configure_dependency_for_has_many

# File activerecord/lib/active_record/associations.rb, line 986
        def configure_dependency_for_has_many(reflection)
          if reflection.options[:dependent] && reflection.options[:exclusively_dependent]
            raise ArgumentError, ':dependent and :exclusively_dependent are mutually exclusive options.  You may specify one or the other.'
          end

          if reflection.options[:exclusively_dependent]
            reflection.options[:dependent] = :delete_all
            #warn "The :exclusively_dependent option is deprecated.  Please use :dependent => :delete_all instead.")
          end

          # See HasManyAssociation#delete_records.  Dependent associations
          # delete children, otherwise foreign key is set to NULL.

          # Add polymorphic type if the :as option is present
          dependent_conditions = %(#{reflection.primary_key_name} = \#{record.quoted_id})
          if reflection.options[:as]
            dependent_conditions += " AND #{reflection.options[:as]}_type = '#{base_class.name}'"
          end

          case reflection.options[:dependent]
            when :destroy, true  
              module_eval "before_destroy '#{reflection.name}.each { |o| o.destroy }'"
            when :delete_all
              module_eval "before_destroy { |record| #{reflection.class_name}.delete_all(%(#{dependent_conditions})) }"
            when :nullify
              module_eval "before_destroy { |record| #{reflection.class_name}.update_all(%(#{reflection.primary_key_name} = NULL),  %(#{dependent_conditions})) }"
            when nil, false
              # pass
            else
              raise ArgumentError, 'The :dependent option expects either :destroy, :delete_all, or :nullify' 
          end
        end