Constraints

Constraints restrict the values an attribute can hold. They are specified after the type in an attribute definition.

Syntax

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

Constraints must appear in this order:

NOT NULL (optionally with ERROR)
UNIQUE (optionally with ERROR)
DEFAULT

NOT NULL

Marks the attribute as required. The value cannot be empty.

Name: String(200) NOT NULL

With a custom error message displayed to the user:

Name: String(200) NOT NULL ERROR 'Name is required'

UNIQUE

Enforces that the value must be unique across all objects of the entity.

Email: String(200) UNIQUE

With a custom error message:

Email: String(200) UNIQUE ERROR 'Email already exists'

DEFAULT

Sets the initial value when a new object is created.

Count: Integer DEFAULT 0
IsActive: Boolean DEFAULT TRUE
Status: Enumeration(Sales.OrderStatus) DEFAULT 'Draft'
Name: String(200) DEFAULT 'Unknown'

Default value syntax varies by type:

Type	Default Syntax	Examples
String	`DEFAULT 'value'`	`DEFAULT ''`, `DEFAULT 'Unknown'`
Integer	`DEFAULT n`	`DEFAULT 0`, `DEFAULT -1`
Long	`DEFAULT n`	`DEFAULT 0`
Decimal	`DEFAULT n.n`	`DEFAULT 0`, `DEFAULT 0.00`, `DEFAULT 99.99`
Boolean	`DEFAULT TRUE/FALSE`	`DEFAULT TRUE`, `DEFAULT FALSE`
AutoNumber	`DEFAULT n`	`DEFAULT 1` (starting value)
Enumeration	`DEFAULT Module.Enum.Value`	`DEFAULT Shop.Status.Active`, `DEFAULT 'Pending'`

Omitting the DEFAULT clause means no default value is set:

OptionalField: String(200)

Combining Constraints

All three constraints can be used together:

Email: String(200) NOT NULL ERROR 'Email is required'
                   UNIQUE ERROR 'Email already exists'
                   DEFAULT ''

More examples:

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',

  -- Default only
  Quantity: Integer DEFAULT 0,

  -- No constraints
  Description: String(unlimited)
);

Validation Rule Mapping

Under the hood, constraints map to Mendix validation rules:

MDL Constraint	Mendix Validation Rule
`NOT NULL`	`DomainModels$RequiredRuleInfo`
`UNIQUE`	`DomainModels$UniqueRuleInfo`

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