而ibatis這方面就比較遜色了,不過對於也支持簡單的關連查詢,如1對1,和1對多。對於一般的情況來說,這兩種已經足夠了,當然不能層疊更新是一個缺陷,看了半天文檔,也沒有找到對象之間的層疊更新,估計是不支持。
以前的版本ibatis處理關連是通過執行兩次sql來實現的,如下的實例:
一對多關聯:
<sqlMap namespace="User">
<typeAlias alias="user" type="com.ibatis.sample.User"/>
<typeAlias alias="address" type="com.ibatis.sample.Address"/>
<resultMap id="get-user-result" class="user">
<result property="id" column="id"/>
<result property="name" column="name"/>
<result property="sex" column="sex"/>
<result property="addresses" column="id" select="User.getAddressByUserId"/>
</resultMap>
<select id="getUsers" parameterClass="java.lang.String" resultMap="get-user-result">
<![CDATA[<![CDATA[</span> </span></li> <li class="alt"><span><span class="cdata">
select id,name,sex</span> </span></li> <li class=""><span><span class="cdata">from t_user
</span> </span></li> <li class="alt"><span><span class="cdata">where id = #id#</span>
</span></li> <li class=""><span><span class="cdata">]> </select> <select id="getAddressByUserId" parameterClass="int" resultClass="address"> <![CDATA[<![CDATA[</span> </span></li> <li class=""><span><span class="cdata">
select address,zipcode</span> </span></li> <li class="alt"><span><span class="cdata">
from t_address</span> </span></li> <li class=""><span><span class="cdata">
where user_id = #userid#</span> </span></li> <li class="alt"><span><span class="cdata">]> </select> </sqlMap>
這裏通過在resultMap 中定義嵌套查詢getAddressByUserId,我們實現了關聯數據的讀取。 select id,name,sex</span> </span></li> <li class=""><span><span class="cdata">from t_user
</span> </span></li> <li class="alt"><span><span class="cdata">where id = #id#</span>
</span></li> <li class=""><span><span class="cdata">]> </select> <select id="getAddressByUserId" parameterClass="int" resultClass="address"> <![CDATA[<![CDATA[</span> </span></li> <li class=""><span><span class="cdata">
select address,zipcode</span> </span></li> <li class="alt"><span><span class="cdata">
from t_address</span> </span></li> <li class=""><span><span class="cdata">
where user_id = #userid#</span> </span></li> <li class="alt"><span><span class="cdata">]> </select> </sqlMap>
需要注意的是,這裏有一個潛在的性能問題,也就是所謂“n+1”Select問題。
一對一關聯:
對於這種情況,我們可以採用一次Select兩張表的方式,避免這樣的性能開銷(假設上面示例中,每個User 只有一個對應的Address記錄):
<resultMap id="get-user-result" class="user">
<result property="id" column="id"/>
<result property="name" column="name"/>
<result property="sex" column="sex"/>
<result property="address" column="t_address.address"/>
<result property="zipCode" column="t_address.zipcode"/>
</resultMap>
<select id="getUsers" parameterClass="java.lang.String" resultMap="get-user-result">
<![CDATA[<![CDATA[</span> </span></li> <li class="alt"><span><span class="cdata">
select *</span> </span></li> <li class=""><span><span class="cdata">from t_user,t_address
</span> </span></li> <li class="alt"><span><span class="cdata">
where t_user.id=t_address.user_id</span> </span></li> <li class=""><span>
<span class="cdata">]> </select>
在現在的版本中,對於n+1問題,ibatis已經很好的解決了。如下的配置:
select *</span> </span></li> <li class=""><span><span class="cdata">from t_user,t_address
</span> </span></li> <li class="alt"><span><span class="cdata">
where t_user.id=t_address.user_id</span> </span></li> <li class=""><span>
<span class="cdata">]> </select>
<resultMap id=”get-product-result” class=”com.ibatis.example.Product”>
<result property=”id” column=”PRD_ID”/>
<result property=”description” column=”PRD_DESCRIPTION”/>
<result property=”category” resultMap=“get-category-result” />
</resultMap>
<resultMap id=”get-category-result” class=”com.ibatis.example.Category”>
<result property=”id” column=”CAT_ID” />
<result property=”description” column=”CAT_DESCRIPTION” />
</resultMap>
<select id=”getProduct” parameterClass=”int” resultMap=”get-product-result”>
select *
from PRODUCT, CATEGORY
where PRD_CAT_ID=CAT_ID
and PRD_ID = #value#
</select>
可以使用內在的resultMap來解決此問題。 同樣一對多如下:
<sqlMap namespace="ProductCategory">
<resultMap id=”categoryResult” class=”com.ibatis.example.Category” groupBy=”id”>
<result property=”id” column=”CAT_ID”/>
<result property=”description” column=”CAT_DESCRIPTION”/>
<result property=”productList” resultMap=”ProductCategory.productResult”/>
</resultMap>
<resultMap id=”productResult” class=”com.ibatis.example.Product”>
<result property=”id” column=”PRD_ID”/>
<result property=”description” column=”PRD_DESCRIPTION”/>
</resultMap>
<select id=”getCategory” parameterClass=”int” resultMap=”categoryResult”>
select C.CAT_ID, C.CAT_DESCRIPTION, P.PRD_ID, P.PRD_DESCRIPTION
from CATEGORY C
left outer join PRODUCT P
on C.CAT_ID = P.PRD_CAT_ID
where CAT_ID = #value#
</select>
</sqlMap>
注意,需要使用增加groupBy屬性來分類