Restrict DuplicateAssociation cop to ActiveRecord

Previously this cop would run against all classes, which led to false
positives when the class was not descended from `ActiveRecord::Base`
and thus might have acceptable use cases for repeating association
declarations in ways that would not be acceptable in AR classes.

changelog/change_restrict_duplicate_association_cop_to.md

@@ -0,0 +1 @@
+* [#994](https://github.com/rubocop/rubocop-rails/pull/994): Restrict DuplicateAssociation cop to ActiveRecord. ([@mjankowski][])

lib/rubocop/cop/rails/duplicate_association.rb

@@ -24,6 +24,7 @@ class DuplicateAssociation < Base
         include RangeHelp
         extend AutoCorrector
         include ClassSendNodeHelper
+        include ActiveRecordHelper
 
         MSG = "Association `%<name>s` is defined multiple times. Don't repeat associations."
 
@@ -32,6 +33,8 @@ class DuplicateAssociation < Base
         PATTERN
 
         def on_class(class_node)
+          return unless active_record?(class_node.parent_class)
+
           offenses(class_node).each do |name, nodes|
             nodes.each do |node|
               add_offense(node, message: format(MSG, name: name)) do |corrector|

spec/rubocop/cop/rails/duplicate_association_spec.rb

@@ -227,4 +227,15 @@ class Post < ApplicationRecord
       RUBY
     end
   end
+
+  describe 'a class that does not descend from Active Record' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<-RUBY)
+        class Post < ActiveModel::Serializer
+          has_many :comments, key: :remarks, if: :formal_mode?
+          has_many :comments, key: :rejoinders, if: :debate_mode?
+        end
+      RUBY
+    end
+  end
 end