Saving entity with one-to-many relationship

Jesper :

I think I have a bad setup for my hibernate database. I have Citizen entities who have one to many relationships with WeeklyCare entities. Below is the relevant code.

Citizen:

@Entity
@Table(name = "citizens")
public class Citizen {
@Id
@Size(max = 10, min = 10, message = "CPR must be exactly 10 characters")
private String cpr;

@OneToMany()
@JoinColumn(name = "cpr")
private List<WeeklyCare> weeklyCare;
}

WeeklyCare:

@Entity
public class WeeklyCare {
@EmbeddedId
private WeeklyCareIdentifier weeklyCareIdentifier;
}

WeeklyCareIdentifier:

@Embeddable
public class WeeklyCareIdentifier implements Serializable {

@NotNull
@Size(max = 10, min = 10, message = "CPR must tbe exactly 10 characters")
private String cpr;

@NotNull
private Integer week;

@NotNull
private Integer year;
}

I have some problems when I want to save data to the database:

1. I can't save WeeklyCare first, because it requires a Citizen.
2. When I send the citizens to my backend, the objects contain a list of WeeklyCare. When I try to save the citizens, it gives me this error: Unable to find Application.Models.WeeklyCare with id Application.Models.WeeklyCareIdentifier@b23ef67b

I can solve the problem by clearing the list of WeeklyCare on the Citizen before saving it, and then saving the list of WeeklyCare after, but that feels like a terrible way to do it.

I guess I want hibernate to ignore the list of WeeklyCare when it saves a Citizen, but acknowledge it when it fetches a Citizen. Is this possible? Or is there an even better way to do it? Thanks.

hovanessyan :

1. I can't save WeeklyCare first, because it requires a Citizen.

You have the "cpr" identifier used in two entities:

• it's the primary Id for Citizen
• it's part of the composite Id for WeeklyCare

You could, theoretically speaking, create a list of WeeklyCare (not with the way it is modeled now though) and later update the associations of each WeeklyCare to Citizen.

2. When I send the citizens to my backend, the objects contain a list of WeeklyCare. When I try to save the citizens, it gives me this error: Unable to find Application.Models.WeeklyCare with id Application.Models.WeeklyCareIdentifier@b23ef67b

The best way to map One-To-Many association is bidirectional. This will also save you from some unnecessary queries Hibernate is generating when using @OneToMany with @JoinColumn only.

1) Remove cpr from WeeklyCareIdentifier class (and probably rename the class).

@Embeddable
public class WeeklyCareIdentifier implements Serializable {

  @NotNull
  private Integer week;

  @NotNull
  private Integer year;

  //constructors, getters, setters
}

2) Remove the composite @EmbeddedId in favor of Long id field:

@Entity
public class WeeklyCare {

  @Id
  @GeneratedValue
  private Long id;

  @Embedded
  private WeeklyCareIdentifier weeklyCareIdentifier;

  //constructors, getters, setters
}

3) Move to bidirectional mapping:

@Entity
@Table(name = "citizens")
public class Citizen {
  @Id
  @Size(max = 10, min = 10, message = "CPR must be exactly 10 characters")
  private String cpr;

  @OneToMany(
      mappedBy = "citizen",
      cascade = CascadeType.ALL, //cascade all operations to children
      orphanRemoval = true //remove orphaned WeeklyCare if they don't have associated Citizen
  )
  private List<WeeklyCare> weeklyCares = new ArrayList<>(); //init collections to avoid nulls

    //constructors, getters, setters

    //add utility methods to manipulate the relationship

    public void addWeeklyCare(WeeklyCare weeklyCare) {
      weeklyCares.add(weeklyCare);
      weeklyCare.setCitizen(this);
    }

    public void removeWeeklyCare(WeeklyCare weeklyCare) {
      weeklyCares.remove(weeklyCare);
      weeklyCare.setCitizen(null);
    }
}

and:

@Entity
public class WeeklyCare {

    @Id
    @GeneratedValue
    private Long id;

    //having reference to the citizen entity from WeeklyCare
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "citizen_cpr")
    private Citizen citizen;

    @Embedded
    private WeeklyCareIdentifier weeklyCareIdentifier;

    //constructors, getters, setters
}

I would also recommend to use Long ids for the entities, even if the cpr is unique and so on. Convert the cpr to a normal column and introduce a DB generated ID column which you use in to join with in your internal domain and treat the cpr as a pure user-facing data column.