execute (DataMapper::Validate::ContextualValidators)

    # File lib/dm-validations/contextual_validators.rb, line 41
          def execute(named_context, target)
            raise(ArgumentError, 'invalid context specified') if !named_context || (contexts.length > 0 && !contexts[named_context])
            target.errors.clear!
            result = true
            context(named_context).each do |validator|
              next unless validator.execute?(target)
              result = false unless validator.call(target)
            end

            result
          end