[Fix rubocop#51] Add ability to whitelist receiver class names

In `Rails/DynamicFindBy`.

Fixes rubocop#51.

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### New features
+
+* [#51](https://github.com/rubocop-hq/rubocop-rails/issues/51): Add ability to whitelist receiver class names in `Rails/DynamicFindBy`. ([@tejasbubane][])
+
 ## 2.3.0 (2019-08-13)
 
 ### New features
@@ -65,3 +69,4 @@
 [@santib]: https://github.com/santib
 [@prathamesh-sonpatki]: https://github.com/prathamesh-sonpatki
 [@MaximeLaurenty]: https://github.com/MaximeLaurenty
+[@tejasbubane]: https://github.com/tejasbubane

config/default.yml

@@ -145,6 +145,7 @@ Rails/DynamicFindBy:
   VersionAdded: '0.44'
   Whitelist:
     - find_by_sql
+    - Gem::Specification
 
 Rails/EnumHash:
   Description: 'Prefer hash syntax over array syntax when defining enums.'

lib/rubocop/cop/mixin/whitelist.rb

@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    # Common functionality for checking whitelisted methods & class names.
+    module Whitelist
+      def whitelisted?(node)
+        whitelisted_method?(node) || whitelisted_receiver?(node)
+      end
+
+      def whitelisted_method?(node)
+        whitelist.include?(node.method_name.to_s)
+      end
+
+      def whitelisted_receiver?(node)
+        return unless node.receiver
+
+        whitelist.include?(node.receiver.source.to_s)
+      end
+
+      private
+
+      def whitelist
+        cop_config['Whitelist']
+      end
+    end
+  end
+end

lib/rubocop/cop/rails/dynamic_find_by.rb

@@ -26,21 +26,21 @@ module Rails
       #   # good
       #   User.find_by!(email: email)
       class DynamicFindBy < Cop
+        include Whitelist
+
         MSG = 'Use `%<static_name>s` instead of dynamic `%<method>s`.'
         METHOD_PATTERN = /^find_by_(.+?)(!)?$/.freeze
 
         def on_send(node)
-          method_name = node.method_name.to_s
-
-          return if whitelist.include?(method_name)
+          return if whitelisted?(node)
 
+          method_name = node.method_name
           static_name = static_method_name(method_name)
-
           return unless static_name
 
           add_offense(node,
                       message: format(MSG, static_name: static_name,
-                                           method: node.method_name))
+                                           method: method_name))
         end
         alias on_csend on_send
 
@@ -68,10 +68,6 @@ def autocorrect_argument_keywords(corrector, node, keywords)
           end
         end
 
-        def whitelist
-          cop_config['Whitelist']
-        end
-
         def column_keywords(method)
           keyword_string = method.to_s[METHOD_PATTERN, 1]
           keyword_string.split('_and_').map { |keyword| "#{keyword}: " }

lib/rubocop/cop/rails_cops.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require_relative 'mixin/target_rails_version'
+require_relative 'mixin/whitelist'
 
 require_relative 'rails/action_filter'
 require_relative 'rails/active_record_aliases'

manual/cops_rails.md

@@ -636,7 +636,7 @@ User.find_by!(email: email)
 
 Name | Default value | Configurable values
 --- | --- | ---
-Whitelist | `find_by_sql` | Array
+Whitelist | `find_by_sql`, `Gem::Specification` | Array
 
 ### References
 

spec/rubocop/cop/rails/dynamic_find_by_spec.rb

@@ -139,6 +139,18 @@
     RUBY
   end
 
+  context 'with whitelisted classname' do
+    let(:cop_config) do
+      { 'Whitelist' => %w[find_by_sql Gem::Specification] }
+    end
+
+    it 'accepts class methods in whitelist' do
+      expect_no_offenses(<<~RUBY)
+        Gem::Specification.find_by_name("backend").gem_dir
+      RUBY
+    end
+  end
+
   context 'when using safe navigation operator' do
     context 'with dynamic find_by_*' do
       let(:source) { 'user&.find_by_name(name)' }