Added Cop to blacklist the use of `dependent:`

This is allowed for existing instances so we don't end up 76 offenses
right away, but for new code one should _only_ use this if they _have_
to remove non database data. Even then it's usually better to do this in
a service class as this gives you more control over how to remove the
data (e.g. in bulk).

2 files changed, 27 insertions, 0 deletions

diff --git a/rubocop/cop/active_record_dependent.rb b/rubocop/cop/active_record_dependent.rb
new file mode 100644
index 00000000000..8d15f150885
--- /dev/null
+++ b/rubocop/cop/active_record_dependent.rb
@@ -0,0 +1,26 @@
+require_relative '../model_helpers'
+
+module RuboCop
+  module Cop
+    # Cop that prevents the use of `dependent: ...` in ActiveRecord models.
+    class ActiveRecordDependent < RuboCop::Cop::Cop
+      include ModelHelpers
+
+      MSG = 'Do not use `dependent: to remove associated data, ' \
+        'use foreign keys with cascading deletes instead'.freeze
+
+      METHOD_NAMES = [:has_many, :has_one, :belongs_to].freeze
+
+      def on_send(node)
+        return unless in_model?(node)
+        return unless METHOD_NAMES.include?(node.children[1])
+
+        node.children.last.each_node(:pair) do |pair|
+          key_name = pair.children[0].children[0]
+
+          add_offense(pair, :expression) if key_name == :dependent
+        end
+      end
+    end
+  end
+end
diff --git a/rubocop/rubocop.rb b/rubocop/rubocop.rb
index 69b4b29507c..1a9bdd9325e 100644
--- a/rubocop/rubocop.rb
+++ b/rubocop/rubocop.rb
@@ -4,6 +4,7 @@ require_relative 'cop/activerecord_serialize'
 require_relative 'cop/redirect_with_status'
 require_relative 'cop/polymorphic_associations'
 require_relative 'cop/project_path_helper'
+require_relative 'cop/active_record_dependent'
 require_relative 'cop/migration/add_column'
 require_relative 'cop/migration/add_column_with_default_to_large_table'
 require_relative 'cop/migration/add_concurrent_foreign_key'