Add unique index merge_request_approval_rules.name

This adds a unique index to [merge_request_id, name] for
merge_request_approval_rules. This makes sure that the users cannot
create the the same approval rule based on a codeowner pattern twice.

db/schema.rb

@@ -233,6 +233,7 @@ ActiveRecord::Schema.define(version: 20190131122559) do
     t.integer "approvals_required", limit: 2, default: 0, null: false
     t.boolean "code_owner", default: false, null: false
     t.string "name", null: false
+    t.index ["merge_request_id", "code_owner", "name"], name: "approval_rule_name_index_for_code_owners", unique: true, where: "(code_owner = true)", using: :btree
     t.index ["merge_request_id", "code_owner"], name: "index_approval_merge_request_rules_1", using: :btree
   end
 

ee/db/migrate/20190128172533_add_index_to_name_on_approval_merge_request_rules.rb

+# frozen_string_literal: true
+
+class AddIndexToNameOnApprovalMergeRequestRules < ActiveRecord::Migration[5.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  disable_ddl_transaction!
+
+  INDEX_NAME = 'approval_rule_name_index_for_code_owners'
+
+  def up
+    add_concurrent_index(
+      :approval_merge_request_rules,
+      [:merge_request_id, :code_owner, :name],
+      unique: true,
+      where: "code_owner = 't'",
+      name: INDEX_NAME
+    )
+  end
+
+  def down
+    remove_concurrent_index(
+      :approval_merge_request_rules,
+      [:merge_request_id, :name],
+      name: INDEX_NAME
+    )
+  end
+end