Mapping Hibernate entity for unknown DiscriminatorValue for InheritanceType.SINGLE_TABLE

Question

I have a classic Hibernate @Inheritance(strategy=InheritanceType.SINGLE_TABLE) with @DiscriminatorFormula. It works fine. However, there are about 500 different values for the @DiscriminatorValue in the database and I need map just about 30 of them to Java classes (children) and the rest of them to map to the parent Java class.

The problem can be modelled as an example inheritance on Animal class.

@Entity
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorFormula("...")
public class Animal implements Serializable {
  ...
  @Column
  public String getName() { ... }
}

So I have have about 30 subclasses of Animal defined in the Java code with @DiscriminatorValue. When Hibernate founds unknown value for the discriminator, then it throws WrongClassException. However, I need to map these unknown discriminator values to one entity, the best it the Animal class. (I need to use only the method getName() in such cases.)

I know one solution is to put a SQL CASE into the @DiscriminatorFormula but then I have to state there all 30 known discriminator values (plus more when I will need to add others). So I am looking for more flexible solution.

P.S. It is a legacy code, so I cannot change the model.

Very good question indeed! I am afraid you are stuck to the SQL CASE solution. Something like a forceDefault=true option (map to root class instead of exception) for the @DiscriminatorColumn or @DiscriminatorFormula annotation would be nice to have. Is the @DiscriminatorValue on the root class also missing in your working code or just in the example? If no, maybe you can force the default mapping this way. Cannot try it out right now but I doubt it, if @DiscriminatorColumn is used a default value is generated for a missing @DiscriminatorValue. — tscho
– tscho, Commented Jun 30, 2012 at 12:00
@DiscriminatorValue has been missing in the code and in the example, too. And this little detail persuaded me to dig into the Hibernate code once again and find the solution. See bellow. — xmedeko
– xmedeko, Commented Jul 2, 2012 at 10:34

Community · Accepted Answer · 2017-05-23 12:14:45Z

9

tscho has pointed me in the right direction, so I was able to find a solution for my case. The @DiscriminatorValue evaluates special values @DiscriminatorValue("null") and @DiscriminatorValue("not null"). The second one is the right for me.

@Entity
@Inheritance(strategy=InheritanceType.SINGLE_TABLE)
@DiscriminatorFormula("...")
@DiscriminatorValue("not null")
public class Animal implements Serializable {
  ...
  @Column
  public String getName() { ... }
}

edited May 23, 2017 at 12:14

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answered Jul 2, 2012 at 10:32

xmedeko

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2 Comments

tscho Over a year ago

Very nice! Can you provide a link where this is documented or is it only documented in the code?

xmedeko Over a year ago

I have found it in the code, see the class SingleTableEntityPersister. You can find some sources by Google search. Otherwise I cannot find it the official docs.

Vlad Mihalcea · Accepted Answer · 2016-07-19 12:43:14Z

5

Good news. I have just documented this behavior in the new User Guide. This is the JIRA issue.

Basically, you have two options:

@DiscriminatorValue("not null") as a fall-back strategy when there is no explicit discriminator value mapping matching the underlying database column value.
@DiscriminatorValue("null") for when the database column value is null. Usually, you'd use this for the base class.

There's a detailed example on the Hibernate blog as well.

edited Jul 19, 2016 at 12:43

answered Jul 18, 2016 at 15:01

Vlad Mihalcea

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