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You have a 1-N (one to many) or N-1 (many to one) when you have one object of a class that has a Set of objects of
another class.
Please note that Sets do not 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 e 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 Set.
The various possible relationships are described below.
This page is aimed at Set fields and so applies to fields of Java type
java.util.HashSet, java.util.LinkedHashSet, java.util.Set, java.util.SortedSet, java.util.TreeSet
We have 2 sample classes
Account
and
Address
. These are related in such a way as
Account
contains a
Set
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
If you define the Meta-Data for these classes as follows
<package name="com.mydomain">
<class name="Account">
<field name="id" primary-key="true">
<column name="ACCOUNT_ID"/>
</field>
...
<field name="addresses" persistence-modifier="persistent">
<collection element-type="com.mydomain.Address"/>
<join/>
</field>
</class>
<class name="Address">
<field name="id" primary-key="true">
<column name="ADDRESS_ID"/>
</field>
...
</class>
</package>
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The crucial part is the
join
element on the field element - this signals to JDO
to use a join table.
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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
attribute on the
class
element
-
To specify the names of the columns where the fields of a class are stored, specify the
column
attribute
on the
field
element.
-
To specify the name of the join table, specify the
table
attribute on the
field
element
with the collection.
-
To specify the names of the join table columns, use the
column
attribute of
join
,
element
elements.
-
To specify the foreign-key between container table and join table, specify <foreign-key> below
the <join> element.
-
To specify the foreign-key between join table and element table, specify <foreign-key> below
either the <field> element or the <element> element.
-
If you wish to share the join table with another relation then use the
DataNucleus "shared join table" extension
-
The join table will, by default, be given a primary key. If you want to omit this then you can turn it off
using the DataNucleus metadata extension "primary-key" (within <join>) set to false.
-
If you want the set to include nulls, you can turn on this behaviour by adding the extension
metadata "allow-nulls" to the <field> set to true
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
<package name="com.mydomain">
<class name="Account">
<field name="id" primary-key="true">
<column name="ACCOUNT_ID"/>
</field>
...
<field name="addresses" persistence-modifier="persistent">
<collection element-type="com.mydomain.Address"/>
<element column="ACCOUNT_ID"/>
</field>
</class>
<class name="Address">
<field name="id" primary-key="true">
<column name="ADDRESS_ID"/>
</field>
...
</class>
</package>
Again there will be 2 tables, one for
Address
, and one for
Account
.
Note that we have no "mapped-by" attribute specified, and also no "join" element. If you wish to specify
the names of the columns used in the schema for the foreign key in the
Address
table you should use
the
element
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
. Also be aware that each
Address
object can have only one owner,
since it has a single foreign key to the
Account
. If you wish to have an
Address
assigned to multiple
Accounts
then you should use the "Join Table" relationship above.
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
attribute on the
class
element
-
To specify the names of the columns where the fields of a class are stored, specify the
column
attribute
on the
field
element.
-
To specify the foreign-key between container table and element table, specify <foreign-key> below
either the <field> element or the <element> element.
We have 2 sample classes
Account
and
Address
. These are related in such a way as
Account
contains a
Set
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
If you define the Meta-Data for these classes as follows
<package name="com.mydomain">
<class name="Account">
<field name="id" primary-key="true">
<column name="ACCOUNT_ID"/>
</field>
...
<field name="addresses" persistence-modifier="persistent" mapped-by="account">
<collection element-type="com.mydomain.Address"/>
<join/>
</field>
</class>
<class name="Address">
<field name="id" primary-key="true">
<column name="ADDRESS_ID"/>
</field>
...
<field name="account" persistence-modifier="persistent">
</field>
</class>
</package>
|
The crucial part is the
join
element on the field element - this signals to JDO
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
attribute on the
class
element
-
To specify the names of the columns where the fields of a class are stored, specify the
column
attribute
on the
field
element.
-
To specify the name of the join table, specify the
table
attribute on the
field
element
with the collection.
-
To specify the names of the join table columns, use the
column
attribute of
join
,
element
elements.
-
To specify the foreign-key between container table and join table, specify <foreign-key> below
the <join> element.
-
To specify the foreign-key between join table and element table, specify <foreign-key> below
either the <field> element or the <element> element.
-
If you wish to share the join table with another relation then use the
DataNucleus "shared join table" extension
-
The join table will, by default, be given a primary key. If you want to omit this then you can turn it off
using the DataNucleus metadata extension "primary-key" (within <join>) set to false.
-
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.
-
If you want the set to include nulls, you can turn on this behaviour by adding the extension
metadata "allow-nulls" to the <field> set to true
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
<package name="com.mydomain">
<class name="Account">
<field name="id" primary-key="true">
<column name="ACCOUNT_ID"/>
</field>
...
<field name="addresses" persistence-modifier="persistent" mapped-by="account">
<collection element-type="com.mydomain.Address"/>
</field>
</class>
<class name="Address">
<field name="id" primary-key="true">
<column name="ADDRESS_ID"/>
</field>
...
<field name="account" persistence-modifier="persistent">
<column name="ACCOUNT_ID"/>
</field>
</class>
</package>
|
The crucial part is the
mapped-by
attribute of the field on the "1" side of the
relationship. This tells the JDO 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
attribute on the
class
element
-
To specify the names of the columns where the fields of a class are stored, specify the
column
attribute
on the
field
element.
-
To specify the foreign-key between container table and element table, specify <foreign-key> below
either the <field> element or the <element> 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.
All of the examples above show a 1-N relationship between 2
PersistenceCapable
classes. DataNucleus can
also cater for a Collection of primitive or Object types. 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 Meta-Data like this
<package name="com.mydomain">
<class name="Account">
<field name="id" primary-key="true">
<column name="ACCOUNT_ID"/>
</field>
...
<field name="addresses" persistence-modifier="persistent">
<collection element-type="java.lang.String"/>
<join/>
<element column="ADDRESS"/>
</field>
</class>
In the datastore the following is created
The ACCOUNT table is as before, but this time we only have the "join table". In our MetaData we used the <element>
tag to specify the column name to use for the actual address String.
Please note that the column ADPT_PK_IDX is added by DataNucleus when the column type of the element is not valid to be
part of a primary key (with the RDBMS being used). If the column type of your element is acceptable for use as part
of a primary key then you will not have this "ADPT_PK_IDX" column.
You can override the default naming of this column by specifying the DataNucleus extension "adapter-column-name" within
the <field> for the Collection.
The above relationship types assume that both classes in the 1-N relation will have their own table.
A variation on this is where you have a join table but you embed the elements of the collection into
this join table. To do this you use the
embedded-element
attribute on the
collection
MetaData element.
This is described in Embedded Collection Elements.
The above relationship types assume that both classes in the 1-N relation will have their own table.
A variation on this is where you have a join table but you serialise the elements of the collection into
this join table in a single column. To do this you use the
serialised-element
attribute on the
collection
MetaData element.
This is described in Serialised Collection Elements
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