JPA : N-1 Relationships

You have a N-to-1 relationship when an object of a class has an associated object of another class (only one associated object) and several of this type of object can be linked to the same associated object. From the other end of the relationship it is effectively a 1-N, but from the point of view of the object in question, it is N-1. You can create the relationship in 2 ways depending on whether the 2 classes know about each other (bidirectional), or whether only the "N" side knows about the other class (unidirectional). These are described below.


Unidirectional

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

so the Account class ("N" side) knows about the User class ("1" side), but not vice-versa, and are using a join table. A particular user could be related to several accounts. If you define the Meta-Data for these classes as follows

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

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

alternatively using annotations

public class Account
{
    ...

    @ManyToOne
    @JoinTable(name="ACCOUNT_USER")
    User user;
}

This will create 3 tables in the database, one for User (with name USER), one for Account (with name ACCOUNT), and a join table (with name ACCOUNT_USER), as shown below.

Note that in the case of non-RDBMS datastores there is no join-table, simply a "column" in the ACCOUNT "table", storing the "id" of the related object


Bidirectional

This relationship is described in the guide for 1-N relationships. In particular there are 2 ways to define the relationship for RDBMS : the first uses a Join Table to hold the relationship, whilst the second uses a Foreign Key in the "N" object to hold the relationship. For non-RDBMS datastores each side will have a "column" (or equivalent) in the "table" of the N side storing the "id" of the related (owning) object. Please refer to the 1-N relationships bidirectional relations since they show this exact relationship.