Add new Rails/MailerPreviews cop

.gitignore

@@ -9,6 +9,8 @@ rdoc
 doc
 .yardoc
 
+tmp
+
 # bundler
 .bundle
 Gemfile.lock

changelog/new_add_mailer_previews_cop.md

@@ -0,0 +1 @@
+* [#654](https://github.com/rubocop/rubocop-rails/issues/654): Add new `Rails/MailerPreviews` cop. ([@fatkodima][])

config/default.yml

@@ -663,6 +663,15 @@ Rails/MailerName:
   Include:
     - app/mailers/**/*.rb
 
+Rails/MailerPreviews:
+  Description: 'Checks for existence of mailer previews.'
+  Enabled: pending
+  VersionAdded: '<<next>>'
+  PreviewPaths:
+    - test/mailers/previews
+  Include:
+    - app/mailers/**/*.rb
+
 Rails/MatchRoute:
   Description: >-
                   Don't use `match` to define any routes unless there is a need to map multiple request types

lib/rubocop-rails.rb

@@ -8,6 +8,7 @@
 require_relative 'rubocop/rails'
 require_relative 'rubocop/rails/version'
 require_relative 'rubocop/rails/inject'
+require_relative 'rubocop/cop/mixin/parsing_helper'
 require_relative 'rubocop/rails/schema_loader'
 require_relative 'rubocop/rails/schema_loader/schema'
 

lib/rubocop/cop/mixin/class_send_node_helper.rb → lib/rubocop/cop/mixin/class_elements_helper.rb

@@ -3,7 +3,7 @@
 module RuboCop
   module Cop
     # A mixin to return all of the class send nodes.
-    module ClassSendNodeHelper
+    module ClassElementsHelper
       def class_send_nodes(class_node)
         class_def = class_node.body
 
@@ -15,6 +15,18 @@ def class_send_nodes(class_node)
           class_def.each_child_node(:send)
         end
       end
+
+      def class_def_nodes(class_node)
+        class_def = class_node.body
+
+        return [] unless class_def
+
+        if class_def.def_type?
+          [class_def]
+        else
+          class_def.each_child_node(:def)
+        end
+      end
     end
   end
 end

lib/rubocop/cop/mixin/parent_class_matchers.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    # A mixin with predefined parent classes matchers
+    module ParentClassMatchers
+      extend NodePattern::Macros
+
+      def_node_matcher :mailer_base_class?, <<~PATTERN
+        {
+          (const (const {nil? cbase} :ActionMailer) :Base)
+          (const {nil? cbase} :ApplicationMailer)
+        }
+      PATTERN
+
+      def_node_matcher :mailer_preview_base_class?, <<~PATTERN
+        (const (const nil? :ActionMailer) :Preview)
+      PATTERN
+    end
+  end
+end

lib/rubocop/cop/mixin/parsing_helper.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    # A mixin with helpers related to source code parsing
+    module ParsingHelper
+      def parse(path, target_ruby_version)
+        klass_name = :"Ruby#{target_ruby_version.to_s.sub('.', '')}"
+        klass = ::Parser.const_get(klass_name)
+        parser = klass.new(RuboCop::AST::Builder.new)
+
+        buffer = Parser::Source::Buffer.new(path, 1)
+        buffer.source = path.read
+
+        parser.parse(buffer)
+      end
+    end
+  end
+end

lib/rubocop/cop/rails/after_commit_override.rb

@@ -32,7 +32,7 @@ module Rails
       #   after_update_commit :log_update_action
       #
       class AfterCommitOverride < Base
-        include ClassSendNodeHelper
+        include ClassElementsHelper
 
         MSG = 'There can only be one `after_*_commit :%<name>s` hook defined for a model.'
 

lib/rubocop/cop/rails/duplicate_association.rb

@@ -32,7 +32,7 @@ module Rails
       class DuplicateAssociation < Base
         include RangeHelp
         extend AutoCorrector
-        include ClassSendNodeHelper
+        include ClassElementsHelper
         include ActiveRecordHelper
 
