TYPO3 Core

If an Extbase model has a class member, which has no public accessor functions and has the type of some other model, then an exception is throws during object validation. (The class member used purely during runtime and not stored permanently.)

Could not get value of property Thing::user", make sure the property is either public or has a getter getUser(), a hasser hasUser() or an isser ',

Until Core v8.7 such class members have been ignored.

Example:

class User {}
class Thing { /** User */ protected $user }