Attributes and Validation Rules

Attributes define the fields on an entity. Each attribute has a name, a type, and optional constraints.

Attribute Syntax

[/** <documentation> */]
<name>: <type> [NOT NULL [ERROR '<message>']] [UNIQUE [ERROR '<message>']] [DEFAULT <value>]

Component	Description
Name	Attribute identifier (follows identifier rules)
Type	One of the primitive types or an enumeration reference
`NOT NULL`	Value is required
`UNIQUE`	Value must be unique across all objects
`DEFAULT`	Initial value on object creation
`/** ... */`	Documentation comment

Attribute Ordering

Attributes are listed in order, separated by commas. The last attribute has no trailing comma:

CREATE PERSISTENT ENTITY Module.Entity (
  FirstAttr: String(200),      -- comma after
  SecondAttr: Integer,         -- comma after
  LastAttr: Boolean DEFAULT FALSE  -- no comma
);

Validation Rules

Validation rules are expressed as attribute constraints. When validation fails, Mendix displays the error message (if provided) or a default message.

Validation	MDL Syntax	Description
Required	`NOT NULL`	Attribute must have a value
Required with message	`NOT NULL ERROR 'message'`	Custom error message on empty
Unique	`UNIQUE`	Value must be unique across all objects
Unique with message	`UNIQUE ERROR 'message'`	Custom error message on duplicate

Example with Validation

CREATE PERSISTENT ENTITY Sales.Product (
  -- Required only
  Name: String(200) NOT NULL,

  -- Required with custom error
  SKU: String(50) NOT NULL ERROR 'SKU is required for all products',

  -- Unique only
  Barcode: String(50) UNIQUE,

  -- Required and unique with custom errors
  ProductCode: String(20) NOT NULL ERROR 'Product code required'
                          UNIQUE ERROR 'Product code must be unique',

  -- Optional field (no validation)
  Description: String(unlimited)
);

Documentation Comments

Attach documentation to individual attributes with /** ... */:

CREATE PERSISTENT ENTITY Sales.Customer (
  /** Unique customer identifier, auto-generated */
  CustomerId: AutoNumber NOT NULL UNIQUE DEFAULT 1,

  /** Full legal name of the customer */
  Name: String(200) NOT NULL,

  /** Primary contact email address */
  Email: String(200) UNIQUE
);

CALCULATED Attributes

An attribute can be marked as calculated, meaning its value is computed by a microflow rather than stored:

FullName: String(400) CALCULATED

Calculated attributes use a CALCULATED BY clause to specify the source microflow. See the MDL quick reference for details.

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