use DoctrineORMMapping as ORM;

/**
 * @ORMEntity
 * @ORMTable(
 *   name="parent"
 * )
 */
class Parent
{
  /**
   * @ORMId
   * @ORMColumn(name="parent_id", type="integer")
   * @ORMGeneratedValue
   */
  private $id;

  /**
   * @ORMOneToMany(targetEntity="Child", mappedBy="parent", cascade={"persist", "remove"})
   */
  private $children;

  // other fields, getters, setters, etc

}

use DoctrineORMMapping as ORM;

/**
 * @ORMEntity
 * @ORMTable(
 *   name="child",
 *   uniqueConstraints={@ORMUniqueConstraint(name="parent_child_type", columns={"parent_id", "child_type"})}
 * )
 */
class Child
{
  /**
   * @ORMId
   * @ORMColumn(name="child_id", type="integer")
   * @ORMGeneratedValue
   */
  private $id;

  /**
   * @ORMManyToOne(targetEntity="Parent", inversedBy="children")
   * @ORMJoinColumn(name="parent_id", referencedColumnName="parent_id")
   */
  private $parent;

  /**
   * @ORMColumn(name="child_type", type="string", length=32)
   */
  private $type;

  // other fields, getters, setters, etc

}

# make sure the collection is valid, which forces validation of each Child
ExampleMyBundleORMModelParent:
    properties:
        children:
            - Valid: ~

# enforce uniqueness between the two fields
ExampleMyBundleORMModelChild:
    constraints:
        - SymfonyBridgeDoctrineValidatorConstraintsUniqueEntity:
            fields: [ parent, type ]
            message: You may only configure one child of each type per parent.