If you have reached this page you are probably a little bit desperate to know WHY?!!! After checking doctrine mappings for a while I've solved the mystery. In my case it was quite easy and stupid:
1. In some command I would create a new instance of MyClass() which has an autoid generated primary key called $id.
2. At some point I would pass it to some repository function: $this->em->searchForMyObject($myNewObject);
3. The repository function would look like:
public function searchForMyObject(MyClass $object): ?SomeOtherClass
{
$result = $this->createQueryBuilder('o')
->select('o')
->where('o.my_property = :object')....}
4. Reason for the error: the object was not persisted to database or retrieved from database, so it had no actual id value.
Reading the error again, now it makes sense: "entities that have an identifier.".
