Add new Rails/RedundantForeignKey cop

The foreign key for an association can be overridden with the
`:foreign_key` option. This cop detects when the specified value is the
same as the default.

Aside from being unnecessary, setting the foreign key prevents the
inverse association from being inferred automatically, which can lead to
unnecessary queries.

CHANGELOG.md

@@ -8,6 +8,7 @@
 * [#211](https://github.com/rubocop-hq/rubocop-rails/issues/211): Add autocorrect to `Rails/RakeEnvironment` cop. ([@tejasbubane][])
 * [#242](https://github.com/rubocop-hq/rubocop-rails/pull/242): Add `Rails/ContentTag` cop. ([@tabuchi0919][])
 * [#249](https://github.com/rubocop-hq/rubocop-rails/pull/249): Add new `Rails/Pick` cop. ([@eugeneius][])
+* [#257](https://github.com/rubocop-hq/rubocop-rails/pull/257): Add new `Rails/RedundantForeignKey` cop. ([@eugeneius][])
 
 ### Bug fixes
 

config/default.yml

@@ -408,6 +408,11 @@ Rails/RedundantAllowNil:
   Include:
     - app/models/**/*.rb
 
+Rails/RedundantForeignKey:
+  Description: 'Checks for associations where the `:foreign_key` option is redundant.'
+  Enabled: true
+  VersionAdded: '2.6'
+
 Rails/RedundantReceiverInWithOptions:
   Description: 'Checks for redundant receiver in `with_options`.'
   Enabled: true

docs/modules/ROOT/pages/cops.adoc

@@ -48,6 +48,7 @@
 * xref:cops_rails.adoc#railsrakeenvironment[Rails/RakeEnvironment]
 * xref:cops_rails.adoc#railsreadwriteattribute[Rails/ReadWriteAttribute]
 * xref:cops_rails.adoc#railsredundantallownil[Rails/RedundantAllowNil]
+* xref:cops_rails.adoc#railsredundantforeignkey[Rails/RedundantForeignKey]
 * xref:cops_rails.adoc#railsredundantreceiverinwithoptions[Rails/RedundantReceiverInWithOptions]
 * xref:cops_rails.adoc#railsreflectionclassname[Rails/ReflectionClassName]
 * xref:cops_rails.adoc#railsrefutemethods[Rails/RefuteMethods]

docs/modules/ROOT/pages/cops_rails.adoc

@@ -2391,6 +2391,44 @@ validates :x, length: { is: 5 }, allow_nil: true, allow_blank: false
 | Array
 |===
 
+== Rails/RedundantForeignKey
+
+|===
+| Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
+
+| Enabled
+| Yes
+| Yes
+| 2.6
+| -
+|===
+
+This cop detects cases where the `:foreign_key` option on associations
+is redundant.
+
+=== Examples
+
+[source,ruby]
+----
+# bad
+class Post
+  has_many :comments, foreign_key: 'post_id'
+end
+
+class Comment
+  belongs_to :post, foreign_key: 'post_id'
+end
+
+# good
+class Post
+  has_many :comments
+end
+
+class Comment
+  belongs_to :author, foreign_key: 'user_id'
+end
+----
+
 == Rails/RedundantReceiverInWithOptions
 
