Add new Rails/DuplicateIgnoredColumns cop

changelog/new_add_new_rails_avoid_duplicate_ignored_colums_cop.md

@@ -0,0 +1 @@
+* [#761](https://github.com/rubocop/rubocop-rails/pull/761): Add new `Rails/DuplicateIgnoredColumns` cop. ([@sikachu][])

config/default.yml

@@ -318,6 +318,11 @@ Rails/DuplicateAssociation:
   Enabled: pending
   VersionAdded: '2.14'
 
+Rails/DuplicateIgnoredColumns:
+  Description: 'Checks for duplicate calls to `ActiveRecord::Base.ignored_columns=` in a single file.'
+  Enabled: 'pending'
+  VersionAdded: '<<next>>'
+
 Rails/DuplicateScope:
   Description: 'Multiple scopes share this same where clause.'
   Enabled: pending

docs/modules/ROOT/pages/cops_rails.adoc

@@ -1420,6 +1420,40 @@ belongs_to :bar
 has_one :foo
 ----
 
+== Rails/DuplicateIgnoredColumns
+
+|===
+| Enabled by default | Safe | Supports autocorrection | Version Added | Version Changed
+
+| Pending
+| Yes
+| No
+| -
+| -
+|===
+
+Checks for duplicate calls to `ActiveRecord::Base.ignored_columns=` as the
+subsequent calls will override the first call.
+
+=== Examples
+
+[source,ruby]
+----
+# bad
+class User < ApplicationRecord
+  self.ignored_columns = %w(profile)
+
+  # ...
+
+  self.ignored_columns = %w(birthday)
+end
+
+# good
+class User < ApplicationRecord
+  self.ignored_columns = %w(profile birthday)
+end
+----
+
 == Rails/DuplicateScope
 
 |===

lib/rubocop/cop/rails/duplicate_ignored_columns.rb

@@ -0,0 +1,62 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # Checks for duplicate calls to `ActiveRecord::Base.ignored_columns=` as the
+      # subsequent calls will override the first call.
+      #
+      # @example
+      #   # bad
+      #   class User < ApplicationRecord
+      #     self.ignored_columns = %w(profile)
+      #     self.ignored_columns = %w(birthday)
+      #   end
+      #
+      #   # good
+      #   class User < ApplicationRecord
+      #     self.ignored_columns = %w(profile birthday)
+      #   end
+      #
+      class DuplicateIgnoredColumns < Base
+        include ActiveRecordHelper
+
+        MESSAGE = '`self.ignored_columns=` has already been called on line %<other_line_number>s.'
+        RESTRICT_ON_SEND = %i[ignored_columns=].freeze
+
+        def initialize(config = nil, options = nil)
+          super
+
+          @found_nodes = {}
+          @reported_lines = []
+        end
+
+        def on_send(node)
+          return unless node.self_receiver?
+          return unless inherit_active_record_base?(node)
+
+          @found_nodes[node.first_line] = node
+          return if @found_nodes.size == 1
+
+          add_offenses
+        end
+
+        private
+
+        def add_offenses
+          # Remove the earliest found node as we don't want to report offense on it.
+          first_occurrence_line_number = @found_nodes.keys.min
+          @found_nodes.delete(first_occurrence_line_number)
+
+          @found_nodes.each do |line_number, node|
+            next if @reported_lines.include?(line_number)
+
+            add_offense(node, message: format(MESSAGE, other_line_number: first_occurrence_line_number))
+
+            @reported_lines << line_number
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -37,6 +37,7 @@
 require_relative 'rails/deprecated_active_model_errors_methods'
 require_relative 'rails/dot_separated_keys'
 require_relative 'rails/duplicate_association'
+require_relative 'rails/duplicate_ignored_columns'
 require_relative 'rails/duplicate_scope'
 require_relative 'rails/duration_arithmetic'
 require_relative 'rails/dynamic_find_by'

spec/rubocop/cop/rails/duplicate_ignored_columns_spec.rb

@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::DuplicateIgnoredColumns, :config do
+  context 'with no duplicate `self.ignored_columns=` call' do
+    it 'does not register an offense' do
+      expect_no_offenses(<<~RUBY)
+        class User < ApplicationRecord
+          self.ignored_columns = %w(profile birthday)
+        end
+      RUBY
+    end
+  end
+
+  context 'with duplicate `self.ignored_columns=` call' do
+    it 'registers an offense' do
+      expect_offense(<<~RUBY)
+        class User < ApplicationRecord
+          self.ignored_columns = %w(profile)
+          self.ignored_columns = %w(birthday)
+          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `self.ignored_columns=` has already been called on line 2.
+        end
+      RUBY
+    end
+  end
+end