Put up a warning sign that mapping may not be inherited from transient classes

[mpdude]

This...

class Parent
{
    /**
     * @Id
     * @Column(type="integer")
     * @GeneratedValue
     */
    protected int $id;

    /**
     * @Column
     */
    protected string $name;

    // ...
}

/** @Entity */
class Child extends Parent
{
    // inherit $id including mapping config

    /**
     * @Column(name="name_override_config")
     */
    protected string $name;

    // ...
}

... seems to work, since it results in a single Child entity class with both fields, and also the $name field is configured with name="name_override_config".

The result comes from the (current) annotations/attribute mapping driver implementation: PHP reflection will report protected and public properties also for subclasses. Since there is no other entity or mapped superclass above Child in this example, the mapping driver assumes that all fields were declared on the Child class initially.

It will not work for e. g. class-level annotations like @Table on Parent, since those are not inherited/reported by the Reflection API for the Child subclass. Also, it does not work for private properties on the Parent class, since those don't show up in the ReflectionClass::getProperties() return value for the Child class.

We should explicitly warn users that this approach to configuration (and yes, I've seen this in the wild) is beyond what the ORM was designed for.

Note that using traits to horizontally reuse mapped fields is nothing different – it pulls fields into classes, including their attributes and annotations (= docblocks). The PHP Reflection API will report those fields as if they had been declared in the class using the trait. Traits have the extra side effect that private fields become part of the class where the trait is used, so they are reported as properties as well.