Entity add custom properties

Hello. I'd like to add a custom property (another object) when requesting find(), findAll() ...
How is this possible within the framework?

If you add public $user - will not be shown in toArray()
The protected $attributes property is not allowed - these are the names of the fields from the table
There is only one way left - to immediately assign $this->user = new User() But I would like to have an initial value as for $attributes = ['user'  => null]

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