Postgres/JSON - 將父子關係轉換爲json

Question

我在postgres數據庫中有以下記錄。 parent_pk與父子關係中的pk相關。

 
pk    name    type   parent_pk 
---   ----    ----   --------- 
1    hnumber101  house   0 
2    hnumber201  house   0 
101   dodge_charger vehicle   1 
102   mustang   vehicle   1 
103   civic   vehicle   2 
301   john    user   101 
302   edward   user   102 
303   john    user   102 
304   john    user   103 

And I want to generate a json out of the above in the following format- 

[ { 
    "id": 1, 
    "name": "hnumber101", 
    "type": "house" 
    "child": [ { 
       "id": 101, 
       "name": "dodge charger", 
       "type": "vehicle" 
       "child": [{ 
         "id": 301, 
         "name": "john", 
         "type": "user" 
         }],        
       }, 
       { 
       "id": 102, 
       "name": "mustang", 
       "type": "vehicle" 
       "child": [{ 
         "id": 303, 
         "name": "john", 
         "type": "user" 
         }, 
         { 
         "id": 302, 
         "name": "edward", 
         "type": "user" 
         }],        
       }], 
    }, 
{ 
    "id": 2, 
    "name": "hnumber201", 
    "type": "house" 
    "child": [ { 
       "id": 103, 
       "name": "civic", 
       "type": "vehicle" 
       "child": [{ 
         "id": 304, 
         "name": "john", 
         "type": "user" 
         }], 

       }], 
    }] 

我一直在使用WITH遞歸查詢的嘗試，但得到的條目列表，然後我必須做大量的循環周圍的/在我的Java代碼中的哈希映射來獲取父裏面那個孩子塊。 什麼是最有效的方法來做到這一點？

Answer 1

這些方針的東西：

SELECT things.pk, things.name, things.type, array_agg(sub.pk) AS children 
FROM things 
LEFT JOIN things sub ON sub.parent_pk = things.pk 
GROUP BY things.pk, things.name, things.type; 

 
pk |  name  | type | children 
-----+---------------+---------+----------- 
    1 | hnumber101 | house | {102,101} 
101 | dodge charger | vehicle | {301} 
102 | mustang  | vehicle | {303,302} 
301 | john   | user | {NULL} 
302 | edward  | user | {NULL} 
303 | john   | user | {NULL} 

收集行成PKMap<Integer, Thing>加密映射。

class Thing { 
    Integer id; 
    String name; 
    String type; 
    Integer[] children; 

    public String printMe(Map<Integer, Thing> allThings) { 
    String ret = "... format json stuff here"; 
    for(Integer childId in children) { 
     Thing child = allThings.get(childId); 
     ret += child.printMe(allThings); 
    } 
    ret += "Format json stuff here"; 
    return ret; 
    } 
};