadata USING btree (project_id) WHERE (deployed = true);
@@ -21876,6 +21879,9 @@ ALTER TABLE ONLY notification_settings
 ALTER TABLE ONLY lists
     ADD CONSTRAINT fk_0d3f677137 FOREIGN KEY (board_id) REFERENCES boards(id) ON DELETE CASCADE;
 
+ALTER TABLE ONLY project_pages_metadata
+    ADD CONSTRAINT fk_0fd5b22688 FOREIGN KEY (pages_deployment_id) REFERENCES pages_deployments(id) ON DELETE SET NULL;
+
 ALTER TABLE ONLY group_deletion_schedules
     ADD CONSTRAINT fk_11e3ebfcdd FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE;