Add unique constraint design_registry.project_id

The index on design_registry.project_id should be unique. There can
only be one row for each project_id.

Closes https://gitlab.com/gitlab-org/gitlab/issues/33545

ee/db/geo/migrate/20191007122326_add_unique_constraint_on_design_registry_project_id.rb

+# frozen_string_literal: true
+
+class AddUniqueConstraintOnDesignRegistryProjectId < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_design_registry_on_project_id'
+
+  def change
+    # `design_registry` is not in use yet, so no need to create index concurrently
+    remove_index :design_registry, column: :project_id, name: INDEX_NAME # rubocop: disable Migration/RemoveIndex
+    add_index    :design_registry,         :project_id, name: INDEX_NAME, unique: true # rubocop: disable Migration/AddIndex
+  end
+end

ee/db/geo/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_09_23_111102) do
+ActiveRecord::Schema.define(version: 2019_10_07_122326) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -38,7 +38,7 @@ ActiveRecord::Schema.define(version: 2019_09_23_111102) do
     t.datetime "retry_at"
     t.datetime "last_synced_at"
     t.datetime "created_at", null: false
-    t.index ["project_id"], name: "index_design_registry_on_project_id"
+    t.index ["project_id"], name: "index_design_registry_on_project_id", unique: true
     t.index ["retry_at"], name: "index_design_registry_on_retry_at"
     t.index ["state"], name: "index_design_registry_on_state"
   end