Q:

Spring data repository's method to find by the field of a field

I've two entities, a user and a registered user. 
A registered user has a field of type user. I would like to have a method in the spring data repository related to this registered user entity to search all registered users by the username of the user that is connected to the registered user.
So, this is the registered user entity with an associated user field:
@Entity
public class RegisteredUser implements Serializable {

    ... 
    @OneToOne
    @JoinColumn(name = "USERNAME_FK")
    private User user;
    ...
}

and this is a user with a username:
@Entity
public class User implements Serializable { 
    ...
    @Id
    @Column(nullable = false)
    protected String username;
    ...
}

A:

Spring Data (at least 1.12.x version) uses PropertyPath#from method to extract path to a property for a predicate constructed from method name. According to sources it uses underscore as "field separator". So first variant is as follows
public interface RegisteredUserRepository extends CrudRepository<RegisteredUser,String> {
    List<RegisteredUser> findRegisteredUserByUser_Username(String username);
}

There is also code which treat an uppercase char as field separator if whole field name is not found. So if you don't have a userUsername field in RegisteredUser second varian is
public interface RegisteredUserRepository extends CrudRepository<RegisteredUser,String> {
    List<RegisteredUser> findRegisteredUserByUserUsername(String username);
}

