I'm working w/legacy data.
One table (A) has a foreign key (with possible empty string values) that maps to a row on another table (B)
Here are the mappings:

(A)
<hibernate-mapping>
<class name="com.amdr.business.model.A table="A">
<cache usage="read-only"/>
<composite-id>
<key-property name="idcodapli"></key-property>
<key-property name="nocod"></key-property>
<key-property name="state"></key-property>
<key-property name="date"></key-property>
</composite-id>
<many-to-one name="reason" column="idReason" not-null="false" class="com.amdr.business.model.B" outer-join="true" />
</hibernate-mapping>

(B)
<hibernate-mapping>
<class name="com.amdr.business.model.B" table="B" >
<id name="idReason" type="string" >
<generator class="assigned"></generator>
</id>
<property column="Description" length="60" name="descripcion" type="string"/>
</class>
</hibernate-mapping>

(the name of the FK in A matchs the name of the PK in B)
Whenever I try to load rows from A I get:
net.sf.hibernate.UnresolvableObjectException: No row with the given identifier exists: " " (empty string) , of class: com.amdr.business.model.B

I need to load thoose values (from B) whenever the FK is not empty.
In sql I achieve this by using LEFT JOIN.

I'd appreciate any help.
Thnx in advance.
-stn

Can't you use NULLs instead of empty string value?
Empty string is a valid string and it is just refering to record with empty string ID in another table.

i'm working with legacy data.
I suppose that I could convert empty string values to null.
But if I can get what I want in SQL/HQL with LEFT JOIN I guess is possible with class mapping. Am I wrong?

I don't think this is doable. As dimas pointed, "" is a valid FK. Even with 2 SQL statement instead of LEFT JOIN, Hibernate will try to load A with "" PK and fail.