JPA 1-N Relationships with Collections

You have a 1-N (one to many) when you have one object of a class that has a Collection of objects of another class. Please note that Collections allow duplicates, and so the persistence process reflects this with the choice of primary keys . There are two ways in which you can represent this in a datastore : Join Table (where a join table is used to provide the relationship mapping between the objects), and Foreign-Key (where a foreign key is placed in the table of the object contained in the Collection.

The various possible relationships are described below.



1-N Collection Unidirectional

We have 2 sample classes Account and Address . These are related in such a way as Account contains a Collection of objects of type Address , yet each Address knows nothing about the Account objects that it relates to. Like this

There are 2 ways that we can persist this relationship. These are shown below

Using Join Table

If you define the Meta-Data for these classes as follows 

<entity-mappings>
    <entity class="Account">
        <table name="ACCOUNT"/>
        <attributes>
            <id name="id">
                <column name="ACCOUNT_ID"/>
            </id>
            ...
            <one-to-many name="addresses" target-entity="com.mydomain.Address">
                <join-table name="ACCOUNT_ADDRESSES">
                    <join-column name="ACCOUNT_ID_OID"/>
                    <inverse-join-column name="ADDRESS_ID_EID"/>
                </join-table>
            </one-to-many>
        </attributes>
    </entity>

    <entity class="Address">
        <table name="ADDRESS"/>
        <attributes>
            <id name="id">
                <column name="ADDRESS_ID"/>
            </id>
            ...
        </attributes>
    </entity>
</entity-mappings>
The crucial part is the join-table element on the field element - this signals to JPA to use a join table.

This will create 3 tables in the database, one for Address , one for Account , and a join table, as shown below.





The join table is used to link the 2 classes via foreign keys to their primary key. This is useful where you want to retain the independence of one class from the other class.

If you wish to fully define the schema table and column names etc, follow these tips

  • To specify the name of the table where a class is stored, specify the table element below the class element
  • To specify the names of the columns where the fields of a class are stored, specify the column attribute on the basic element.
  • To specify the name of the join table, specify the join-table element below the one-to-many element with the collection.
  • To specify the names of the join table columns, use the join-column and inverse-join-column elements below the join-table element.
  • The join table will NOT be given a primary key (since a Collection can have duplicates).


Using Foreign-Key

In strict JPA1 you cannot have a 1-N unidirectional relation using a ForeignKey - they must have a JoinTable. Consequently to use this relation as specified below you must specify the property "datanucleus.jpa.level" as "JPA2"/"DataNucleus"

In this relationship, the Account class has a List of Address objects, yet the Address knows nothing about the Account . In this case we don't have a field in the Address to link back to the Account and so DataNucleus has to use columns in the datastore representation of the Address class. So we define the MetaData like this 

<entity-mappings>
    <entity class="Account">
        <table name="ACCOUNT"/>
        <attributes>
            <id name="id">
                <column name="ACCOUNT_ID"/>
            </id>
            ...
            <one-to-many name="addresses" target-entity="com.mydomain.Address">
                <join-column name="ACCOUNT_ID"/>
            </one-to-many>
        </attributes>
    </entity>

    <entity class="Address">
        <table name="ADDRESS"/>
        <attributes>
            <id name="id">
                <column name="ADDRESS_ID"/>
            </id>
            ...
        </attributes>
    </entity>
</entity-mappings>

There will be 2 tables, one for Address , and one for Account . If you wish to specify the names of the column(s) used in the schema for the foreign key in the Address table you should use the join-column element within the field of the collection.





In terms of operation within your classes of assigning the objects in the relationship. You have to take your Account object and add the Address to the Account collection field since the Address knows nothing about the Account .

If you wish to fully define the schema table and column names etc, follow these tips

  • To specify the name of the table where a class is stored, specify the table element below the class element
  • To specify the names of the columns where the fields of a class are stored, specify the column attribute on the basic element.

Limitation : Since each Address object can have at most one owner (due to the "Foreign Key") this mode of persistence will not allow duplicate values in the Collection. If you want to allow duplicate Collection entries, then use the "Join Table" variant above.



