chore: Add custom RuboCop cop to enforce one class per file (#12947)

.rubocop.yml

@@ -7,6 +7,7 @@ plugins:
 require:
   - ./rubocop/use_from_email.rb
   - ./rubocop/custom_cop_location.rb
+  - ./rubocop/one_class_per_file.rb
 
 Layout/LineLength:
   Max: 150
@@ -205,6 +206,9 @@ UseFromEmail:
 CustomCopLocation:
   Enabled: true
 
+Style/OneClassPerFile:
+  Enabled: true
+
 AllCops:
   NewCops: enable
   Exclude:

db/migrate/20230515051424_update_article_image_keys.rb

@@ -44,6 +44,7 @@ class ArticleKeyConverter
   end
 end
 
+# rubocop:disable Style/OneClassPerFile
 class UpdateArticleImageKeys < ActiveRecord::Migration[7.0]
   def change
     # Iterate through all articles
@@ -53,3 +54,4 @@ class UpdateArticleImageKeys < ActiveRecord::Migration[7.0]
     end
   end
 end
+# rubocop:enable Style/OneClassPerFile

rubocop/one_class_per_file.rb

@@ -0,0 +1,86 @@
+require 'rubocop'
+
+# Enforces having only one class definition per file
+# Excludes common Ruby patterns:
+# - Nested Scope classes (Pundit pattern)
+# - Single-line exception classes inheriting from StandardError or other exceptions
+# - Nested classes within exception/error hierarchies
+class RuboCop::Cop::Style::OneClassPerFile < RuboCop::Cop::Base
+  MSG = 'Define only one class per file.'.freeze
+
+  def on_new_investigation
+    return unless processed_source.ast
+
+    class_nodes = processed_source.ast.each_node(:class).to_a
+    return if class_nodes.size <= 1
+
+    class_nodes[1..].each do |node|
+      next if allowed_nested_class?(node)
+
+      add_offense(node, message: MSG)
+    end
+  end
+
+  private
+
+  def allowed_nested_class?(node)
+    # Allow nested Scope classes (Pundit pattern)
+    return true if scope_class?(node)
+
+    # Allow nested Request classes (Rack::Attack pattern)
+    return true if rack_attack_request_class?(node)
+
+    # Allow exception classes (single-line or multi-line)
+    return true if exception_class?(node)
+
+    # Allow classes within CustomExceptions modules
+    return true if in_custom_exceptions_module?(node)
+
+    # Allow common nested patterns: Builder, Factory, Result, Response, Params, etc.
+    return true if common_nested_pattern?(node)
+
+    false
+  end
+
+  def scope_class?(node)
+    class_name = node.identifier.source
+    class_name == 'Scope'
+  end
+
+  def rack_attack_request_class?(node)
+    class_name = node.identifier.source
+    return false unless class_name == 'Request'
+
+    # Check if we're inside a Rack::Attack class
+    node.each_ancestor(:class).any? do |ancestor|
+      ancestor.identifier.source.include?('Rack::Attack')
+    end
+  end
+
+  def exception_class?(node)
+    # Check if the class inherits from StandardError or ends with 'Error'
+    return false unless node.parent_class
+
+    parent_class = node.parent_class.source
+    parent_class.include?('Error') || parent_class.include?('StandardError')
+  end
+
+  def in_custom_exceptions_module?(node)
+    # Check if any parent node is a module containing 'CustomExceptions'
+    node.each_ancestor(:module).any? do |ancestor|
+      ancestor.identifier.source.include?('CustomExceptions')
+    end
+  end
+
+  def common_nested_pattern?(node)
+    class_name = node.identifier.source
+
+    # Common nested class patterns in Ruby/Rails
+    nested_patterns = %w[Builder Factory Result Response Params Config Configuration
+                         Context Query Form Validator Serializer Presenter Decorator
+                         Command Handler]
+
+    # Check if class name ends with any of these patterns
+    nested_patterns.any? { |pattern| class_name.end_with?(pattern) }
+  end
+end