         MSG = "Association `%<name>s` is defined multiple times. Don't repeat associations."

lib/rubocop/cop/rails/duplicate_scope.rb

@@ -17,7 +17,7 @@ module Rails
       #   scope :hidden, -> { where(visible: false) }
       #
       class DuplicateScope < Base
-        include ClassSendNodeHelper
+        include ClassElementsHelper
 
         MSG = 'Multiple scopes share this same where clause.'
 

lib/rubocop/cop/rails/mailer_name.rb

@@ -29,16 +29,10 @@ module Rails
       #
       class MailerName < Base
         extend AutoCorrector
+        include ParentClassMatchers
 
         MSG = 'Mailer should end with `Mailer` suffix.'
 
-        def_node_matcher :mailer_base_class?, <<~PATTERN
-          {
-            (const (const {nil? cbase} :ActionMailer) :Base)
-            (const {nil? cbase} :ApplicationMailer)
-          }
-        PATTERN
-
         def_node_matcher :class_definition?, <<~PATTERN
           (class $(const _ !#mailer_suffix?) #mailer_base_class? ...)
         PATTERN

lib/rubocop/cop/rails/mailer_previews.rb

@@ -0,0 +1,78 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # Enforces the existence of mailer previews.
+      #
+      # @example
+      #   # bad
+      #   # app/mailer/user_mailer.rb
+      #   class UserMailer < ApplicationMailer
+      #     def welcome_email
+      #     end
+      #   end
+      #
+      #   # No file exists in mailer previews directory.
+      #
+      #   # good
+      #   # app/mailer/user_mailer.rb
+      #   class UserMailer < ApplicationMailer
+      #     def welcome_email
+      #     end
+      #   end
+      #
+      #   # test/mailers/previews/user_mailer_preview.rb
+      #   class UserMailer < ActionMailer::Preview
+      #     def welcome_email
+      #     end
+      #   end
+      #
+      class MailerPreviews < Base
+        include ParentClassMatchers
+        include ClassElementsHelper
+        include ParsingHelper
+        include VisibilityHelp
+
+        MSG = 'Add a mailer preview for `%<action_name>s`.'
+
+        def on_class(node)
+          return unless mailer_base_class?(node.parent_class)
+
+          actions(node).each do |action_node|
+            mailer_name = node.identifier.source
+            action_name = action_node.method_name
+            message = format(MSG, action_name: action_name)
+
+            add_offense(action_node, message: message) unless preview_action_exists?(mailer_name, action_name)
+          end
+        end
+
+        private
+
+        def preview_action_exists?(mailer_name, action_name)
+          preview_files(mailer_name).any? do |preview_path|
+            if preview_path.exist?
+              node = parse(preview_path, target_ruby_version)
+
+              node&.class_type? &&
+                mailer_preview_base_class?(node.parent_class) &&
+                actions(node).map(&:method_name).include?(action_name)
+            end
+          end
+        end
+
+        def actions(class_node)
+          class_def_nodes(class_node).select { |def_node| node_visibility(def_node) == :public }
+        end
+
+        def preview_files(class_name)
+          path = Pathname.pwd
+          Array(cop_config['PreviewPaths']).map do |preview_path|
+            path.join(preview_path, "#{class_name.underscore}_preview.rb")
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -2,11 +2,12 @@
 
 require_relative 'mixin/active_record_helper'
 require_relative 'mixin/active_record_migrations_helper'
-require_relative 'mixin/class_send_node_helper'
+require_relative 'mixin/class_elements_helper'
 require_relative 'mixin/database_type_resolvable'
 require_relative 'mixin/enforce_superclass'
 require_relative 'mixin/index_method'
 require_relative 'mixin/migrations_helper'
+require_relative 'mixin/parent_class_matchers'
 require_relative 'mixin/target_rails_version'
 
