[Fix rubocop#833] Fix a false positive for Rails/Pluck

Fixes rubocop#833.

This commit fixes a false positive for `Rails/Pluck` when receiver is not block argument for `[]`.

Author: koic

changelog/fix_a_false_positive_for_rails_pluck.md

@@ -0,0 +1 @@
+* [#833](https://github.com/rubocop/rubocop-rails/issues/833): Fix a false positive for `Rails/Pluck` when receiver is not block argument for `[]`. ([@koic][])

lib/rubocop/cop/rails/pluck.rb

@@ -55,6 +55,8 @@ def use_one_block_argument?(argument)
         end
 
         def use_block_argument_in_key?(block_argument, key)
+          return false if block_argument == key.source
+
           key.each_descendant(:lvar).none? { |lvar| block_argument == lvar.source }
         end
 

spec/rubocop/cop/rails/pluck_spec.rb

@@ -58,6 +58,15 @@
         end
       end
 
+      context 'when receiver is not block argument for `[]`' do
+        it 'does not register an offense' do
+          expect_no_offenses(<<~RUBY)
+            lvar = do_something
+            x.#{method} { |id| lvar[id] }
+          RUBY
+        end
+      end
+
       context 'when there are multiple block arguments' do
         it 'does not register an offense' do
           expect_no_offenses(<<~RUBY)