java - Hibernate, delete from collection -

i have database contain 2 tables connect one-to-many relationship. enteract database through hibernate.

this hbm.xml files:

question.hbm.xml

<?xml version="1.0" encoding="utf-8"?> <!doctype hibernate-mapping public "-//hibernate/hibernate mapping dtd 3.0//en" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">  <hibernate-mapping package="app">     <class name="question" table="question">         <id name="id" column="id" type="java.lang.long">             <generator class="native"/>         </id>         <property name="text" column="text" type="java.lang.string" not-null="true"/>                          <set name="answers" cascade="all">             <key column="question_id"/>             <one-to-many class="answer"/>         </set>                </class> </hibernate-mapping> 

answer.hbm.xml

<?xml version="1.0" encoding="utf-8"?> <!doctype hibernate-mapping public "-//hibernate/hibernate mapping dtd 3.0//en" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">  <hibernate-mapping package="app">       <class name="answer" table="answer">         <id name="id" column="id" type="java.lang.long">             <generator class="native"/>         </id>         <property name="text" column="text" type="java.lang.string" not-null="true"/>                 <many-to-one name="question" column="question_id" class="question" not-null="true"/>     </class> </hibernate-mapping> 

and java files:

question.java

package app;   import java.util.set;   public class question {     long id = null;     string text = "";     set<answer> answers = null;      // getters , setters } 

answer.java

package app;   public class answer {     long id = null;         string text = "";     boolean istrue = false;     question question = null;      // getters , setters } 

so question can have answers , question instance has collection fo answers instances.

this main method:

package application;   import app.answer; import app.question; import java.util.hashset; import org.hibernate.session; import org.hibernate.sessionfactory; import org.hibernate.cfg.configuration;   public class application {      public static void main(string[] args) {          sessionfactory sessionfactory = new configuration().configure().buildsessionfactory();         session session = sessionfactory.opensession();                question question = new question();                 question.settext("question1");          answer answer1 = new answer();                 answer1.settext("answer1");         answer1.setquestion(question);         answer answer2 = new answer();                 answer2.settext("answer2");         answer2.setquestion(question);          hashset<answer> answers = new  hashset<>();         answers.add(answer1);         answers.add(answer2);                 question.setanswers(answers);          session.begintransaction();                 try {                         session.saveorupdate(question);                session.gettransaction().commit();         } catch (exception ex) {             ex.printstacktrace();             session.gettransaction().rollback();         }          answers.remove(answer2);          session.begintransaction();                 try {                         session.saveorupdate(question);                session.gettransaction().commit();         } catch (exception ex) {             ex.printstacktrace();             session.gettransaction().rollback();         }     } } 

so create question instance, add 2 answers instances collection in question instance , save question instance in database. work correctly - question , answers writen database. remome 1 of answers instance collection in question instance , try save question instance in database again. catch exception. it's in native language, sqlserverexception , this: "can't insert null in column "question_id" in table "test.dbo.answer", in column null forbidden. error in update"

what's wrong? how can delete 1 of answer question?

in bidirectional relationship 1 side has inverse one. in one-to-many association one- side:

<set name="answers" cascade="all" inverse="true">   <key column="question_id"/>   <one-to-many class="answer"/> </set> 

this way many- side take care of relationship.

in case, both sides it, , since foreign key in answer class/table, hibernate updates null when flushing association changes in question class.