PostgreSQL unique constraint handler: it sets the record as invalid if a PostgreSQL unique constraint error is raised on record saving

unique_constraint_handler.rb

# Usage example:
#
#   class User < ActiveRecord::Base
#     include UniqueConstraintHandler
#     handle_unique_constraint_on :email
#   end
#
#   User.create email: 'test@example.com'
#   user = User.create email: 'test@example.com'
#   user.new_record? #=> true
#   user.errors.added? :email, :taken #=> true

module UniqueConstraintHandler
  UNIQUE_CONSTRAINT_ERROR_REGEX_TEMPLATE =
    %[\\Aduplicate key value violates unique constraint "%s"\\z]

  extend ActiveSupport::Concern

  module ClassMethods
    def handle_unique_constraint_on(attribute, options = {})
      index_name = options[:index_name] || "index_#{table_name}_on_#{attribute}" 

      regexp = Regexp.new UNIQUE_CONSTRAINT_ERROR_REGEX_TEMPLATE % index_name

      prepend Module.new {
        private

        define_method :create_or_update do
          begin
            super()
          rescue ActiveRecord::RecordNotUnique => exception
            raise exception unless _unique_constraint_error? exception, regexp
            
            errors.add attribute, :taken

            false
          end
        end
      }
    end
  end

  private

  def _unique_constraint_error?(exception, regexp)
    original_exception = exception.try(:original_exception)                         and
    original_exception.is_a?(PG::UniqueViolation)                                   and
    result = original_exception.try(:result)                                        and
    error_primary_message = result.error_field(PG::Result::PG_DIAG_MESSAGE_PRIMARY) and
    error_primary_message =~ regexp
  end
end