[Fix rubocop#551] Fix a false positive for Rails/FindEach

Fixes rubocop#551.

This PR fixes a false positive for `Rails/FindEach`
when using `model.errors.where` in Rails 6.1.

changelog/fix_a_false_positive_for_rails_find_each.md

@@ -0,0 +1 @@
+* [#551](https://github.com/rubocop/rubocop-rails/issues/551): Fix a false positive for `Rails/FindEach` when using `model.errors.where` in Rails 6.1. ([@koic][])

lib/rubocop/cop/rails/find_each.rb

@@ -43,9 +43,20 @@ def on_send(node)
         private
 
         def ignored?(node)
+          return true if active_model_error_where?(node.receiver)
+
           method_chain = node.each_node(:send).map(&:method_name)
+
           (cop_config['IgnoredMethods'].map(&:to_sym) & method_chain).any?
         end
+
+        def active_model_error_where?(node)
+          node.method?(:where) && active_model_error?(node.receiver)
+        end
+
+        def active_model_error?(node)
+          node.send_type? && node.method?(:errors)
+        end
       end
     end
   end

spec/rubocop/cop/rails/find_each_spec.rb

@@ -42,6 +42,13 @@
     expect_no_offenses('User.all.find_each { |u| u.x }')
   end
 
+  # Active Model Errors slice from the new query interface introduced in Rails 6.1.
+  it 'does not register an offense when using `model.errors.where`' do
+    expect_no_offenses(<<~RUBY)
+      model.errors.where(:title).each { |error| do_something(error)  }
+    RUBY
+  end
+
   it 'auto-corrects each to find_each' do
     expect_offense(<<~RUBY)
       User.all.each { |u| u.x }