Added ReflectionClassName cop

This PR add a cop to check if the value of the option `class_name` of a [ActiveRecord::Reflection](https://apidock.com/rails/ActiveRecord/Reflection/AbstractReflection/class_name) is a string.

_Why this cop:_

I was working in a Rails project where often there was a constant used for `class_name` value. That caused a issue which was to preload unnecessary models use in the definition of a reflection.

Author: Bhacaz

config/enabled.yml

@@ -172,6 +172,10 @@ Rails/RedundantReceiverInWithOptions:
   Description: 'Checks for redundant receiver in `with_options`.'
   Enabled: true
 
+Rails/ReflectionClassName:
+  Description: 'Use a string for `class_name` option value in the definition of a reflection.'
+  Enabled: true
+
 Rails/RefuteMethods:
   Description: 'Use `assert_not` methods instead of `refute` methods.'
   Enabled: true

lib/rubocop/cop/rails/reflection_class_name.rb

@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # This cop checks if the value of the option `class_name`, in
+      # the definition of a reflection is a string.
+      #
+      # @example
+      #   # bad
+      #   has_many :accounts, class_name: Account
+      #   has_many :accounts, class_name: Account.name
+      #
+      #   # good
+      #   has_many :accounts, class_name: 'Account'
+      class ReflectionClassName < Cop
+        MSG = 'Use a string has value for a class_name.'.freeze
+
+        def_node_matcher :association_with_options?, <<-PATTERN
+          (send nil? {:has_many :has_one :belongs_to} _ (hash $...))
+        PATTERN
+
+        def_node_search :reflection_class_name, <<-PATTERN
+          (pair (sym :class_name) !str)
+        PATTERN
+
+        def on_send(node)
+          return unless association_with_options?(node)
+
+          reflection_class_name = reflection_class_name(node).first
+          return unless reflection_class_name
+
+          add_offense(node, location: reflection_class_name.loc.expression)
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -45,6 +45,7 @@ module Cop
 require_relative 'rails/present'
 require_relative 'rails/read_write_attribute'
 require_relative 'rails/redundant_receiver_in_with_options'
+require_relative 'rails/reflection_class_name'
 require_relative 'rails/refute_methods'
 require_relative 'rails/relative_date_constant'
 require_relative 'rails/request_referer'

manual/cops_rails.md

@@ -1608,6 +1608,26 @@ with_options options: false do |merger|
 end
 ```
 
+## Rails/ReflectionClassName
+
+Enabled by default | Supports autocorrection
+--- | ---
+Enabled | No
+
+This cop checks if the value of the option `class_name`, in
+the definition of a reflection is a string.
+
+### Examples
+
+```ruby
+# bad
+has_many :accounts, class_name: Account
+has_many :accounts, class_name: Account.name
+
+# good
+has_many :accounts, class_name: 'Account'
+```
+
 ## Rails/RefuteMethods
 
 Enabled by default | Supports autocorrection

spec/rubocop/cop/rails/reflection_class_name_spec.rb

@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::ReflectionClassName do
+  subject(:cop) { described_class.new(config) }
+
+  let(:config) { RuboCop::Config.new }
+
+  context "registers an offense when using `foreign_key: 'account_id'`" do
+    it 'has_many' do
+      expect_offense(<<-RUBY.strip_indent)
+      has_many :accounts, class_name: Account, foreign_key: :account_id
+                          ^^^^^^^^^^^^^^^^^^^ Use a string has value for a class_name.
+      RUBY
+    end
+
+    it '.name' do
+      expect_offense(<<-RUBY.strip_indent)
+      has_many :accounts, class_name: Account.name
+                          ^^^^^^^^^^^^^^^^^^^^^^^^ Use a string has value for a class_name.
+      RUBY
+    end
+
+    it 'has_one' do
+      expect_offense(<<-RUBY.strip_indent)
+      has_one :account, class_name: Account
+                        ^^^^^^^^^^^^^^^^^^^ Use a string has value for a class_name.
+      RUBY
+    end
+
+    it 'belongs_to' do
+      expect_offense(<<-RUBY.strip_indent)
+      belongs_to :account, class_name: Account
+                           ^^^^^^^^^^^^^^^^^^^ Use a string has value for a class_name.
+      RUBY
+    end
+  end
+
+  it 'does not register an offense when using `foreign_key :account_id`' do
+    expect_no_offenses(<<-RUBY.strip_indent)
+      has_many :accounts, class_name: 'Account', foreign_key: :account_id
+      has_one :account, class_name: 'Account'
+      belongs_to :account, class_name: 'Account'
+    RUBY
+  end
+end