Many to Many Relationship using junction table..

Question

Hello im testing a many to many relation ship on j2ee and found this issue.
Creating a relation ship table until now without succes to update the relationship table:
code fragment as is:
On application flow my session bean call the following method according with the specification i have no reason to call persist in my writer just on parent Object: Any clue on what is wrong ?
@TransactionAttribute(TransactionAttributeType.REQUIRED)
public void saveWriter (Book book , Writer writer){
try {
Book atBook = entityManager.merge(book);
Writer atWriter = entityManager.merge(writer);
book.getWriters().add(atWriter);
entityManager.persist(atBook);
entityManager.flush();
}catch (Exception ex)
ex.printStackTrace();
<entity class="com.octech.biblio.domain.Book" name="book">
<description></description>
<table name="books" />
<attributes>
<id name="id">
<generated-value strategy="IDENTITY"/>
</id>
<basic fetch="EAGER" name="bookName">
<column insertable="true" length="128" name="bookname" table="" updatable="true"/>
</basic>
<many-to-one name="Library" fetch="LAZY" target-entity="com.octech.biblio.domain.Library">
<join-table name="library_books">
<join-column name="books_id"/>
<inverse-join-column name="library_id"/>
</join-table>
<cascade/>
</many-to-one>
<many-to-many name="Writers" fetch="LAZY" target-entity="com.octech.biblio.domain.Writer">
<join-table name="writers_books">
<join-column name="books_id"/>
<inverse-join-column name="writer_id"/>
</join-table>
<cascade/>
</many-to-many>
</attributes>
</entity>
<entity class="com.octech.biblio.domain.Writer" name="writer">
<description></description>
<table name="writers" />
<attributes>
<id name="id">
<generated-value strategy="IDENTITY"/>
</id>
<basic fetch="EAGER" name="writerName">
<column insertable="true" length="128" name="writer_name" table="" updatable="true"/>
</basic>
<many-to-many name="Books" fetch="LAZY" target-entity="com.octech.biblio.domain.Book">
<join-table name="writers_books">
<join-column name="writer_id"/>
<inverse-join-column name="books_id"/>
</join-table>
<cascade/>
</many-to-many>
</attributes>
</entity>

Answer 1

sybrand_b wrote:
The way you write it yes, but there is one minor detail.
You can have a 0,1 or many relationship: one employee has zero, one or many phone numbers
but you can not have the opposite, as it doesn't make sense.
0, 1 or many phone numbers belong to 1 employee.
It is customary to use 0,1 or m when the relationship is optional.
Sybrand Bakker
Senior Oracle DBAIs this correct now ?
one to many : one employee has 0,1 or multiple phone numbers
many to one : 1 or multiple phone numbers to one employee