java - Hibernate and no PK

Question

Is it possible to create a table (from a JPA annotated Hibernate @Entity) that does not contain a primary key / Id?

I know this is not a good idea; a table should have a primary key.

Answer 1

Roger's self-answer is correct. To elaborate a bit on what is meant (I wasn't clear on it at first and figured this would help):

Say you have you have a table Foo as such:

TABLE Foo (
bar varchar(20),
bat varchar(20)
)

Normally, you can write a class w/Annotations to work with this table:

// Technically, for this example, the @Table and @Column annotations 
// are not needed, but don't hurt. Use them if your column names 
// are different than the variable names.

@Entity
@Table(name = "FOO")
class Foo {

  private String bar;
  private String bat;


  @Column(name = "bar")
  public String getBar() {
   return bar;    
  }

  public void setBar(String bar) {
   this.bar = bar;    
  }

  @Column(name = "bat")
  public String getBat() {
   return bat;    
  }

  public void setBat(String bat) {
   this.bat = bat;    
  }

}

.. But, darn. This table has nothing we can use as an id, and it's a legacy database that we use for [insert vital business function]. I don't think they'll let me start modifying tables in order for me to use hibernate.

You can, instead, split the object up into a hibernate-workable structure which allows the entire row to be used as the key. (Naturally, this assumes that the row is unique.)

Split the Foo object into two thusly:

@Entity
@Table(name = "FOO")
class Foo {

  @Id
  private FooKey id;

  public void setId(FooKey id) {
    this.id = id;
  }

  public void getId() {
    return id;
  }
}

and

@Embeddable
class FooKey implements Serializable {
  private String bar;
  private String bat;

  @Column(name = "bar")
  public String getBar() {
   return bar;    
  }

  public void setBar(String bar) {
   this.bar = bar;    
  }

  @Column(name = "bat")
  public String getBat() {
   return bat;    
  }

  public void setBat(String bat) {
   this.bat = bat;    
  }

}

.. And that should be it. Hibernate will use the Embeddable key for its required identity and you can make a call as normal:

Query fooQuery = getSession().createQuery("from Foo");

Hope this helps first-timers with getting this working.