1-N Collection Bidirectional

We have 2 sample classes Account and Address . These are related in such a way as Account contains a Collection of objects of type Address , and each Address has a reference to the Account object that it relates to. Like this

There are 2 ways that we can persist this relationship. These are shown below

Using Join Table

If you define the Meta-Data for these classes as follows 

<entity-mappings>
    <entity class="Account">
        <table name="ACCOUNT"/>
        <attributes>
            <id name="id">
                <column name="ACCOUNT_ID"/>
            </id>
            ...
            <one-to-many name="addresses" target-entity="com.mydomain.Address" mapped-by="account">
                <join-table name="ACCOUNT_ADDRESSES">
                    <join-column name="ACCOUNT_ID_OID"/>
                    <inverse-join-column name="ADDRESS_ID_EID"/>
                </join-table>
            </one-to-many>
        </attributes>
    </entity>

    <entity class="Address">
        <table name="ADDRESS"/>
        <attributes>
            <id name="id">
                <column name="ADDRESS_ID"/>
            </id>
            ...
            <many-to-one name="account">
            </many-to-one>
        </attributes>
    </entity>
</entity-mappings>
The crucial part is the join element on the field element - this signals to JPA to use a join table.

This will create 3 tables in the database, one for Address , one for Account , and a join table, as shown below.





The join table is used to link the 2 classes via foreign keys to their primary key. This is useful where you want to retain the independence of one class from the other class.

If you wish to fully define the schema table and column names etc, follow these tips

  • To specify the name of the table where a class is stored, specify the table element below the class element
  • To specify the names of the columns where the fields of a class are stored, specify the column attribute on the basic element.
  • To specify the name of the join table, specify the join-table element below the one-to-many element with the collection.
  • To specify the names of the join table columns, use the join-column and inverse-join-column elements below the join-table element.
  • The join table will NOT be given a primary key (since a Collection can have duplicates).
  • 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.


Using Foreign-Key

Here we have the 2 classes with both knowing about the relationship with the other.

If you define the Meta-Data for these classes as follows 

<entity-mappings>
    <entity class="Account">
        <table name="ACCOUNT"/>
        <attributes>
            <id name="id">
                <column name="ACCOUNT_ID"/>
            </id>
            ...
            <one-to-many name="addresses" target-entity="com.mydomain.Address" mapped-by="account">
                <join-column name="ACCOUNT_ID"/>
            </one-to-many>
        </attributes>
    </entity>

    <entity class="Address">
        <table name="ADDRESS"/>
        <attributes>
            <id name="id">
                <column name="ADDRESS_ID"/>
            </id>
            ...
            <many-to-one name="account">
            </many-to-one>
        </attributes>
    </entity>
</entity-mappings>
The crucial part is the mapped-by attribute of the field on the "1" side of the relationship. This tells the JPA implementation to look for a field called account on the Address class.

This will create 2 tables in the database, one for Address (including an ACCOUNT_ID to link to the ACCOUNT table), and one for Account . Notice the subtle difference to this set-up to that of the Join Table relationship earlier.





If you wish to fully define the schema table and column names etc, follow these tips

  • To specify the name of the table where a class is stored, specify the table element below the class element
  • To specify the names of the columns where the fields of a class are stored, specify the column attribute on the basic element.
  • 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.

Limitation : Since each Address object can have at most one owner (due to the "Foreign Key") this mode of persistence will not allow duplicate values in the Collection. If you want to allow duplicate Collection entries, then use the "Join Table" variant above.



1-N Collection of non-Entity objects

In JPA1 you cannot have a 1-N collection of non-Entity objects. This is available in JPA2. All of the examples above show a 1-N relationship between 2 PersistenceCapable classes. If you want the element to be primitive or Object types then follow this section. For example, when you have a Collection of Strings. This will be persisted in the same way as the "Join Table" examples above. A join table is created to hold the collection elements. Let's take our example. We have an Account that stores a Collection of addresses. These addresses are simply Strings. We define the annotations like this 

@Entity
public class Account
{
    ...

    @ElementCollection(targetClass=String.class)
    @CollectionTable(name="ACCOUNT_ADDRESSES")
    Collection addresses;
}

In the datastore the following is created

The ACCOUNT table is as before, but this time we only have the "join table". Use @Column on the field/method to define the column details of the element in the join table.