based on: https://www.vzurauskas.com/2019/04/07/two-layer-repositories-in-spring/amp/

TODO

Typical Anemic Domain Model

@Data
@Entity
public class Account {
    @Id 
	@Column 
	private String iban;
    
	@Column 
	private int ownerId;
}

@Service
public final class AccountService {
    public boolean isAccountOwnedBy(Account account, int customerId) {
        return account.getOwnerId() == customerId;
    }
}

@Repository
public interface AccountRepository extends CrudRepository<Account, String> {
}

public void openNewAccount(String iban, int customerId) {
    repository.save(new Account(iban, customerId));
}

public String getBalance(String iban, int requestorId) {
    Account account = repository.findById("LT601010012345678901").orElseThrow(...);

    if (accountService.isAccountOwnedBy(account, requestorId)) {
        // Calculate balance...
    } else {
        throw new IllegalArgumentException(
            "Account is not owned by " + requestorId
        );
    }
}

move isAccountOwnedBy into model class

@Data
@Entity
public final class Account {
    @Id
	@Column
	private String iban;
    
	@Column
	private int ownerId;
    
    // Constructors...

    public boolean isOwnedBy(int customerId) {
        return this.ownerId == customerId;
    }
}

public String getBalance(String iban, int requestorId) {
    Account account = repository.findById("LT601010012345678901").orElseThrow(...);
    if (account.isOwnedBy(requestorId)) {
        // Calculate balance...
    } else {
        throw new IllegalArgumentException(
            "Account is not owned by " + requestorId
        );
    }
}