postgresql - 去扫描 Postgres array_agg

Question

我在 postgres 中有一对多的关系(事件有很多 EventUser)，并且想扫描并存储到结构 Event .

// EventUser struct
type EventUser struct {
    ID              int64
    CheckedIn       bool
    PaidAmount      float32
}

// Event struct
type Event struct {
    ID         int64  `json:"id"`
    Name       string `json:"name"`
    StartTime  string `json:"startTime"`
    EventUsers string
}

这是查询:

SELECT events.id, events.name, events."startTime", e."eventUsers" as "eventUsers"
FROM "Events" as events
LEFT JOIN (
        SELECT events.id as id, array_to_json(array_agg(eu.*)) as "eventUsers"
        FROM "EventUsers" as eu
        JOIN "Events" AS "events" ON events.id = eu."eventId"
        WHERE eu.status = 'RESERVED'
        GROUP BY events.id
    ) AS e USING (id)
WHERE events.status = 'COMPLETED' 

查询返回:

{
  id: 2,
  name: "2 Events are 48 days from now",
  startTime: 1590471343345,
  eventUsers: [
    {
      id: 2,
      checkedIn: false,
      paidAmount: 8
    },
    {
      id: 3,
      checkedIn: false, 
      paidAmount: 8,
    },
  ],
};

这就是我想做的，直接扫描 eventUsers下的每一项放入结构中，并存储在 Event的结构。

got := []Event{}

for rows.Next() {
    var r Event
    err = rows.Scan(&r.ID, &r.Name, &r.StartTime, &r.EventUsers)
    if err != nil {
        panic(err)
    }
}

我想我可以通过将数组存储为字符串然后 unmarshal 来实现这一点它。

我想要的是类似于 sql.NullString .

Answer 1

你可以定义一个 slice 类型来实现 sql.Scanner 界面。请注意，当您将实现 Scanner 的类型的实例传递给 (*sql.Rows).Scan或 (*sql.Row).Scan调用，执行者的Scan方法将被自动调用。

type EventUserList []*EventUser

func (list *EventUserList) Scan(src interface{}) error {
    if data, ok := src.([]byte); ok && len(data) > 0 {
        if err := json.Unmarshal(data, list); err != nil {
            return err
        }
    }
    return nil
}

然后，假设 e."eventUsers"在 select 查询中是一个 json 数组，你可以像这样使用它:

// EventUser struct
type EventUser struct {
    ID         int64
    CheckedIn  bool
    PaidAmount float32
}

// Event struct
type Event struct {
    ID         int64         `json:"id"`
    Name       string        `json:"name"`
    StartTime  string        `json:"startTime"`
    EventUsers EventUserList `json:"eventUsers"`
}

// ...

var events []*Event
for rows.Next() {
    e := new(Event)
    if err := rows.Scan(&e.ID, &e.Name, &e.StartTime, &e.EventUsers); err != nil {
        return err
    }
    events = append(events, e)
}