Add new Rails/ArelStar cop

CHANGELOG.md

@@ -7,6 +7,8 @@
 * [#345](https://github.com/rubocop-hq/rubocop-rails/issues/345): Fix error of `Rails/AfterCommitOverride` on `after_commit` with a lambda. ([@pocke][])
 * [#349](https://github.com/rubocop-hq/rubocop-rails/pull/349): Fix errors of `Rails/UniqueValidationWithoutIndex`. ([@Tietew][])
 
+* [#344](https://github.com/rubocop-hq/rubocop-rails/pull/344): Add new `Rails/ArelStar` cop which checks for quoted literal asterisks in `arel_table` calls. ([@flanger001][])
+
 ## 2.8.0 (2020-09-04)
 
 ### New features
@@ -283,3 +285,4 @@
 [@bubaflub]: https://github.com/bubaflub
 [@dvandersluis]: https://github.com/dvandersluis
 [@Tietew]: https://github.com/Tietew
+[@flanger001]: https://github.com/flanger001

config/default.yml

@@ -98,6 +98,14 @@ Rails/ApplicationRecord:
   VersionAdded: '0.49'
   VersionChanged: '2.5'
 
+Rails/ArelStar:
+  Description: 'Enforces `Arel.star` instead of `"*"` for expanded columns.'
+  Enabled: true
+  Safe: false
+  SafeAutoCorrect: false
+  AutoCorrect: false
+  VersionAdded: '2.9'
+
 Rails/AssertNot:
   Description: 'Use `assert_not` instead of `assert !`.'
   Enabled: true

docs/modules/ROOT/pages/cops.adoc

@@ -26,6 +26,7 @@ based on the https://rails.rubystyle.guide/[Rails Style Guide].
 * xref:cops_rails.adoc#railsapplicationjob[Rails/ApplicationJob]
 * xref:cops_rails.adoc#railsapplicationmailer[Rails/ApplicationMailer]
 * xref:cops_rails.adoc#railsapplicationrecord[Rails/ApplicationRecord]
+* xref:cops_rails.adoc#railsarelstar[Rails/ArelStar]
 * xref:cops_rails.adoc#railsassertnot[Rails/AssertNot]
 * xref:cops_rails.adoc#railsbelongsto[Rails/BelongsTo]
 * xref:cops_rails.adoc#railsblank[Rails/Blank]

docs/modules/ROOT/pages/cops_rails.adoc

@@ -374,6 +374,46 @@ class Rails4Model < ActiveRecord::Base
 end
 ----
 
+== Rails/ArelStar
+
+|===
+| Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
+
+| Enabled
+| No
+| Yes (Unsafe)
+| 2.9
+| -
+|===
+
+This cop prevents usage of `"*"` on an Arel::Table column reference.
+
+Using `arel_table["*"]` causes the outputted string to be a literal
+quoted asterisk (e.g. <tt>`my_model`.`*`</tt>). This causes the
+database to look for a column named <tt>`*`</tt> (or `"*"`) as opposed
+to expanding the column list as one would likely expect.
+
+=== Examples
+
+[source,ruby]
+----
+# bad
+MyTable.arel_table["*"]
+
+# good
+MyTable.arel_table[Arel.star]
+----
+
+=== Configurable attributes
+
+|===
+| Name | Default value | Configurable values
+
+| AutoCorrect
+| `false`
+| Boolean
+|===
+
 == Rails/AssertNot
 
 |===

lib/rubocop/cop/rails/arel_star.rb

@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # This cop prevents usage of `"*"` on an Arel::Table column reference.
+      #
+      # Using `arel_table["*"]` causes the outputted string to be a literal
+      # quoted asterisk (e.g. <tt>`my_model`.`*`</tt>). This causes the
+      # database to look for a column named <tt>`*`</tt> (or `"*"`) as opposed
+      # to expanding the column list as one would likely expect.
+      #
+      # @example
+      #   # bad
+      #   MyTable.arel_table["*"]
+      #
+      #   # good
+      #   MyTable.arel_table[Arel.star]
+      #
+      class ArelStar < Cop
+        MSG = 'Use `Arel.star` instead of `"*"` for expanded column lists.'
+
+        RESTRICT_ON_SEND = %i[[]].freeze
+
+        def_node_matcher :star_bracket?, <<~PATTERN
+          (send {const {_ :arel_table}} :[] $(str "*"))
+        PATTERN
+
+        def on_send(node)
+          return unless (star = star_bracket?(node))
+
+          add_offense(star)
+        end
+
+        def autocorrect(node)
+          ->(corrector) { corrector.replace(node.loc.expression, 'Arel.star') }
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -14,6 +14,7 @@
 require_relative 'rails/application_job'
 require_relative 'rails/application_mailer'
 require_relative 'rails/application_record'
+require_relative 'rails/arel_star'
 require_relative 'rails/assert_not'
 require_relative 'rails/belongs_to'
 require_relative 'rails/blank'

spec/rubocop/cop/rails/arel_star_spec.rb

@@ -0,0 +1,53 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::ArelStar do
+  subject(:cop) { described_class.new }
+
+  it 'registers an offense and corrects when an asterisk is used on an Arel::Table column reference' do
+    expect_offense(<<~RUBY)
+      class MyModel < ApplicationRecord
+        scope :my_scope, -> { select(arel_table["*"]) }
+                                                ^^^ Use `Arel.star` instead of `"*"` for expanded column lists.
+      end
+    RUBY
+
+    expect_correction(<<~RUBY)
+      class MyModel < ApplicationRecord
+        scope :my_scope, -> { select(arel_table[Arel.star]) }
+      end
+    RUBY
+  end
+
+  it 'registers an offense on the `arel_table` object in a void' do
+    expect_offense(<<~RUBY)
+      arel_table["*"]
+                 ^^^ Use `Arel.star` instead of `"*"` for expanded column lists.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      arel_table[Arel.star]
+    RUBY
+  end
+
+  it 'registers an offense on the `arel_table` object on a model' do
+    expect_offense(<<~RUBY)
+      MyModel.arel_table["*"]
+                         ^^^ Use `Arel.star` instead of `"*"` for expanded column lists.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      MyModel.arel_table[Arel.star]
+    RUBY
+  end
+
+  it 'registers an offense for ArelExtensions asterisks' do
+    expect_offense(<<~RUBY)
+      MyModel["*"]
+              ^^^ Use `Arel.star` instead of `"*"` for expanded column lists.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      MyModel[Arel.star]
+    RUBY
+  end
+end