Data Type Mapping Table

Comprehensive mapping between MDL data types, Mendix internal types, and backend representations.

Primitive Types

MDL Type	Description	Range/Limits
`String`	Variable-length text (default length 200)	1 to `unlimited` characters
`String(n)`	Variable-length text with explicit length	1 to `unlimited` characters
`Integer`	32-bit signed integer	-2,147,483,648 to 2,147,483,647
`Long`	64-bit signed integer	-9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
`Decimal`	High-precision decimal number	Up to 20 digits total
`Boolean`	True/false value	`true` or `false` (must have DEFAULT)
`DateTime`	Date and time with timezone awareness	UTC timestamp
`Date`	Date only (no time component)	Internally stored as DateTime
`AutoNumber`	Auto-incrementing integer	Typically combined with NOT NULL UNIQUE
`Binary`	Binary data (files, images)	Configurable max size
`HashedString`	Securely hashed string (passwords)	One-way hash, cannot be retrieved

Complex Types

MDL Type	Description	Example
`Enumeration(Module.EnumName)`	Reference to an enumeration type	`Status: Enumeration(Sales.OrderStatus)`

MDL to Backend Type Mapping

MDL Type	BSON $Type	Go Type (SDK)
`String`	`DomainModels$StringAttributeType`	`*StringAttributeType`
`String(n)`	`DomainModels$StringAttributeType` + Length	`*StringAttributeType{Length: n}`
`Integer`	`DomainModels$IntegerAttributeType`	`*IntegerAttributeType`
`Long`	`DomainModels$LongAttributeType`	`*LongAttributeType`
`Decimal`	`DomainModels$DecimalAttributeType`	`*DecimalAttributeType`
`Boolean`	`DomainModels$BooleanAttributeType`	`*BooleanAttributeType`
`DateTime`	`DomainModels$DateTimeAttributeType`	`*DateTimeAttributeType`
`Date`	`DomainModels$DateTimeAttributeType`	`*DateTimeAttributeType`
`AutoNumber`	`DomainModels$AutoNumberAttributeType`	`*AutoNumberAttributeType`
`Binary`	`DomainModels$BinaryAttributeType`	`*BinaryAttributeType`
`Enumeration`	`DomainModels$EnumerationAttributeType`	`*EnumerationAttributeType`
`HashedString`	`DomainModels$HashedStringAttributeType`	`*HashedStringAttributeType`

Default Value Mapping

MDL Default	BSON Structure	Go Structure
`DEFAULT 'text'`	`Value: {$Type: "DomainModels$StoredValue", DefaultValue: "text"}`	`Value: &AttributeValue{DefaultValue: "text"}`
`DEFAULT 123`	`Value: {$Type: "DomainModels$StoredValue", DefaultValue: "123"}`	`Value: &AttributeValue{DefaultValue: "123"}`
`DEFAULT TRUE`	`Value: {$Type: "DomainModels$StoredValue", DefaultValue: "true"}`	`Value: &AttributeValue{DefaultValue: "true"}`
(calculated)	`Value: {$Type: "DomainModels$CalculatedValue", Microflow: <id>}`	`Value: &AttributeValue{Type: "CalculatedValue", MicroflowID: id}`

Default Value Syntax 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'`

Constraints

Constraint	Syntax	Description
Not Null	`NOT NULL`	Value is required
Not Null with Error	`NOT NULL ERROR 'message'`	Required with custom error
Unique	`UNIQUE`	Value must be unique
Unique with Error	`UNIQUE ERROR 'message'`	Unique with custom error

Constraints must appear in this order:

NOT NULL [ERROR '...']
UNIQUE [ERROR '...']
DEFAULT <value>

Type Compatibility

MDL does not perform implicit type conversions. Types must match exactly.

Compatible type changes:

String(100) to String(200) – Increasing length
Integer to Long – Widening numeric type

Incompatible type changes:

String to Integer
Boolean to String
Any type to AutoNumber (if not empty)

Complete Example

/** Example entity demonstrating all data types */
@Position(100, 100)
CREATE PERSISTENT ENTITY Demo.AllTypes (
  /** Auto-generated ID */
  Id: AutoNumber NOT NULL UNIQUE DEFAULT 1,

  /** Short text field */
  Code: String(10) NOT NULL UNIQUE,

  /** Standard text field */
  Name: String(200) NOT NULL,

  /** Long text field */
  Description: String(unlimited),

  /** Integer counter */
  Counter: Integer DEFAULT 0,

  /** Large number */
  BigNumber: Long,

  /** Money amount */
  Amount: Decimal DEFAULT 0.00,

  /** Flag field */
  IsActive: Boolean DEFAULT TRUE,

  /** Timestamp */
  CreatedAt: DateTime,

  /** Date only */
  BirthDate: Date,

  /** File attachment */
  Attachment: Binary,

  /** Status from enumeration */
  Status: Enumeration(Demo.Status) DEFAULT 'Active'
)
INDEX (Code)
INDEX (Name, CreatedAt DESC);

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