Scanning nested structs implementing Scan and Value

[DallanQ]

I'm wondering if it's possible to scan nested structs when the nested struct implements Scan and Value.

My database table definition is

create table test(
	id serial not null constraint test_pkey primary key,
	body jsonb,
	insert_time timestamp with time zone default CURRENT_TIMESTAMP,
	last_update_time timestamp with time zone default CURRENT_TIMESTAMP
);

My struct definitions are:

type Body struct {
	One string
	Two string
}
func (b Body) Scan(val interface{}) error {
	return json.Unmarshal(val.([]byte), &b)
}
func (b Body) Value() (driver.Value, error) {
	return json.Marshal(b)
}
type Wrapper struct {
	ID int64                 `db:"id"`
	Body                     `db:"body"`
	InsertTime time.Time     `db:"insert_time"`
	LastUpdateTime time.Time `db:"last_update_time"`
}

But the following code gives an error: non-struct dest type struct with >1 columns (4)

ws := []Wrapper{}
err = db.SelectContext(ctx, &ws, "SELECT * FROM test")

Am I trying to do something that's not possible, or do I have an error somewhere?

[DallanQ]

I know I can achieve the result I'm looking for by scanning each row separately

rows, err := db.QueryContext(ctx, "SELECT * FROM test")
if err != nil {
	log.Fatalf("Select %v\n", err)
}
ws := []Wrapper{}
for rows.Next() {
	w := Wrapper{}
	err = rows.Scan(&w.ID, &w.Body, &w.InsertTime, &w.LastUpdateTime)
	if err != nil {
		log.Fatalf("Scan %v\n", err)
	}
	ws = append(ws, w)
}

But I'm hoping to use the Select function to avoid the extra lines of code.

[lkebin]

I guess that because you are using embedding struct

Try this ?

type Wrapper struct {
	ID int64 `db:"id"`
	Body Body `db:"body"`   // <-- Try give it a field name
	InsertTime time.Time `db:"insert_time"`
	LastUpdateTime time.Time `db:"last_update_time"`
}

[p12s]

Works for me with *

// 1
type First struct {
	Id          string      `json:"id" db:"id"`
	...
	Second      *second `json:"params" db:"params"` // <----- nested 2 level
	...	
	LaunchedAt  *time.Time  `json:"launchedAt" db:"launched_at"`
}

// 2
type second struct {
	DeviceName   string       `json:"deviceName" db:"deviceName"`
	...
	Third  *third `json:"preparation" db:"preparation"` // <---- nester 3 level
	...
}

func (t *second) Scan(val interface{}) error { // need only for second
	return json.Unmarshal(val.([]byte), &t)
}

func (t *second) Value() (driver.Value, error) { // need only for second
	return json.Marshal(t)
}

// 3
type third struct {  
	Name       string `json:"name"`
	Value      string `json:"value"`
}
// for third - not need Scan() and Value()

[DallanQ]

Thank you!