[Fix rubocop#1145] Fix a false positive for Rails/DuplicateAssociation

Fixes rubocop#1145.

This PR fixes a false positive for `Rails/DuplicateAssociation`
when using duplicate `belongs_to` associations of same class without other arguments.

changelog/fix_a_false_positive_for_rails_duplicate_association.md

@@ -0,0 +1 @@
+* [#1145](https://github.com/rubocop/rubocop-rails/issues/1145): Fix a false positive for `Rails/DuplicateAssociation` when using duplicate `belongs_to` associations of same class without other arguments. ([@koic][])

lib/rubocop/cop/rails/duplicate_association.rb

@@ -87,7 +87,8 @@ def duplicated_association_name_nodes(association_nodes)
         end
 
         def duplicated_class_name_nodes(association_nodes)
-          grouped_associations = association_nodes.group_by do |node|
+          filtered_nodes = association_nodes.reject { |node| node.method?(:belongs_to) }
+          grouped_associations = filtered_nodes.group_by do |node|
             arguments = association(node).last
             next unless arguments.count == 1
 

spec/rubocop/cop/rails/duplicate_association_spec.rb

@@ -210,7 +210,7 @@ class Post < ApplicationRecord
       RUBY
     end
 
-    it 'registers offenses when using duplicate associations of same class without other arguments' do
+    it 'registers offenses when using duplicate `has_*` associations of same class without other arguments' do
       expect_offense(<<~RUBY)
         class Post < ApplicationRecord
           has_many :foos, class_name: 'Foo'
@@ -234,6 +234,15 @@ class Post < ApplicationRecord
       RUBY
     end
 
+    it 'does not register an offenses when using duplicate `belongs_to` assocs of same class without other args' do
+      expect_no_offenses(<<~RUBY)
+        class Post < ApplicationRecord
+          belongs_to :foos, class_name: 'Foo'
+          belongs_to :bars, class_name: 'Foo'
+        end
+      RUBY
+    end
+
     it 'does not register an offense when using duplicate associations of same class with other arguments' do
       expect_no_offenses(<<~RUBY)
         class Post < ApplicationRecord