JPA : 1-1 Relationships

You have a 1-to-1 relationship when an object of a class has an associated object of another class (only one associated object). It could also be between an object of a class and another object of the same class (obviously). You can create the relationship in 2 ways depending on whether the 2 classes know about each other (bidirectional), or whether only one of the classes knows about the other class (unidirectional). These are described below.

For RDBMS a 1-1 relation is stored as a foreign-key column(s). For non-RDBMS it is stored as a String "column" storing the 'id' (possibly with the class-name included in the string) of the related object.


Unidirectional

For this case you could have 2 classes, User and Account, as below.

so the Account class knows about the User class, but not vice-versa. If you define the XML metadata for these classes as follows 

<entity-mappings>
    <entity class="User">
        <table name="USER"/>
        <attributes>
            <id name="id">
                <column name="USER_ID"/>
            </id>
            ...
    </entity>

    <entity class="Account">
        <table name="ACCOUNT"/>
        <attributes>
            <id name="id">
                <column name="ACCOUNT_ID"/>
            </id>
            ...
            <one-to-one name="user">
                <join-column name="USER_ID"/>
            </one-to-one>
        </attributes>
    </entity>
</entity-mappings>

or alternatively using annotations 

public class Account
{
    ...

    @OneToOne
    @JoinColumn(name="USER_ID")
    User user;
}

public class User
{
    ...
}

This will create 2 tables in the database, one for User (with name USER), and one for Account (with name ACCOUNT and a column USER_ID), as shown below.

Things to note :-

  • Account has the object reference to User (and so is the "owner" of the relation) and so its table holds the foreign-key
  • If you call EntityManager.remove() on the end of a 1-1 unidirectional relation without the relation and that object is related to another object, an exception will typically be thrown (assuming the RDBMS supports foreign keys). To delete this record you should remove the other objects association first.

Bidirectional

For this case you could have 2 classes, User and Account again, but this time as below. Here the Account class knows about the User class, and also vice-versa.


We create the 1-1 relationship with a single foreign-key. To do this you define the XML metadata as 

<entity-mappings>
    <entity class="User">
        <table name="USER"/>
        <attributes>
            <id name="id">
                <column name="USER_ID"/>
            </id>
            ...
            <one-to-one name="account" mapped-by="user"/>
        </attributes>
    </entity>

    <entity class="Account">
        <table name="ACCOUNT"/>
        <attributes>
            <id name="id">
                <column name="ACCOUNT_ID"/>
            </id>
            ...
            <one-to-one name="user">
                <join-column name="USER_ID"/>
            </one-to-one>
        </attributes>
    </entity>
</entity-mappings>

or alternatively using annotations 

public class Account
{
    ...

    @OneToOne
    @JoinColumn(name="USER_ID")
    User user;
}

public class User
{
    ...

    @OneToOne(mappedBy="user")
    Account account;

    ...
}
The difference is that we added mapped-by to the field of User making it bidirectional (and putting the FK at the other side for RDBMS)

This will create 2 tables in the database, one for User (with name USER), and one for Account (with name ACCOUNT). For RDBMS it includes a USER_ID column in the ACCOUNT table, like this


For other types of datastore it will have a USER_ID column in the ACCOUNT table and a ACCOUNT column in the USER table.

Things to note :-

  • When forming the relation please make sure that you set the relation at BOTH sides since DataNucleus would have no way of knowing which end is correct if you only set one end.