使用QBC的方式应用多对多关系中的查询

关系描述

         User实体和Group实体是多对多的关系，实体和表的对应关系如下：

Java Bean	表
User	tb_user
Group	tb_group

tb_user表的字为：ID,UserName,FirstName,LastName

tb_group表的字段为：ID,Name

其中关联关系中间表为 :tb_userGroup ，其字段为：ID,UserID(外键，引用tb_user表的id),GroupID(外键，引用tb_group表的id)

User实体的代码如下：                             

public class User{         private  Long  id ;         private String userName;         private String firstName;         private String lastName;         private Set<Group> groupList = new HashSet<Group>();         // Constructors         /** default constructor */       public  User() {       }         public  Long getId() {             return this.id;       }         public void setId(Long id) {             this.id = id;       }         public  String getUserName() {             return this.userName;       }         public void setUserName(String userName) {             this.userName = userName;       }         public  String getFirstName() {             return this.firstName;       }         public void setFirstName(String firstName) {             this.firstName = firstName;       }         public  String getLastName() {             return this.lastName;       }         public void setLastName(String lastName) {             this.lastName = lastName;       }         public  String getUsername() {             return this.userName;       }         public Set<Group> getGroupList() {             return  groupList ;       }         public void setGroupList(Set<Group> groupList) {             this.groupList = groupList;       }   }

 

Group实体代码如下:

public class Group implements java.io.Serializable {      private Long id;      private String name;      private Set<User> userList = new HashSet<User>();      // Constructors      /** default constructor */      public Group() {      }      public String getName() {           return this.name;      }      public Long getId() {           return id;      }      public void setId(Long id) {           this.id = id;      }      public void setName(String name) {           this.name = name;      }      public Set<User> getUserList() {           return userList;      }      public void setUserList(Set<User> userList) {           this.userList = userList;      } }

User实体的映射文件如下：

<? xml  version = "1.0"  encoding = "utf-8" ?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" > <!-- Mapping file autogenerated by MyEclipse Persistence Tools --> < hibernate-mapping >       <class name="com.xxx.User" table="tb_user">             <id name="id" type="java.lang.Long">                   <column name="ID" />                   <generator class="native" />             </id>             <property name="userName" type="java.lang.String">                   <column name="UserName" length="20" not-null="true" />             </property>             <property name="firstName" type="java.lang.String">                   <column name="FirstName" length="20" not-null="true" />             </property>             <property name="lastName" type="java.lang.String">                   <column name="LastName" length="20" not-null="true" />             </property>             <set name="groupList" inverse="true" table="tb_userGroup">                   <key>                         <column name="UserID" not-null="true" />                   </key>                   <many-to-many class="com.xxx. Group">                         <column name="GroupID" />                   </many-to-many>             </set>       </class> </ hibernate-mapping >

Group实体的映射文件如下：

<? xml  version = "1.0"  encoding = "utf-8" ?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" > <!-- Mapping file autogenerated by MyEclipse Persistence Tools --> < hibernate-mapping >       <class name="com.xxx. Group" table="tb_group">             <id name="id" type="java.lang.Long">                   <column name="ID" />                   <generator class="native" />             </id>             <property name="name" type="java.lang.String">                   <column name="Name" length="20" />             </property>             <set name="userList" table="tb_userGroup">                   <key>                         <column name="GroupID" not-null="true" />                   </key>                   <many-to-many class="com.xxx.User">                         <column name="UserID" not-null="true"/>                   </many-to-many>             </set>       </class> </ hibernate-mapping >

那么查询在某个Group的User集合的QBC写法为：

public Set<User> getUnBelongUserList(Long groupId) {             DetachedCriteria userCriteria = DetachedCriteria                         .forClass(TmsUser.class);             userCriteria.createAlias("groupList",  "groupList" , Criteria.LEFT_JOIN);             Property tempGroupId = Property.forName("groupList.id");             userCriteria.add(tempGroupId.eq(groupId)); return this.getHibernateTemplate().findByCriteria(userCriteria);      } 说明： l  createAlias 方法不光能为实体属性取别名，他同时也会立即加载该属性，相当于setFetchMode方法。这样写后Hibernate会关联去关联tb_userGroup和tb_group表 l  用面向对象的思想，通过User实体下的Group集合groupList.id来匹配查询条件，这样写后，Hibernate会判断groupList集合中，哪一个group的id等于参数groupId

生成的Sql语句大概为：

select         *     from         tb_user this_     left outer join         tb_userGroup grouplist3_             on this_.ID=grouplist3_.UserID     left outer join         tb_group grouplist1_             on grouplist3_.GroupID=grouplist1_.ID     where      grouplist1_.ID=?