我们再用mybatis做嵌套查询时,有时会遇到子集合只有1条数据的情况,例如下这样:
数据库查询结果
xml
XML
<resultMap id="userMap" type="com.springboot.demo.test.entity.User">
<id column="uid" property="uid"/>
<result column="username" property="username"/>
<result column="tel" property="tel"/>
<result column="age" property="age"/>
<collection property="userRoleList" ofType="com.springboot.demo.test.entity.UserRole">
<result column="roleid" property="roleid"/>
</collection>
</resultMap>
<select id="selectUser" resultMap="userMap">
SELECT
a.uid,
a.username,
a.tel,
a.age,
b.roleid
FROM
user a
LEFT JOIN user_role b ON a.uid = b.userid
</select>
返回结果
XML
[{
"uid": 33,
"username": "jon",
"tel": "123",
"age": 23,
"userRoleList": [{
"roleid": 1
}]
},
{
"uid": 31,
"username": "xiaomi",
"tel": "110",
"age": 20,
"userRoleList": [{
"roleid": 13
}]
},
{
"uid": 35,
"username": "WANG",
"tel": "222",
"age": 33,
"userRoleList": [{
"roleid": 1
}]
},
{
"uid": 34,
"username": "xiaocai",
"tel": "111",
"age": 32,
"userRoleList": [{
"roleid": 1
}]
}
]
很明显,这不是我们期望结果。如果遇到这种情况,可以在子集合的映射里面把id放进去,这样mybatis就会避免上述的情况,如下
XML
<resultMap id="userMap" type="com.springboot.demo.test.entity.User">
<id column="uid" property="uid"/>
<result column="username" property="username"/>
<result column="tel" property="tel"/>
<result column="age" property="age"/>
<collection property="userRoleList" ofType="com.springboot.demo.test.entity.UserRole">
<id column="urid" property="urid"/>
<result column="roleid" property="roleid"/>
</collection>
</resultMap>
<select id="selectUser" resultMap="userMap">
SELECT
a.uid,
a.username,
a.tel,
a.age,
b.roleid,
b.urid
FROM
user a
LEFT JOIN user_role b ON a.uid = b.userid
</select>
结果
XML
[{
"uid": 33,
"username": "jon",
"tel": "123",
"age": 23,
"userRoleList": [{
"urid": 47,
"roleid": 9
},
{
"urid": 48,
"roleid": 1
}
]
},
{
"uid": 31,
"username": "xiaomi",
"tel": "110",
"age": 20,
"userRoleList": [{
"urid": 81,
"roleid": 9
},
{
"urid": 82,
"roleid": 10
},
{
"urid": 83,
"roleid": 16
},
{
"urid": 84,
"roleid": 1
},
{
"urid": 85,
"roleid": 13
}
]
},
{
"uid": 35,
"username": "WANG",
"tel": "222",
"age": 33,
"userRoleList": [{
"urid": 86,
"roleid": 11
},
{
"urid": 87,
"roleid": 1
}
]
},
{
"uid": 34,
"username": "xiaocai",
"tel": "111",
"age": 32,
"userRoleList": [{
"urid": 92,
"roleid": 1
}]
}
]