validation-microflows

Validation Microflows Skill

This skill provides guidance for creating validation microflows in MDL that validate user input on NewEdit pages and provide feedback to users.

When to Use This Skill

Use this skill when:

• Creating validation logic for NewEdit pages
• Implementing attribute validation with feedback messages
• Building conditional validation chains
• Creating action microflows that call validation microflows

Nanoflow validation: The same patterns apply to nanoflows (create nanoflow instead of create microflow). Use nanoflows for client-side validation when server roundtrips are unnecessary — validation feedback renders instantly without a network call.

The Validation Pattern

Mendix validation follows a two-microflow pattern:

1. VAL_Entity_Action - The validation microflow that:

   • Takes an entity object as parameter
   • Validates each required field
   • Shows validation feedback on invalid fields
   • Returns a Boolean indicating overall validity

2. ACT_Entity_Action - The action microflow that:

   • Calls the validation microflow
   • Only proceeds with save/commit if validation passes
   • Closes the page on success

MDL Syntax

VALIDATION FEEDBACK Statement

validation feedback $VariableName/attributename message 'Error message';

With template arguments (for dynamic messages):

validation feedback $VariableName/attributename message '{1}' objects [$MessageVariable];

CLOSE PAGE Statement

close page;

Or to close multiple pages:

close page 2;

Complete Example

Validation Microflow (VAL_Car_NewEdit)

/**
 * Validates a Car entity for NewEdit operations
 *
 * Performs validation on all required fields and displays
 * appropriate error messages to the user.
 *
 * @param $Car The Car entity to validate
 * @returns Boolean indicating if all validations passed
 */
create microflow MdlTemplates.VAL_Car_NewEdit (
  $Car: MdlTemplates.Car
)
returns boolean as $IsValid
folder 'OverviewPages'
begin
  -- Initialize validation flag
  declare $IsValid boolean = true;

  -- Validate Brand (required text field)
  if trim($Car/Brand) = '' then
    set $IsValid = false;
    validation feedback $Car/Brand message 'Brand is required';
  end if;

  -- Validate Model (required text field)
  if trim($Car/model) = '' then
    set $IsValid = false;
    validation feedback $Car/model message 'Model is required';
  end if;

  -- Validate Price (required, must be positive)
  if $Car/Price = empty then
    set $IsValid = false;
    validation feedback $Car/Price message 'Price is required';
  else
    if $Car/Price <= 0 then
      set $IsValid = false;
      validation feedback $Car/Price message 'Price must be greater than 0';
    end if;
  end if;

  -- Validate enumeration (required)
  if $Car/CarType = empty then
    set $IsValid = false;
    validation feedback $Car/CarType message 'Car type is required';
  end if;

  return $IsValid;
end;
/