go scan Postgres array_agg - postgresql

Question

I have a one-to-many relationship in postgres (Event has many EventUser), and would like to scan and store into a struct 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
}
Here is the query:
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'
The query returns this:
{
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,
},
],
};
This is what I am trying to do, which directly scan each item under eventUsers into struct, and store in the Event's struct.
got := []Event{}
for rows.Next() {
var r Event
err = rows.Scan(&r.ID, &r.Name, &r.StartTime, &r.EventUsers)
if err != nil {
panic(err)
}
}
I think I could achieve this by storing the array as a string and then unmarshal it.
What I want is something similar to sql.NullString.

Answer 1

You could define a slice type that implements the sql.Scanner interface. Note that when you pass an instance of a type that implements Scanner to a (*sql.Rows).Scan or (*sql.Row).Scan call, the impelmenter's Scan method will be invoked automatically.
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
}
Then, assuming that the e."eventUsers" in the select query is a json array, you could use it like this:
// 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)
}