 require_relative 'rails/action_controller_flash_before_render'
@@ -74,6 +75,7 @@
 require_relative 'rails/lexically_scoped_action_filter'
 require_relative 'rails/link_to_blank'
 require_relative 'rails/mailer_name'
+require_relative 'rails/mailer_previews'
 require_relative 'rails/match_route'
 require_relative 'rails/migration_class_name'
 require_relative 'rails/negate_include'

lib/rubocop/rails/schema_loader.rb

@@ -5,6 +5,7 @@ module Rails
     # It loads db/schema.rb and return Schema object.
     # Cops refers database schema information with this module.
     module SchemaLoader
+      include Cop::ParsingHelper
       extend self
 
       # It parses `db/schema.rb` and return it.
@@ -45,17 +46,6 @@ def load!(target_ruby_version)
         ast = parse(path, target_ruby_version)
         Schema.new(ast) if ast
       end
-
-      def parse(path, target_ruby_version)
-        klass_name = :"Ruby#{target_ruby_version.to_s.sub('.', '')}"
-        klass = ::Parser.const_get(klass_name)
-        parser = klass.new(RuboCop::AST::Builder.new)
-
-        buffer = Parser::Source::Buffer.new(path, 1)
-        buffer.source = path.read
-
-        parser.parse(buffer)
-      end
     end
   end
 end

spec/rubocop/cop/rails/mailer_previews_spec.rb

@@ -0,0 +1,86 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::MailerPreviews, :config do
+  include FileHelper
+
+  let(:cop_config) { { 'PreviewPaths' => 'tmp/mailers/previews' } }
+
+  after { FileUtils.rm_rf('tmp') }
+
+  it 'registers an offense when there is no mailer preview file' do
+    expect_offense(<<~RUBY)
+      class UserMailer < ApplicationMailer
+        def welcome_email
+        ^^^^^^^^^^^^^^^^^ Add a mailer preview for `welcome_email`.
+        end
+      end
+    RUBY
+  end
+
+  it 'registers an offense when there is no mailer preview method' do
+    create_preview(<<~RUBY)
+      class UserMailerPreview < ActionMailer::Preview
+      end
+    RUBY
+
+    expect_offense(<<~RUBY)
+      class UserMailer < ApplicationMailer
+        def welcome_email
+        ^^^^^^^^^^^^^^^^^ Add a mailer preview for `welcome_email`.
+        end
+      end
+    RUBY
+  end
+
+  it 'registers an offense when there is a private mailer preview method' do
+    create_preview(<<~RUBY)
+      class UserMailerPreview < ActionMailer::Preview
+        private
+        def welcome_email
+        end
+      end
+    RUBY
+
+    expect_offense(<<~RUBY)
+      class UserMailer < ApplicationMailer
+        def welcome_email
+        ^^^^^^^^^^^^^^^^^ Add a mailer preview for `welcome_email`.
+        end
+      end
+    RUBY
+  end
+
+  it 'registers an offense when there is no mailer preview in the file' do
+    create_preview
+
+    expect_offense(<<~RUBY)
+      class UserMailer < ApplicationMailer
+        def welcome_email
+        ^^^^^^^^^^^^^^^^^ Add a mailer preview for `welcome_email`.
+        end
+      end
+    RUBY
+  end
+
+  it 'does not register an offense when there is mailer preview' do
+    create_preview(<<~RUBY)
+      class UserMailerPreview < ActionMailer::Preview
+        def welcome_email
+        end
+      end
+    RUBY
+
+    expect_no_offenses(<<~RUBY)
+      class UserMailer < ApplicationMailer
+        def welcome_email
+        end
+      end
+    RUBY
+  end
+
+  private
+
+  def create_preview(content = '')
+    create_file('tmp/mailers/previews/user_mailer_preview.rb', content)
+  end
+end