java如何使用mybatis实现递归映射？

Question

我有一个java类

class Node {
  String id;
  String name;
  List<Node> children;
}

MySQL中的表：

NODES:
| ID | NAME |
| 1  | name1 |
| 2  | name2 |
| 3  | name3 |
| 4  | name4 |

还有另一张关系桌

RELATIONS:
| PARENT_ID | CHILD_ID |
|    1      |     2    |
|    2      |     3    |
|    2      |     4    |

通常我们可以使用left join和collection将它们连接在一起，如下所示

<resultMap id="node" type="org.hello.Node>
  <id property="id" column="ID"/>
  <result property="name" column="NAME"/>
  <collection property="children" ofType="org.hello.Node" resultMap="node"/>
</resultMap>

<select id="select" resultMap="node">
 SELECT ID, NAME FROM NODES N
 LEFT JOIN RELATIONS R ON N.ID = R.PARENT_ID
 LEFT JOIN NODES N1 ON R.CHILD_ID = N1.ID
</select>

如果N1只是另一个具有另一个模型的表，那么它应该可以工作。问题是，我们如何编写mybatis xml配置，以递归地映射到指向自身的结构之上

Answer 1

您只需要指定适当的列别名并使用columnPrefix

<resultMap type="org.hello.Node" id="node">
  <id property="id" column="ID" />
  <result property="name" column="NAME" />
  <collection property="children" resultMap="node"
    columnPrefix="C_"/>
</resultMap>

<select id="select" resultMap="node">
  SELECT
    N1.ID, N1.NAME,
    N2.ID C_ID, N2.NAME C_NAME,
  FROM NODES N1
  LEFT JOIN RELATIONS R1 ON R1.PARENT_ID = N1.ID
  LEFT JOIN NODES N2 ON N2.ID = R1.CHILD_ID
</select>

我还应该提到，上面的结果映射适用于任何级别的节点，因为columnPrefix是递归应用的

<select id="select" resultMap="node">
  SELECT
    N1.ID, N1.NAME,
    N2.ID C_ID, N2.NAME C_NAME,
    N3.ID C_C_ID, N3.NAME C_C_NAME
  FROM NODES N1
  LEFT JOIN RELATIONS R1 ON R1.PARENT_ID = N1.ID
  LEFT JOIN NODES N2 ON N2.ID = R1.CHILD_ID
  LEFT JOIN RELATIONS R2 ON R2.PARENT_ID = N2.ID
  LEFT JOIN NODES N3 ON N3.ID = R2.CHILD_ID
</select>