I'm trying to write a hibernate adapter for an old database schema. This schema does not have a dedicated id column, but uses about three other columns to join data.
On some tables, I need to use coalesce. This is what I came up with so far:
About the definition:
COALESCE(NULLIF())
@Entity
@Table(name="CAR")
public class Car extends TableEntry implements Serializable {
@Id
@Column(name="DEP_NR")
private int depnr;
@Id
@Column(name="USER_NAME")
@Type(type="TrimmedString")
private String username;
@ManyToOne(fetch = FetchType.EAGER, targetEntity=CarGroup.class)
@JoinColumns(value={
@JoinColumn(name="GROUP_NAME"),
@JoinColumn(name="DEP_NR"),
@JoinColumn(name="state"),
})
private CarGroup group;
@OneToMany(fetch=FetchType.EAGER, targetEntity=CarUserElement.class, mappedBy="car")
private Set<CarUserElement> elements;
}
@Entity
@Table(name="CAR_GROUP")
public class CarGroup extends TableEntry implements Serializable {
@Id
@Column(name="DEP_NR")
private int depnr;
@Id
@Column(name="GROUP_NAME")
@Type(type="TrimmedString")
private String group;
@ManyToOne(fetch = FetchType.EAGER, targetEntity=Car.class)
@JoinColumns(value={
@JoinColumn(name="GROUP_NAME"),
@JoinColumn(name="DEP_NR"),
@JoinColumn(name="state"),
})
private Set<Car> cars;
@OneToMany(fetch=FetchType.EAGER, targetEntity=CarGroupElement.class, mappedBy="car")
private Set<CarGroupElement> elements;
}
@MappedSuperclass
public class CarElement extends TableEntry {
@Id
@ManyToOne(fetch = FetchType.EAGER, targetEntity=Element.class)
@JoinColumns(value={
@JoinColumn(name="ELEMENT_NAME"),
@JoinColumn(name="state"),
})
private Element element;
}
@Entity
@Table(name="CAR_TO_ELEMENT")
public class CarUserElement extends CarElement {
@Id
@Column(name="DEFINITION")
private char definition;
@Id
@ManyToOne(fetch = FetchType.LAZY)
@JoinColumnsOrFormulas(value = {
@JoinColumnOrFormula(formula=@JoinFormula(value="COALESCE(NULLIF(DEP_NR, 0), USR_DEP_NR)", referencedColumnName="DEP_NR")),
@JoinColumnOrFormula(column=@JoinColumn(name="FORIGN_ELEMENT", referencedColumnName="USER_NAME")),
@JoinColumnOrFormula(column=@JoinColumn(name="STATE", referencedColumnName="STATE"))
})
private Car car;
}
@Entity
@Table(name="CAR_TO_ELEMENT")
public class CarGroupElement extends CarElement {
@Id
@Column(name="DEFINITION")
private char definition;
@Id
@ManyToOne(fetch = FetchType.LAZY)
@JoinColumnsOrFormulas(value = {
@JoinColumnOrFormula(formula=@JoinFormula(value="COALESCE(NULLIF(DEP_NR, 0), USR_DEP_NR)", referencedColumnName="DEP_NR")),
@JoinColumnOrFormula(column=@JoinColumn(name="FORIGN_ELEMENT", referencedColumnName="GROUP_NAME")),
@JoinColumnOrFormula(column=@JoinColumn(name="STATE", referencedColumnName="STATE"))
})
private Car car;
}
@JoinColumnsOrFormulas
Exception in thread "main" java.lang.ClassCastException: org.hibernate.mapping.Formula cannot be cast to org.hibernate.mapping.Column
at org.hibernate.cfg.annotations.TableBinder.bindFk(TableBinder.java:351)
at org.hibernate.cfg.annotations.CollectionBinder.bindCollectionSecondPass(CollectionBinder.java:1338)
at org.hibernate.cfg.annotations.CollectionBinder.bindOneToManySecondPass(CollectionBinder.java:791)
at org.hibernate.cfg.annotations.CollectionBinder.bindStarToManySecondPass(CollectionBinder.java:719)
at org.hibernate.cfg.annotations.CollectionBinder$1.secondPass(CollectionBinder.java:668)
at org.hibernate.cfg.CollectionSecondPass.doSecondPass(CollectionSecondPass.java:66)
at org.hibernate.cfg.Configuration.originalSecondPassCompile(Configuration.java:1597)
at org.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:1355)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1737)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1788)
coalesce(nullif())
Join formulas are very fragile in Hibernate for the time being; I always had a difficult time to get them work properly.
The workaround that helped me often was to create database views which exposed the proper columns (including foreign keys that don't exist in the original tables). Then I mapped the entities to the views using classing Hibernate/JPA mappings.
Sometimes there are redundant joins in the generated SQL when using such entities, but the database optimizes such queries in most cases so that the execution plan is optimal anyway.
Another approach could be using @Subselect
s, which are some kind of Hibernate views, but I expect them to be less performant than the classic database views.