Best approach to handle conditional fields in a model

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I have 3 models Person, Role and Position representing people in a football club like

public class Person {
    private Long id;
    private String name;
    private Role role;
}

public class Role {
    private Long id;
    private String name; //like manager, goalkeeping_coach, player
}

public class Position {
    private Long id;
    private String name; //striker, midfielder, defender, goalkeeper
}

The problem is that position only makes sense for a person if their role is player. So if I do

public class Person {
    private Long id;
    private String name;
    private Role role;
    private Position position;
}

then for all person instances that don't have the role of player, the position field will store null values. Similarly, there may be other attributes that make sense only for manager and/or goalkeeping_coach instances.

I tried making the Person class abstract

@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public abstract class Person {
    private Long id;
    private String name;
    private Role role;
}

public class Player extends Person {
    private Position position;
}

public class Manager extends Person {

}

This will cause the player and manager instances to be saved in different tables. But the problem now arises if the role of the person changes (say a player retires and becomes the manager of the club). Then I'd have to move the row from one table to another (say delete the player instance and create a new manager instance using the same data excluding the position data), which doesn't seem to be a good way to go.

So what would be the best approach to handle this scenario? Is it ok to have null values as in the first case?

The requirement is that the connection between a Person and Role can be changed. The model can reflect that if you make a table for this connection, as well as the connection between Person and Position

// Person table has only Person details 
public class Person {
    private Long id;
    private String name;
}

// Role table has only Role details 
public class Role {
    private Long id;
    private String name; //like manager, goalkeeping_coach, player
}

// Position table has only ... 
public class Position {
    private Long id;
    private String name; //striker, midfielder, defender, goalkeeper
}

// connection of Person and Role 
public class PersonRole {
    private Long person_id;
    private Long role_id;
}

// connection of Person and Position
public class PersonPosition {
    private Long person_id;
    private Long position_id;
}

This design solves the requirement of person changing roles and also of the conditional relevance of position. it also caters for future requirements like a person having multiple roles and positions

EDIT: I figured that what I described is actually the database model. in the Java you can model the connection tables as many-to-many relation (if you use some ORM)

How to handle "conditional fields" in Java?, tl;dr As others have said, #6 is, design-wise, the best approach, and likely the best approach on other axes as well. The remainder of this is� The linear regression model is probably the oldest, best understood and most widely used conditional model.

The first essential differentiation here is whether we are talking java side code or database side code.

For the Java-representation: Deriving Player and Manager from Person seems right. No nullary fields there. The behavior will take care of whether it is a Player or Manager, and you can write transfer-constructors that make a player a manager (or vice versa, if that makes any sense)

For the Database-representation: It is okay to have nullary fields (they are cheap in terms of space and you can query by treating them with null checks). Whether you want to map them in an own table depends on whether you want to query them separately, or combined. Sometimes you want a query over Person instead of only Player, for example to estimate a total salary. Then you have to union (assuming you use some form of SQL-Database).

So essentially the right answer is, that depending on your entire architecture and purpose of your software, there is no one right answer, but you will have to make adaptions either way, in the critical places, if and when it comes to database interaction.

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One way to solve this is to have the same table serving both person and manager and actually any other type of person. You would have rows which are unused depending on the person type but all common fields would remain in that singe table. This way there is no need to delete rows and create them on another table when the role changes. You will have to add a field for the person type.

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You are running into a common problem. Although a Manager meets the "is a Person" criterion, this may only apply for a specific time period, and the database records typically apply to a much longer time period.

The correct way to model this is to have Person and Role tables (which you already have), plus a PersonRole linking table which will have personId, roleId, startDate and endDate fields. This will fix the data modelling problem but will complicate your code.

However, many applications are only concerned with an instant in time (e.g. Next game date, next payday). These can use your suggested "Manager extends Person" heirarchy as it holds true at an instance in time.

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Comments
  • Instead of using the name field, you could make a class for player, a class for manager etc.