hibernate - mutiple many-to-one to the same entity column

Question

I have a relationship between buildings and floors: some floors can only be in 1 building (these floors will have only 1 foreign key to "building" table) and there are some floors sit between 2 buildings (these floors will have 2 foreign keys to "building" table, one is startBuildingId and the second is endBuildingId ).I have this code to represent this relationship:

public class Building implements Serializable {

    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @OneToMany(mappedBy = "building")
    private Set<Floor> floors = new HashSet<>();
}

public class Floor implements Serializable {

    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne
    @JsonIgnoreProperties("floors")
    @JoinColumn(name = "building_id")
    private Building building;

    @ManyToOne
    @JsonIgnoreProperties("")
    @JoinColumn(name = "start_building_id")
    private Building startBuildingId;

    @ManyToOne
    @JsonIgnoreProperties("")
    @JoinColumn(name = "end_building_id")
    private Building endBuildingId;
}

When I create a floor that is sits between 2 buildings, I've chosen the startBuildingId and endBuildingId , but when I retrieve all the floors from the building's private Set<Floor> floors , it only shows the other floors, not those floors that sit between 2 buildings. How can I achieve this?

Answer 1

My suggestion:

• Three @OneToMany fields
• a method that returns a union of them

I assume you don't do any modifications on the floors list (adding a Floor would not be clearly defined - does it belong to a single building, or is it shared). In this case, it can be a lazy, unmodifiable view on the actual sets.

Alternatively (and preferably), you can easily create a JPQL/Criteria query that finds all the floors by building id.