How to Use Reflection to Call a Variadic `Rows.Scan()` Function?

How to Utilize Reflection to Call Scan Variadic Function

When operating with database results, it may be necessary to call the Rows.Scan() function using reflection. However, this can be a challenge due to the function's requirement for a variable number of pointers. In this article, we will delve into a practical solution that addresses this need.

By utilizing reflection, we aim to populate a slice with values obtained from a database query. The process involves determining the number of columns and allocating a slice of empty interfaces to hold the data points. This approach provides flexibility in handling values without prior knowledge of their types.

Sample Code and Implementation

The following code exemplifies the implementation of this approach:

package main

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

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)
    valuePtrs := make([]interface{}, count)

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

        rows.Scan(valuePtrs...)

        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)
        }
    }
}

Key to Success

The key to this approach lies in employing two slices: one stores the values and the other contains pointers corresponding to each value. After the pointers have been used, the values slice is populated with the data, providing access to the actual data points for further processing.

By leveraging reflection, we empower our code with the ability to handle values whose types are not known in advance. This flexibility enhances the versatility and reusability of our database interactions.

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