Inheritance

Inheritance is an object-oriented concept without a direct database equivalent, so an ORM has to emulate it. Propulsion supports two strategies:

Single Table Inheritance — the cheapest at query time, but limited to a small, fixed set of inherited columns.
Class Table Inheritance — one table per class, joined together. Propulsion doesn’t implement this directly; the delegate behavior gives you the same result.

Propulsion doesn’t have a dedicated Concrete Table Inheritance feature — a concrete_inheritance behavior did this in 2.x, but it’s gone in 3.0 (see Upgrading from 2.x to 3.0). Model the relationship with a foreign key to the parent table instead, or use single-table inheritance if the columns fit.

Single table inheritance

One table backs every subclass, so the table needs every column any subclass might use. Propulsion generates a stub subclass for each.

Consider Book, with two subclasses, Essay and Comic. A discriminator column (conventionally class_key, but any name works) needs the inheritance="single" attribute and one <inheritance> child per subclass:

<table name="book">
  <column name="id" type="integer" primaryKey="true" autoIncrement="true"/>
  <column name="title" type="varchar" size="100"/>
  <column name="class_key" type="integer" inheritance="single">
    <inheritance key="1" class="Book"/>
    <inheritance key="2" class="Essay" extends="Book"/>
    <inheritance key="3" class="Comic" extends="Book"/>
  </column>
</table>

Rebuilding generates Book, Essay, Comic (each extending Book), and matching BookQuery, EssayQuery, ComicQuery classes (each extending BookQuery). An <inheritance> element can itself extends another inherited class, so a Manga type could extend Comic instead of Book directly.

Use the generated classes exactly like any other model class — Propulsion sets the discriminator column for you:

<?php
$book = new Book();
$book->setTitle('War And Peace');
$book->save();

$essay = new Essay();
$essay->setTitle('On the Duty of Civil Disobedience');
$essay->save();

id | title                             | class_key
---|-----------------------------------|----------
1  | War And Peace                     | Book
2  | On the Duty of Civil Disobedience | Essay

Querying the parent class hydrates the correct subclass automatically:

<?php
$books = BookQuery::create()->find();
foreach ($books as $book) {
    echo get_class($book) . ': ' . $book->getTitle() . "\n";
}
// Book: War And Peace
// Essay: On the Duty of Civil Disobedience

Query with a subclass’s own Query class (ComicQuery::create()->find()) to restrict results to just that subclass. Mark a table abstract="true" to prevent instantiating the base class directly, forcing callers through a subclass:

<table name="book" abstract="true">
  <!-- ... -->
</table>

Class table inheritance via `delegate`

Propulsion has no dedicated class-table-inheritance feature; the delegate behavior gets you the same result by proxying method calls from a child table to a “parent” table it doesn’t actually inherit from in PHP terms.

<table name="player">
  <column name="id" type="integer" primaryKey="true" autoIncrement="true"/>
  <column name="first_name" type="varchar" size="100"/>
  <column name="last_name" type="varchar" size="100"/>
</table>
<table name="basketballer">
  <column name="id" required="true" primaryKey="true" autoIncrement="true" type="integer"/>
  <column name="points" type="integer"/>
  <column name="player_id" type="integer"/>
  <foreign-key foreignTable="player">
    <reference local="player_id" foreign="id"/>
  </foreign-key>
  <behavior name="delegate">
    <parameter name="to" value="player"/>
  </behavior>
</table>

Basketballer delegates unknown method calls to its related Player — setting Player columns directly on a Basketballer instance:

<?php
$basketballer = new Basketballer();
$basketballer->setPoints(101);
$basketballer->setFirstName('Michael'); // delegated to a Player instance
$basketballer->setLastName('Giordano');
$basketballer->save(); // saves both the basketballer and the player row

If no Player is already attached, delegate creates one automatically. Delegation is single-level — a deeper hierarchy needs delegation to every ancestor (to="basketballer, player"), simulating multiple inheritance. The delegated methods are real, typed methods on the delegating class, so IDEs and PHPStan resolve them the same as any other generated method.

Inheritance

Single table inheritance

Class table inheritance via delegate

Class table inheritance via `delegate`