老师快帮帮我，mybatis的association关联查询不执行

在查询科室的时候我想用association关联查询，把科室相应的分类也查询出来，结果就是查询不出来，

然后我用log4j的DEBUG模式查看了一下，结果发现association关联查询根本就没有执行。。。。。。。

下面是相应的两个映射文件

===============================================================

DepartmentDao.xml

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.4//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="hdm.dao.DepartmentDao">
 

  <resultMap id="resultMap" type="Department">
     <id property="id" column="id" javaType="Integer" />
     <result property="categoryId" column="category_id" javaType="Integer" />
     <result property="name" column="name" javaType="String" />
     <result property="createTime" column="create_time" javaType="java.util.Date" />
     <result property="updateTime" column="update_time" javaType="java.util.Date" />

     <association property="category" column="categoryId" javaType="Category" select="hdm.dao.CategoryDao.selectById"/>

  </resultMap>

  <insert id="insert" parameterType="Department">
     insert into dept (category_id,name,create_time,update_time)values(#{categoryId},#{name},#{createTime},#{updateTime})
  </insert>

  <delete id="delete" parameterType="Integer">
     delete from dept where id=#{id}
  </delete>

  <select id="selectById" parameterType="Integer" resultMap="resultMap">
     select * from dept where id=#{id}
  </select>

  <select id="selectAll" resultMap="resultMap">
     select * from dept
  </select>

  <select id="selectByCategoryId" resultMap="resultMap" parameterType="Integer">
     select * from dept where category_id=#{cid}
  </select>

  <update id="update" parameterType="Department">
     update from dept set category_id=#{categoryId},name=#{name},update_time=#{updateTime} where id=#{id}
  </update>

</mapper>

=================================================================================================

CategoryDao.xml

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.4//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="hdm.dao.CategoryDao">
 

  <resultMap id="resultMap" type="Category">
     <id property="id" column="id" javaType="Integer" />
     <result property="name" column="name" javaType="String"/>
     <result property="createTime" column="create_time" javaType="java.util.Date" />
     <result property="updateTime" column="update_time" javaType="java.util.Date" />
  </resultMap>

  <insert id="insert" parameterType="Category" useGeneratedKeys="true">
     insert into category(name,create_time,update_time) values(#{name},#{createTime},#{updateTime})
  </insert>

  <delete id="delete" parameterType="Integer">
     delete from category where id=#{id}
  </delete>

  <select id="selectAll" resultMap="resultMap">
     select * from category
  </select>

  <select id="selectById" parameterType="Integer" resultMap="resultMap">
     select * from category  where id=#{id}
  </select>

  <update id="update" parameterType="Category">
     update category set name=#{name},update_time=#{updateTime} where id=#{id}
  </update>


</mapper>