[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