net.sf.hibernate.JDBCException: Could not synchronize databa

net.sf.hibernate.JDBCException: Could not synchronize database state with session: General error: Column 'ClassesId' cannot be null

为什么会出现这种同步数据库的错误

<?xml version="1.0"?>

<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 2.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">

<hibernate-mapping>
    <class
        name="com.hellking.study.hibernate.Student"
        table="Students"
        dynamic-update="false"
    >

        <id
            name="id"
            column="StudentId"
            type="string"
            unsaved-value="any"
        >
            <generator class="assigned"/>
        </id>

         <property
            name="name"
            type="string"
            update="true"
            insert="true"
            column="Name"
        />
<property
name="classid"
type="string"
update="true"
insert="true"
column="ClassesId"
/>


         <set
            name="courses"
            table="Student_Course_Link"
            lazy="false"
            inverse="false"
            cascade="all"
            sort="unsorted"
          >

              <key
                  column="StudentId"
              />

  <many-to-many
    class="com.hellking.study.hibernate.Course"
    column="CourseId"
    outer-join="auto"
  />

        </set>
       
       
          <many-to-one
            name="classes"
            class="com.hellking.study.hibernate.Classes"
            cascade="none"
            outer-join="auto"
            update="true"
            insert="true"
            column="ClassesId"
        />
       
         <one-to-one
            name="address"
            class="com.hellking.study.hibernate.Address"
            cascade="none"
            outer-join="auto"
            constrained="false"
         />      
    </class>

</hibernate-mapping>
我的文件是从本网站上down下来的，之后改了一个配置，如下
我在这里面又加了一个ClassesId这个property

哪位高手能帮忙啊

为什么要这样？ClassesId列不是已经映射了吗？

我加的那一段已经删掉了，但是还是没有解决啊

那个ClassId通过页面从class表里得到的