First column in sql rows.Scan is always zero

Saied Seghatoleslami

unread,

Jun 8, 2020, 7:03:14 PM6/8/20

to golang-nuts

I am scanning database tables into a structure explicitly (with &struct.field) that works just fine (working path below).

But my tables have different columns, so I am working to generalize the API. There are two methods that build and reverse a []interface{} to be supplied to scan. I have listed them in the problem path. The last item is the people data in the use case.

It scans every column correctly except the first one that is the ID.

The problem path always returns zero for the ID field while the working path works fine. I have been staring at it for a day and experimenting with different ideas but no success

Any thoughts?


//=============================. Working Path =============================

//Person is the target for scan.
type Person struct {
	ID             int
	Name           string
	Email          string
	HashedPassword string
	Created        time.Time
	Role           string
	Active         bool
	Online         bool
}

//inside the "get" method to get the data

person := broker.Person{}
for rows.Next() {
			err := rows.Scan(&person.ID, &person.Name, &person.Email, //g...)
				&person.HashedPassword, &person.Created, &person.Active)
			if err != nil {
				if errors.Is(err, sql.ErrNoRows) {
					return broker.ErrNoRecord
				}
				return err
			}
			newPeople = append(newPeople, person)
//=============================. Problem Path =============================

person := broker.Person{}
g := person.GetItems(e.Get)
for rows.Next() {
			err := rows.Scan(g...)
			if err != nil {
				if errors.Is(err, sql.ErrNoRows) {
					return broker.ErrNoRecord
				}
				return err
			}
			person.GetBack(e.Get, g)
			newPeople = append(newPeople, person)	




//note that iD, Name, Email... are constants

//GetItems uses the get string to generate an interface to be passed to the
//sql.Execute statement for the INSERT sql command.
func (p *Person) GetItems(get []string) []interface{} {
	var g = []interface{}{}
	for _, sp := range get {
		switch sp {
		case iD:
			g = append(g, &p.ID)
		case Name:
			g = append(g, &p.Name)
		case Email:
			g = append(g, &p.Email)
		case HashedPassword:
			g = append(g, &p.HashedPassword)
		case Created:
			g = append(g, &p.Created)
		case Role:
			g = append(g, &p.Role)
		case Active:
			g = append(g, &p.Active)
		case Online:
			g = append(g, &p.Online)
		}
	}
	return g
}

//GetBack reverses the get item and takes the interface items and gets the
//underlying data back.
func (p *Person) GetBack(get []string, g []interface{}) error {
	var ok bool
	for i, sp := range get {
		switch sp {
		case iD:
			p.ID, ok = g[i].(int)
			if !ok {
				return fmt.Errorf("ID (int) type assertion failed")
			}
		case Name:
			p.Name, ok = g[i].(string)
			if !ok {
				return fmt.Errorf("Name (string) type assertion failed")
			}
		case Email:
			p.Email, ok = g[i].(string)
			if !ok {
				return fmt.Errorf("Email (string) type assertion failed")
			}
		case HashedPassword:
			p.HashedPassword, ok = g[i].(string)
			if !ok {
				return fmt.Errorf("Hashed Password (string) type assertion failed")
			}
		case Created:
			p.Created, ok = g[i].(time.Time)
			if !ok {
				return fmt.Errorf("Created (time.Time) type assertion failed")
			}
		case Role:
			p.Role, ok = g[i].(string)
			if !ok {
				return fmt.Errorf("Role (string) type assertion failed")
			}
		case Active:
			p.Active, ok = g[i].(bool)
			if !ok {
				return fmt.Errorf("Active (bool) type assertion failed")
			}
		case Online:
			p.Online, ok = g[i].(bool)
			if !ok {
				return fmt.Errorf("Online (bool) type assertion failed")
			}
		}
	}
	return nil
}

exchange := Exchange{
		Table: table,
		Put:   []string{},
		Spec:  []string{iD},
		Get: []string{iD, Name, Email, HashedPassword, Created,
			Active, Online},
		People: []Person{
			Person{ID: id},
		},
		Action: "get",
	}

David Suarez

unread,

Jun 8, 2020, 11:19:48 PM6/8/20

to golang-nuts

I havemf eun tour code bit the lower case "eye" jumps out as something to look at... "case iD".

Hope that helps. A debugger to walk through should help!

David Suarez

unread,

Jun 10, 2020, 3:14:29 PM6/10/20

to golang-nuts

Lol! Lack of sleep is real... apologies for the horrible keyboard offset below. What I was trying to suggest was to check the case on your ID field. In your struct it is all capitalized. The constant and my guess the dereferenced string is a lower case i with a capital D. Running your code through a debugger at that point might confirm why it isnt falling into your expected area vs my above guess based on the case mismatch of your ahortened example.
Good luck! I am sure you will figure it out soon... David

Tamás Gulácsi

unread,

Jun 10, 2020, 4:25:17 PM6/10/20

to golang-nuts

I don't see anything obviously wrong here.

Sprinkle it with log.Printfs around everywhere iD is, and make sure that it IS getting set, with the correct value!

Daniel Theophanes

unread,

Jun 10, 2020, 9:00:54 PM6/10/20

to golang-nuts

Can you try to use

github.com/golang-sql/table

? That might help.

Saied Seghatoleslami

unread,

Jun 10, 2020, 11:24:00 PM6/10/20

to golang-nuts

Thank you very much for your help. Following your suggestions and more experimentation, the problem ended up being that I was asserting int, string and bool types rather than *int, *string and *bool types. I should have gotten a not ok back, but apparently not. I will have to figure out why. The other odd thing was that after I fixed the ID field as below, the problem moved to the name field. After that, I ended up changing everything as you see below.

func (p *Person) GetBack(get []string, g []interface{}) error {

	for i, sp := range get {
		switch sp {
		case iD:

xID, ok := g[i].(*int)

			if !ok {
				return fmt.Errorf("ID (int) type assertion failed")
			}

			p.ID = *xID
		case Name:
			xName, ok := g[i].(*string)

			if !ok {
				return fmt.Errorf("Name (string) type assertion failed")
			}

			p.Name = *xName
		case Email:
			xEmail, ok := g[i].(*string)

			if !ok {
				return fmt.Errorf("Email (string) type assertion failed")
			}

			p.Email = *xEmail
		case HashedPassword:
			xPass, ok := g[i].(*string)

			if !ok {
				return fmt.Errorf("Hashed Password (string) type assertion failed")
			}

			p.HashedPassword = *xPass
		case Created:
			xCreated, ok := g[i].(*time.Time)

			if !ok {
				return fmt.Errorf("Created (time.Time) type assertion failed")
			}

			p.Created = *xCreated
		case Role:
			xRole, ok := g[i].(*string)

			if !ok {
				return fmt.Errorf("Role (string) type assertion failed")
			}

			p.Role = *xRole
		case Active:
			xActive, ok := g[i].(*bool)

			if !ok {
				return fmt.Errorf("Active (bool) type assertion failed")
			}

			p.Active = *xActive
		case Online:
			xOnline, ok := g[i].(*bool)

			if !ok {
				return fmt.Errorf("Online (bool) type assertion failed")
			}

			p.Online = *xOnline
		}
	}
	return nil
}

Reply all

Reply to author

Forward