What is discriminator in Hibernate?

What is discriminator in Hibernate?

In case of table per class hierarchy an discriminator column is added by the hibernate framework that specifies the type of the record. It is mainly used to distinguish the record. To specify this, discriminator subelement of class must be specified. The subclass subelement of class, specifies the subclass.

What are the types of inheritance in Hibernate?

There are three inheritance mapping strategies defined in the hibernate:

• Table Per Hierarchy.
• Table Per Concrete class.
• Table Per Subclass.

What is a discriminator column?

The discriminator column is always in the table of the base entity. It holds a different value for records of each class, allowing the JPA runtime to determine what class of object each row represents. The DiscriminatorColumn annotation represents a discriminator column.

How do you find the discriminator column?

The entity definition: @Entity(name=”Port”) @DiscriminatorColumn(name=”type”, discriminatorType=DiscriminatorType. STRING, length=10) @DiscriminatorValue(value=”port”) @Table(name=”vPorts”) @XmlRootElement(name=”port”) public class PortEntity { …

Which persistent strategy uses discriminator?

The javax. persistence. DiscriminatorValue annotation may be used to set the value entered into the discriminator column for each entity in a class hierarchy. You may decorate only concrete entity classes with @DiscriminatorValue.

What is a discriminator value?

The discriminator value is used to define the type of each row. The discriminator value can be generated using 3 different strategies in the backend configuration wizard: {hash} –the default strategy. When {hash} or nothing is specified the discriminator column value will be the same as the class id.

What is collection mapping in Hibernate?

Hibernate supports collection mapping as value type. In this mapping, the collection are mapped into a separate table. When we have to persist property of a class, which is of data type like integer, long, float etc. or objects of wrapper classes or Strings, these properties are stored in the table of class directly.

Which inheritance mapping strategy uses discriminator column?

Single Table Inheritance. Single Table Inheritance is an inheritance mapping strategy where all classes of a hierarchy are mapped to a single database table. In order to distinguish which row represents which type in the hierarchy a so-called discriminator column is used.

What is Cascade in Hibernate?

Cascading is a feature in Hibernate, which is used to manage the state of the mapped entity whenever the state of its relationship owner (superclass) affected. When the relationship owner (superclass) is saved/ deleted, then the mapped entity associated with it should also be saved/ deleted automatically.

What is a discriminator in database?

The discriminator (or partial key) of a weak entity set is the set of attributes that distinguishes among all the entities of a weak entity set on one particular strong entity. The primary key of a weak entity set.

What is a discriminator key?

The partial Key of the weak entity set is also known as a discriminator. It is just a part of the key as only a subset of the attributes can be identified using it. It is partially unique and can be combined with other strong entity set to uniquely identify the tuples.

What is mapping in Hibernate?

hibernate mappings are one of the key features of hibernate . they establish the relationship between two database tables as attributes in your model. that allows you to easily navigate the associations in your model and criteria queries. one to one — it represents the one to one relationship between two tables.

What is the use of discriminator in inheritance?

Discriminator is commonly used in SINGLE_TABLE inheritance because you need a column to identify the type of the record. Example: You have a class Student and 2 sub-classes: GoodStudent and BadStudent.

Does hibernate index the Discriminator column in JPQL queries?

However, for JPQL query such as this one: Hibernate generates a SQL query which filters by the associated discriminator column (e.g. DTYPE by default): So, because we are filtering by the discriminator column, we might want to index it or include it to speed up queries.

How to understand discriminator in SQL?

To understand discriminator, first you must understand the inheritance strategies: SINGLE_TABLE, JOINED, TABLE_PER_CLASS. Discriminator is commonly used in SINGLE_TABLE inheritance because you need a column to identify the type of the record. Example: You have a class Student and 2 sub-classes: GoodStudent and BadStudent.

When to use the Discriminator column in a single table?

When you have an entity inheritance using the single table strategy, and you want the value of the discriminator column to be something other than the name of the class of the entity concrete class, or when the type of the discriminator column is not STRING. This is explained, with an example, in the javadoc.