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proposal: database/sql: add interface for Scan method on Row and Rows #33786

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Fanarito opened this issue Aug 22, 2019 · 5 comments
Closed

proposal: database/sql: add interface for Scan method on Row and Rows #33786

Fanarito opened this issue Aug 22, 2019 · 5 comments
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@Fanarito
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Problem

Every time that I start a new project that uses database/sql I end up creating an interface for the Scan method that is present on both sql.Row and sql.Rows this allows me to make a function that accepts both sql.Row and sql.Rows and uses the Scan method to populate a struct.

Solution

I feel like this interface should be present in the database/sql package rather than it being defined by me.

type RowScanner interface {
	Scan(dest ...interface{}) error
}
@gopherbot gopherbot added this to the Proposal milestone Aug 22, 2019
@ianlancetaylor
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CC @kardianos

@kardianos
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If I was to guess, you are manually hydrating each struct item through a method. That's an interesting method.

I assume you are doing something like this:

type MyDataRow struct {
    Item1 string
    Item2 int64
}
func (r *MyDataRow) fill(s RowsScanner) error {
    return s.Scan(&r.Item1, &r.Item2)
}

Can you confirm?

@Fanarito
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Yeah that's right

@Fanarito
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A full example would look something like this

type Todo struct {
	Id    string
	Title string
	Done  bool
}

const todoFields = "id, title, done"

func populateTodo(todo *Todo, s RowsScanner) error {
	return s.Scan(&todo.Id, &todo.Title, &todo.Done)
}

func FindAll(db *sql.DB) []Todo {
	query := fmt.Sprintf("SELECT %s FROM todos", todoFields)
	todos := make([]Todo, 0)
	rows, _ := db.Query(query)
	for rows.Next() {
		var todo Todo
		_ := populateTodo(&todo)
		todos = append(todos, todo)
	}
	return todos
}

func FindSingle(db *sql.DB, id string) Todo {
	query := fmt.Sprintf("SELECT %s FROM todos WHERE id = $1", todoFields)
	row := db.Query(query, id)
	var todo Todo
	_ := populateTodo(&todo, row)
	return todo
}

@kardianos
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Thank you for the use case.

  1. I think this illustrates is a great aspect of Go. An interface can be implicit.
  2. I don't think this is a universal issue. For instance, we still haven't added a Queryer interface yet.

I could easily see an interface defined as:

type Scanner interface {
    Scan(rows *sql.Rows) error
}

type TODO struct {...}
func (t *TODO) Scan(rows *sql.Rows) error { ... }

But this isn't to say your version would be worse or better, but I would perceive the value is more application dependent. So if you choose to marshal data in this way, make a common package that defines it for you.

I think a RowScanner would have some value, but I don't think it would pull its own weight.

@golang golang locked and limited conversation to collaborators Aug 22, 2020
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@kardianos @ianlancetaylor @Fanarito @gopherbot