 |===

lib/rubocop/cop/rails/redundant_foreign_key.rb

@@ -0,0 +1,80 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # This cop detects cases where the `:foreign_key` option on associations
+      # is redundant.
+      #
+      # @example
+      #   # bad
+      #   class Post
+      #     has_many :comments, foreign_key: 'post_id'
+      #   end
+      #
+      #   class Comment
+      #     belongs_to :post, foreign_key: 'post_id'
+      #   end
+      #
+      #   # good
+      #   class Post
+      #     has_many :comments
+      #   end
+      #
+      #   class Comment
+      #     belongs_to :author, foreign_key: 'user_id'
+      #   end
+      class RedundantForeignKey < Cop
+        include RangeHelp
+
+        MSG = 'Specifying the default value for `foreign_key` is redundant.'
+
+        def_node_matcher :association_with_foreign_key, <<~PATTERN
+          (send nil? ${:belongs_to :has_one :has_many :has_and_belongs_to_many} ({sym str} $_)
+            $(hash <$(pair (sym :foreign_key) ({sym str} $_)) ...>)
+          )
+        PATTERN
+
+        def on_send(node)
+          association_with_foreign_key(node) do |type, name, options, foreign_key_pair, foreign_key|
+            if redundant?(node, type, name, options, foreign_key)
+              add_offense(node, location: foreign_key_pair.loc.expression)
+            end
+          end
+        end
+
+        def autocorrect(node)
+          _type, _name, _options, foreign_key_pair, _foreign_key = association_with_foreign_key(node)
+          range = range_with_surrounding_space(range: foreign_key_pair.source_range, side: :left)
+          range = range_with_surrounding_comma(range, :left)
+
+          lambda do |corrector|
+            corrector.remove(range)
+          end
+        end
+
+        private
+
+        def redundant?(node, association_type, association_name, options, foreign_key)
+          foreign_key.to_s == default_foreign_key(node, association_type, association_name, options)
+        end
+
+        def default_foreign_key(node, association_type, association_name, options)
+          if association_type == :belongs_to
+            "#{association_name}_id"
+          elsif (as = find_as_option(options))
+            "#{as}_id"
+          else
+            node.parent_module_name&.foreign_key
+          end
+        end
+
+        def find_as_option(options)
+          options.pairs.find do |pair|
+            pair.key.sym_type? && pair.key.value == :as
+          end&.value&.value
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -50,6 +50,7 @@
 require_relative 'rails/rake_environment'
 require_relative 'rails/read_write_attribute'
 require_relative 'rails/redundant_allow_nil'
+require_relative 'rails/redundant_foreign_key'
 require_relative 'rails/redundant_receiver_in_with_options'
 require_relative 'rails/reflection_class_name'
 require_relative 'rails/refute_methods'

