[Fix #137] Make Rails/HasManyOrHasOneDependent aware of readonly?…

… is `true`

Fixes #137.

This PR makes `Rails/HasManyOrHasOneDependent` aware of `readonly?` is `true`.

There may be false positives or false negatives with just `def readonly? = true`,
but it makes it possible to detect perhaps general definition of `readonly?` method.

CHANGELOG.md

@@ -18,6 +18,7 @@
 * [#103](https://github.com/rubocop/rubocop-rails/issues/103): Fix a false positive for `Rails/FindEach` when not inheriting `ActiveRecord::Base` and using `all.each`. ([@koic][])
 * [#466](https://github.com/rubocop/rubocop-rails/pull/466): Fix a false positive for `Rails/DynamicFindBy` when not inheriting `ApplicationRecord` and without no receiver. ([@koic][])
 * [#147](https://github.com/rubocop/rubocop-rails/issues/147): Fix a false positive for `Rails/HasManyOrHasOneDependent` when specifying default `dependent: nil` strategy. ([@koic][])
+* [#137](https://github.com/rubocop/rubocop-rails/issues/137): Make `Rails/HasManyOrHasOneDependent` aware of `readonly?` is `true`. ([@koic][])
 
 ### Changes
 

docs/modules/ROOT/pages/cops_rails.adoc

@@ -1570,8 +1570,9 @@ This cop checks for the use of the has_and_belongs_to_many macro.
 
 This cop looks for `has_many` or `has_one` associations that don't
 specify a `:dependent` option.
+
 It doesn't register an offense if `:through` or `dependent: nil`
-was specified.
+is specified, or if the model is read-only.
 
 === Examples
 
@@ -1590,6 +1591,15 @@ class User < ActiveRecord::Base
   has_many :articles, dependent: nil
   has_many :patients, through: :appointments
 end
+
+class User < ActiveRecord::Base
+  has_many :comments
+  has_one :avatar
+
+  def readonly?
+    true
+  end
+end
 ----
 
 === Configurable attributes

lib/rubocop/cop/rails/has_many_or_has_one_dependent.rb

@@ -5,8 +5,9 @@ module Cop
     module Rails
       # This cop looks for `has_many` or `has_one` associations that don't
       # specify a `:dependent` option.
+      #
       # It doesn't register an offense if `:through` or `dependent: nil`
-      # was specified.
+      # is specified, or if the model is read-only.
       #
       # @example
       #   # bad
@@ -22,6 +23,15 @@ module Rails
       #     has_many :articles, dependent: nil
       #     has_many :patients, through: :appointments
       #   end
+      #
+      #   class User < ActiveRecord::Base
+      #     has_many :comments
+      #     has_one :avatar
+      #
+      #     def readonly?
+      #       true
+      #     end
+      #   end
       class HasManyOrHasOneDependent < Base
         MSG = 'Specify a `:dependent` option.'
         RESTRICT_ON_SEND = %i[has_many has_one].freeze
@@ -59,8 +69,14 @@ class HasManyOrHasOneDependent < Base
             (args) ...)
         PATTERN
 
+        def_node_matcher :readonly?, <<~PATTERN
+          (def :readonly?
+            (args)
+            (true))
+        PATTERN
+
         def on_send(node)
-          return if active_resource?(node.parent)
+          return if active_resource?(node.parent) || readonly_model?(node)
           return if !association_without_options?(node) && valid_options?(association_with_options?(node))
           return if valid_options_in_with_options_block?(node)
 
@@ -69,6 +85,12 @@ def on_send(node)
 
         private
 
+        def readonly_model?(node)
+          return false unless (parent = node.parent)
+
+          parent.each_descendant(:def).any? { |def_node| readonly?(def_node) }
+        end
+
         def valid_options_in_with_options_block?(node)
           return true unless node.parent
 

spec/rubocop/cop/rails/has_many_or_has_one_dependent_spec.rb

@@ -224,6 +224,33 @@ class User < ActiveResource::Base
     end
   end
 
+  context 'when defining `readonly?` method' do
+    it 'does not register an offense for `readonly?` is `true`' do
+      expect_no_offenses(<<~RUBY)
+        class Person < ActiveRecord::Base
+          has_one :foo
+
+          def readonly?
+            true
+          end
+        end
+      RUBY
+    end
+
+    it 'registers an offense for `readonly?` is not `true`' do
+      expect_offense(<<~RUBY)
+        class Person < ActiveRecord::Base
+          has_one :foo
+          ^^^^^^^ Specify a `:dependent` option.
+
+          def readonly?
+            false
+          end
+        end
+      RUBY
+    end
+  end
+
   context 'when an Active Record model does not have any associations' do
     it 'does not register an offense' do
       expect_no_offenses(<<~RUBY)