Add PrependTrueOptionOnBeforeDestroy cop

Prevents unexpected deletion of associated records

changelog/new_add_prepend_true_option_on_before_destroy_cop.md

@@ -0,0 +1 @@
+* [#922](https://github.com/rubocop/rubocop-rails/pull/922): Add PrependTrueOptionOnBeforeDestroy cop. ([@SHinGo-Koba][])

config/default.yml

@@ -736,6 +736,11 @@ Rails/PluralizationGrammar:
   Enabled: true
   VersionAdded: '0.35'
 
+Rails/PrependTrueOptionOnBeforeDestroy:
+  Description: 'Add `prepend: true` option on `before_destroy`.'
+  Enabled: pending
+  VersionAdded: '<<next>>'
+
 Rails/Presence:
   Description: 'Checks code that can be written more easily using `Object#presence` defined by Active Support.'
   Enabled: true

lib/rubocop/cop/rails/prepend_true_option_on_before_destroy.rb

@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # Looks for `before_destroy` without `prepend: true` option.
+      #
+      # `dependent: :destroy` might cause unexpected deletion of associated records
+      # unless `before_destroy` callback had `prepend: true` option.
+      #
+      # This could cause because `dependent: :destroy` is one of callbacks as well as `before_destroy`,
+      # therefore they are run in the order they are defined.
+      #
+      # Adding `prepend: true` option on `before_destroy` is one of solutions of this happening.
+      #
+      # @example
+      #   # bad
+      #   class User < ActiveRecord::Base
+      #     has_many :comments, dependent: :destroy
+      #
+      #     before_destroy :prevent_deletion_if_comments_exists
+      #   end
+      #
+      #   # good
+      #   class User < ActiveRecord::Base
+      #     has_many :comments, dependent: :destroy
+      #
+      #     before_destroy :prevent_deletion_if_comments_exists, prepend: true
+      #   end
+      class PrependTrueOptionOnBeforeDestroy < Base
+        extend AutoCorrector
+
+        RESTRICT_ON_SEND = %i[before_destroy].freeze
+
+        def_node_matcher :match_before_destroy_with_prepend_true_option, <<~PATTERN
+          (send _ :before_destroy ...
+            (hash <(pair (sym :prepend) true) ...>)
+          )
+        PATTERN
+
+        MSG = 'Add `prepend: true` option on `before_destroy` to prevent unexpected deletion of associated records.'
+
+        def on_send(node)
+          return if node.receiver
+
+          matched_node = match_before_destroy_with_prepend_true_option(node)
+
+          return if matched_node
+
+          add_offense(node) do |corrector|
+            corrector.insert_after(node, ', prepend: true')
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -83,6 +83,7 @@
 require_relative 'rails/pluck_id'
 require_relative 'rails/pluck_in_where'
 require_relative 'rails/pluralization_grammar'
+require_relative 'rails/prepend_true_option_on_before_destroy'
 require_relative 'rails/presence'
 require_relative 'rails/present'
 require_relative 'rails/rake_environment'

spec/rubocop/cop/rails/prepend_true_option_on_before_destroy_spec.rb

@@ -0,0 +1,32 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::PrependTrueOptionOnBeforeDestroy, :config do
+  it 'registers an offense when `before_destroy` without `prepend: true` option.' do
+    expect_offense(<<~RUBY)
+      class User < ActiveRecord::Base
+        has_many :comments, dependent: :destroy
+
+        before_destroy :prevent_deletion_if_comments_exists
+        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Add `prepend: true` option on `before_destroy` to prevent unexpected deletion of associated records.
+      end
+    RUBY
+
+    expect_correction(<<~RUBY)
+      class User < ActiveRecord::Base
+        has_many :comments, dependent: :destroy
+      
+        before_destroy :prevent_deletion_if_comments_exists, prepend: true
+      end
+    RUBY
+  end
+
+  it 'registers no offense when `before_destroy` with `prepend: true` option' do
+    expect_no_offenses(<<~RUBY)
+      class User < ActiveRecord::Base
+        has_many :comments, dependent: :destroy
+
+        before_destroy :prevent_deletion_if_comments_exists, prepend: true
+      end
+    RUBY
+  end
+end