java - cascade save in hibernate, foregin key is saved as null -

i have class follows

class a{  set<b> = new hashset<b>();  }  class b{   a;  } 

now primary key of autogenerated cant set before in or b.b has inverse mapping of a.and object within b null originally. maps 2 tables , b in db.now if have object set contains 2 records when save object 2 records gets created in b.now code working fine.but when see recods in b find foreign key corresponding blank.how come?

table b structure bid aid bname   

//aid foreign key tablles primary key , aid stored null, why so?i want aid should automatically stored in b

you need correctly set both side of relation

parenta.getbchilds().add(childb);   childb.setparenta(parenta); <-- important see link below working bi 

you can create link management methods in parent correctly set both sides.

public class parenta { ...     public void addchildb(b pchild) {     this.childsb.add(pchild);     pchild.setparenta( ); }     ... } 

set 'inverse' attribute true on collection relation

<set name="bchilds" inverse="true">     <key column="bid"/>     <one-to-many class="childb"/> </set> 

maybe you're mixing between inverse , cascade. explanation can read blog entry on hibernate inverse attribute hibernates bizarre interpretation of inverse ;) , hibernate doc on working bi-directional links