[Fix rubocop#401] Fix an error for Rails/WhereEquals

Fixes rubocop#401.

This PR fixes an error for `Rails/WhereEquals` when
using only named placeholder template without replacement argument.

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### Bug fixes
+
+* [#401](https://github.com/rubocop-hq/rubocop-rails/issues/401): Fix an error for `Rails/WhereEquals` using only named placeholder template without replacement argument. ([@koic][])
+
 ## 2.9.0 (2020-12-09)
 
 ### New features

spec/rubocop/cop/rails/where_equals_spec.rb

@@ -160,4 +160,12 @@
  User.where('name = ? AND age = ?', 'john', 19)
  RUBY
  end
+
+ it 'does not register an offense when using only named placeholder template without replacement argument' do
+ expect_no_offenses(<<~'RUBY')
+ sql = User.where('name = :name').select(:id).to_sql
+
+ User.where("id IN (#{sql})", name: 'Lastname').first
+ RUBY
+ end
 end