Generalization

Generalization (inheritance) allows an entity to extend another entity, inheriting all of its attributes and associations. The child entity can add its own attributes on top.

Syntax

CREATE PERSISTENT ENTITY <Module>.<Name>
  EXTENDS <ParentEntity>
(
  <additional-attributes>
);

Both EXTENDS (preferred) and GENERALIZATION (legacy) keywords are supported.

Examples

Extending System.User

The most common use of generalization is creating an application-specific user entity:

/** Employee extends User with additional fields */
CREATE PERSISTENT ENTITY HR.Employee EXTENDS System.User (
  EmployeeNumber: String(20) NOT NULL UNIQUE,
  Department: String(100),
  HireDate: Date
);

The HR.Employee entity inherits all attributes from System.User (Name, Password, etc.) and adds EmployeeNumber, Department, and HireDate.

Extending System.Image

For entities that store images:

/** Product photo entity */
CREATE PERSISTENT ENTITY Catalog.ProductPhoto EXTENDS System.Image (
  Caption: String(200),
  SortOrder: Integer DEFAULT 0
);

Extending System.FileDocument

For entities that store file attachments:

/** File attachment entity */
CREATE PERSISTENT ENTITY Docs.Attachment EXTENDS System.FileDocument (
  Description: String(500)
);

Common System Generalizations

Parent Entity	Purpose
`System.User`	User accounts with authentication
`System.FileDocument`	File storage (name, size, content)
`System.Image`	Image storage (extends FileDocument with dimensions)

Inheritance Rules

A persistent entity can extend another persistent entity
The child entity inherits all attributes and associations of the parent
The child entity can add new attributes but cannot remove inherited ones
Associations to the parent entity also apply to child objects
Database queries on the parent entity include child objects

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