How to Dynamically Call Variadic Rows.Scan() Using Reflection in Go?

Calling Variadic Functions with Reflection for SQL Scan

When utilizing the Rows.Scan() function for SQL database querying, it becomes necessary to use reflection to account for the variable number of pointers it accepts. This allows for dynamically populating slices with data from a query result.

Steps to Dynamically Scan Using Reflection

To call a variadic function like Rows.Scan() using reflection, follow these steps:

1. Obtain Column Information: Use rows.Columns() to retrieve the column names.
2. Create Slices: Create two slices: one for values (values) and one forpointers (valuePtrs), both matching the length of the columns.
3. Scan Data: In the loop for each row in the result set, iterate over each column and assign pointers to the valuePtrs slice. Then, invoke rows.Scan(valuePtrs...).
4. Retrieve Values: After scanning, the data will be available in the values slice. Convert the []byte into strings where necessary.

Code Example

The following code snippet demonstrates how to dynamically scan using reflection:

package main

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

func main() {
    db, _ := sql.Open("postgres", "connection_string")
    rows, _ := db.Query("SELECT * FROM my_table")

    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)
        }
    }
}

By implementing this method, you can dynamically populate slices with query results without specifying the data types in advance.

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