I can understand relationships between entities (and classes) easily from a DDL or a UML diagram. But Hibernate confuses me totally. I've tried generating various DDL's but when I think I understand, I get stuck again.

Could someone explain how to realize the following in Hibernate mapping files? I'd like an explanation on these cases with and without bi-directionality as well.

I guess this is a lot to ask, but trying doesn't hurt... ;-)

Using UML notation:



I think that pretty much cover all the cases.

An example on MANY-TO-MANY (your case 4)

User 0...* ---------> 0...* Group (many-to-many)

Schema:

table user (
id integer PRIMARY KEY,
login VARCHAR(100),
...
)

table group (
id integer PRIMARY KEY,
name VARCHAR(100),
....
)

table user_group (
user_id INTEGER,
group_id INTEGER,
CONSTRAINT ug_pkey PRIMARY KEY (user_id,group_id),
CONSTRAINT ug_u_fkey FOREIGN KEY (user_id) REFERENCES user(id),
CONSTRAINT ug_g_fkey FOREIGN KEY (group_id) REFERENCES group(id)
)

Mapping:

<class name="com.estobel.model.User" table="user">

<id name="id" type="int" unsaved-value="null" >
<column name="id" sql-type="INTEGER" not-null="true"/>
<generator class="assigned"/>
</id>

<property name="login">
<column name="login" sql-type="varchar(20)" not-null="true" unique="true"/>
</property>

<set name="groups" table="user_group" lazy="true">
<key>
<column name="user_id" not-null="true"/>
</key>
<many-to-many class="com.estobel.model.Group">
<column name="group_id" not-null="true"/>
</many-to-many>
</set>
</class>

<class name="com.estobel.model.Group" table="group">

<id name="id" type="int" unsaved-value="null" >
<column name="id" sql-type="INTEGER" not-null="true"/>
<generator class="assigned"/>
</id>

<!-- If you want an bidirecional .... -->
<set name="users" table="user_group" inverse="true" lazy="true">
<key>
<column name="group_id" not-null="true"/>
</key>
<many-to-many class="com.estobel.model.User">
<column name="user_id" not-null="true"/>
</many-to-many>
</set>

</class>

Beans:

public class User {
int id;
String name;
Set groups;
.....
}

public class Group {
int id;
String name;
Set users; // If you want an bidirecional
}

An example on MANY-TO-ONE.

Query ------------ 0...1 DataSource

I have many datasources and I can use them in many queries ... an query has one datasource.

Schema:

table query (
id INTEGER,
datasource_name VARCHAR(20),
CONSTRAINT query_ds_fkey FOREIGN KEY (datasource_name) REFERENCES datasource(name)
)

table datasource (
name VARCHAR(20),
url VARCHAR(200)
)

Mapping:

<class name="com.estobel.model.Query" table="query">
<id name="id" type="int" column="id">
<generator class="native">
<param name="sequence">nom_query_id_seq</param>
</generator>

</id>

<many-to-one name="dataSource" column="datasource_name" class="com.estobel.model.DataSource"/>
</class>

<class name="com.estobel.model.DataSource" table="nom_datasource">
<id name="name" type="string">
<column name="name" length="20"/>
<generator class="assigned"/>
</id>
<property name="url" column="username" type="string"/>
</class>

Beans:

public class Query {
int id;
DataSource datasource;
}

public class DataSource {
String name;
String url;
}

An example ONE-TO-MANY ...

query 1 -------- 0...* parameter

I have a query that can have many parameters ... parameters exists only for that query:

Schema:

table query (
id INTEGER
name VARCHAR
)

table query_parameter (
query_id INTEGER NOT NULL,
param_id INTEGER NOT NULL,
name VARCHAR(100),
CONSTRAINT qp_pkey PRIMARY KEY (query_id,param_id),
CONSTRAINT qp_q_fkey FOREIGN KEY (query_id) REFERENCES nom_query(id)
)

Mapping:

<class name="com.estobel.model.Query" table="query">
<id name="id" type="int" column="id">
<generator class="native">
<param name="sequence">nom_query_id_seq</param>
</generator>

</id>
<set name="parameters" table="query_parameter" inverse="true" lazy="true"
order-by="param_id asc" cascade="delete">
<key>
<column name="query_id"/>
</key>
<one-to-many class="com.estobel.model.QueryParameter"/>
</set>

</class>

<class name="com.estobel.model.QueryParameter" table="nom_query_parameter">
<!-- The parameter id is a composite id -->
<composite-id>
<key-property name="id" column="param_id" type="int"/>
<key-many-to-one name="query" class="com.estobel.model.Query" column="query_id"/>
</composite-id>


<property name="name" />
</class>


Beans:

public class Query {
int id;
Set parameters;
}

public class QueryParameter {
int id;
Query query;
String name;
}

I took these cases from my application ... I think that is a good start to clarify some doubts.

THANKS! That was very useful.

Could you explain inverse="true" in UML terms as well?