[Fix rubocop#419] Fix an error for Rails/UniqueValidationWithoutIndex

Fixes rubocop#419.

This PR fixes an error for `Rails/UniqueValidationWithoutIndex` when using
a unique index and `check_constraint` that has `nil` first argument.

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Bug fixes
+
+* [#419](https://github.com/rubocop/rubocop-rails/issues/419): Fix an error for `Rails/UniqueValidationWithoutIndex` when using a unique index and `check_constraint` that has `nil` first argument. ([@koic][])
+
 ## 2.10.1 (2021-05-06)
 
 ### Bug fixes

lib/rubocop/rails/schema_loader/schema.rb

@@ -114,7 +114,7 @@ class Column
         attr_reader :name, :type, :not_null
 
         def initialize(node)
-          @name = node.first_argument.value
+          @name = node.first_argument.str_content
           @type = node.method_name
           @not_null = nil
 

spec/rubocop/cop/rails/unique_validation_without_index_spec.rb

@@ -89,6 +89,26 @@ class User
       end
     end
 
+    context 'with a unique index and `check_constraint` that has `nil` first argument' do
+      let(:schema) { <<~RUBY }
+        ActiveRecord::Schema.define(version: 2020_02_02_075409) do
+          create_table "users", force: :cascade do |t|
+            t.string "account", null: false
+            t.index ["account"], name: "index_users_on_account", unique: true
+            t.check_constraint nil, 'expression', name: "constraint_name"
+          end
+        end
+      RUBY
+
+      it 'does not register an offense' do
+        expect_no_offenses(<<~RUBY)
+          class User
+            validates :account, uniqueness: true
+          end
+        RUBY
+      end
+    end
+
     context 'when the validation is for two columns' do
       context 'without proper index' do
         let(:schema) { <<~RUBY }