How to Use Reflection to Call the Database/SQL Rows.Scan() Variadic Function?

Calling the Scan() Variadic Function with Reflection

The Rows.Scan() function provides a convenient way to retrieve data from a database query. However, it utilizes a variable number of pointers as its arguments, which can be challenging to incorporate using reflection. Consider the following scenario:

You want to dynamically populate a slice with values obtained from a query and utilize Rows.Scan() to extract the data. In this case, you'll need to determine the number of columns and create a slice to store the values.

A Common Pitfall:
An attempt to use reflection to call the Scan() function may lead to unexpected results. This is because Rows.Scan() expects pointers to the values, and simply passing a slice of interface{} values would result in nil references.

The Solution:

To successfully call Scan() with reflection, a dual-slice approach is employed. The first slice, values, holds the actual data, while the second slice, valuesPtrs, contains pointers to each element in values.

For each column in the query result, a pointer in valuesPtrs is mapped to the corresponding element in values. Subsequently, Rows.Scan() can be invoked with valuesPtrs as its argument, effectively filling values.

Working Example:

package main

import (
    "fmt"
    _ "github.com/lib/pq"
    "database/sql"
)

func main() {
    db, _ := sql.Open(
        "postgres",
        "user=postgres dbname=go_testing password=pass sslmode=disable")

    rows, _ := db.Query("SELECT * FROM _user;")

    columns, _ := rows.Columns()
    count := len(columns)
    values := make([]interface{}, count)
    valuesPtrs := make([]interface{}, count)

    for rows.Next() {
        for i := range columns {
            valuesPtrs[i] = &values[i]
        }

        rows.Scan(valuesPtrs...)

        for i, col := range columns {
            val := values[i]

            b, ok := val.([]byte)
            var v interface{}
            if (ok) {
                v = string(b)
            } else {
                v = val
            }

            fmt.Println(col, v)
        }
    }
}

In this example, rows.Scan() is invoked with the pointer slice valuesPtrs, populating the values slice with the retrieved data. The loop then prints each column name and its corresponding value.

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