spec/rubocop/cop/rails/redundant_foreign_key_spec.rb

@@ -0,0 +1,174 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::RedundantForeignKey do
+  subject(:cop) { described_class.new }
+
+  context '`belongs_to` associations' do
+    it 'registers an offense when the `:foreign_key` option is redundant' do
+      expect_offense(<<~RUBY)
+        class Comment
+          belongs_to :post, foreign_key: 'post_id'
+                            ^^^^^^^^^^^^^^^^^^^^^^ Specifying the default value for `foreign_key` is redundant.
+        end
+      RUBY
+
+      expect_correction(<<~RUBY)
+        class Comment
+          belongs_to :post
+        end
+      RUBY
+    end
+
+    it 'does not register an offense when the `:foreign_key` option is not redundant' do
+      expect_no_offenses(<<~RUBY)
+        class Comment
+          belongs_to :author, foreign_key: 'user_id'
+        end
+      RUBY
+    end
+
+    it 'does not register an offense when the `:foreign_key` option is absent' do
+      expect_no_offenses(<<~RUBY)
+        class Comment
+          belongs_to :author
+        end
+      RUBY
+    end
+
+    it 'registers an offense even when other options are used' do
+      expect_offense(<<~RUBY)
+        class Comment
+          belongs_to :post, class_name: 'SpecialPost', foreign_key: 'post_id', dependent: :destroy
+                                                       ^^^^^^^^^^^^^^^^^^^^^^ Specifying the default value for `foreign_key` is redundant.
+        end
+      RUBY
+
+      expect_correction(<<~RUBY)
+        class Comment
+          belongs_to :post, class_name: 'SpecialPost', dependent: :destroy
+        end
+      RUBY
+    end
+
+    it 'registers an offense even when defined in a block' do
+      expect_offense(<<~RUBY)
+        class_methods do
+          belongs_to :post, foreign_key: 'post_id'
+                            ^^^^^^^^^^^^^^^^^^^^^^ Specifying the default value for `foreign_key` is redundant.
+        end
+      RUBY
+
+      expect_correction(<<~RUBY)
+        class_methods do
+          belongs_to :post
+        end
+      RUBY
+    end
+  end
+
+  %w[has_one has_many has_and_belongs_to_many].each do |association_type|
+    context "`#{association_type}` associations" do
+      it 'registers an offense when the `:foreign_key` option is redundant' do
+        expect_offense(<<~RUBY, association_type: association_type)
+          class Book
+            %{association_type} :chapter, foreign_key: 'book_id'
+            _{association_type}           ^^^^^^^^^^^^^^^^^^^^^^ Specifying the default value for `foreign_key` is redundant.
+          end
+        RUBY
+
+        expect_correction(<<~RUBY)
+          class Book
+            #{association_type} :chapter
+          end
+        RUBY
+      end
+
+      it 'does not register an offense when the `:foreign_key` option is not redundant' do
+        expect_no_offenses(<<~RUBY)
+          class Book
+            #{association_type} :chapter, foreign_key: 'publication_id'
+          end
+        RUBY
+      end
+
+      it 'does not register an offense when the `:foreign_key` option is absent' do
+        expect_no_offenses(<<~RUBY)
+          class Book
+            #{association_type} :chapter
+          end
+        RUBY
+      end
+
+      it 'registers an offense even when other options are used' do
+        expect_offense(<<~RUBY, association_type: association_type)
+          class Book
+            %{association_type} :chapter, class_name: 'SpecialChapter', foreign_key: 'book_id', dependent: :destroy
+            _{association_type}                                         ^^^^^^^^^^^^^^^^^^^^^^ Specifying the default value for `foreign_key` is redundant.
+          end
+        RUBY
+
+        expect_correction(<<~RUBY)
+          class Book
+            #{association_type} :chapter, class_name: 'SpecialChapter', dependent: :destroy
+          end
+        RUBY
+      end
+
+      it 'registers an offense even when multiple associations are defined' do
+        expect_offense(<<~RUBY, association_type: association_type)
+          class Book
+            belongs_to :series
+
+            %{association_type} :chapter, foreign_key: 'book_id'
+            _{association_type}           ^^^^^^^^^^^^^^^^^^^^^^ Specifying the default value for `foreign_key` is redundant.
+          end
+        RUBY
+
+        expect_correction(<<~RUBY)
+          class Book
+            belongs_to :series
+
+            #{association_type} :chapter
+          end
+        RUBY
+      end
+
+      it 'does not register an offense when defined in a block' do
+        expect_no_offenses(<<~RUBY)
+          class_methods do
+            #{association_type} :chapter, foreign_key: 'book_id'
+          end
+        RUBY
+      end
+
+      it 'registers an offense when the `:foreign_key` options is redundant with the `:as` option' do
+        expect_offense(<<~RUBY, association_type: association_type)
+          class Book
+            %{association_type} :chapter, as: :publishable, foreign_key: 'publishable_id'
+            _{association_type}                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Specifying the default value for `foreign_key` is redundant.
+          end
+        RUBY
+
+        expect_correction(<<~RUBY)
+          class Book
+            #{association_type} :chapter, as: :publishable
+          end
+        RUBY
+      end
+
+      it 'does not register an offense when the `:foreign_key` option is not redundant with the `:as` option' do
+        expect_no_offenses(<<~RUBY)
+          class Book
+            #{association_type} :chapter, as: :publishable, foreign_key: 'book_id'
+          end
+        RUBY
+      end
+
+      it 'does not register an offense when the class cannot be determined' do
+        expect_no_offenses(<<~RUBY)
+          #{association_type} :chapter, foreign_key: 'book_id'
+        RUBY
+      end
+    end
+  end
+end