c# - NHibernate save object, a column is null -

the database has 2 table,one mb_user table , other mb_comment table。a user have comments.when insert comment mb_comment ,the comment_user_id of mb_comment null.i can't save value of comment_user_id.please me!

map file mb_user map file

<?xml version="1.0" encoding="utf-8" ?> <hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="miserblogdata" namespace="miserblogdata.entities" default-lazy="false">   <class name="a_person" table="mb_user" discriminator-value="0">     <id name="id" column ="user_id" >       <generator class ="native"/>     </id>     <discriminator column="user_role" type="int" />     <property name="state" column="user_state" />     <property name ="name" column="user_name" />     <property name ="pwd" column="user_pwd" />     <property name ="cdate" column="user_cdate" />     <property name ="udate" column="user_udate" />     <property name ="role" column="user_role" />     <property name ="face" column="user_face" />     <bag name="commentlist"  >       <key column="comment_user_id" foreign-key="fk_personcomment" />       <one-to-many class="comment" />     </bag>   </class> </hibernate-mapping> 

mb_comment map file

<?xml version="1.0" encoding="utf-8" ?> <hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="miserblogdata" namespace="miserblogdata.entities">   <class name ="comment" table="mb_comment">     <id name="id" column ="comment_id">       <generator class ="native"/>     </id>     <property name ="articleid" column="comment_article_id" />     <property name ="content" column="comment_content" />     <property name ="state" column="comment_state" />     <property name ="cdate" column="comment_cdate" />      <many-to-one name="person"  column="comment_user_id" class="a_person" not-null="true" foreign-key="fk_personcomment"   />   </class> </hibernate-mapping> 

c# code

    protected isession _session = nhibernatehelper.getcurrentsession();      public virtual object save<t>(t obj) t : class     {         return _session.save(obj);     } 

error :not-null property references null or transient valuemiserblogdata.entities.comment.person

i know,if remove not-null="true" in mb_comment file ,system good.but comment_user_id column null.how solve？

you should assign property person on comment valid a_person instance. nhibernate know user id write in table.