Golang server: send JSON with SQL query result that has variable number of columns - sql

Question

I'm creating a little implementation of RESTful API with Go server.
I'm extracting query parameters from URL (I know it's not safe, I'll try to fix this later on, but if you have any recommendations even on this topic, they would be helpful).
I have table name, desired columns and some conditions saved in 3 sring variables.
I'm using this query:
rows, _ := db.Query(fmt.Sprintf("SELECT %s FROM %s WHERE %s", columns, table, conditions))
I want to send the query result back to my frontend, as JSON. I have variable number of unknown columns, so I can't do it "standard" way.
One solution I can think of is to build a JSON string "manually" from from query result and rows.Columns().
But I'd like to do this in a more sofisticated way using something like variadic interface and stuff like that. The problem is, even after trying a lot, I still dont understand how it works.
I tried using following code
cols, err := rows.Columns() // Get the column names; remember to check err
vals := make([]sql.RawBytes, len(cols)) // Allocate enough values
ints := make([]interface{}, len(cols)) // Make a slice of []interface{}
for i := range ints {
vals[i] = &ints[i] // Copy references into the slice
}
for rows.Next() {
err := rows.Scan(vals...)
// Now you can check each element of vals for nil-ness,
// and you can use type introspection and type assertions
// to fetch the column into a typed variable.
}
from this tutorial but it doesn't work, I'm getting errors like
cannot use &ints[i] (type *interface {}) as type sql.RawBytes in assignment
And even if it'd work, I dont understand it.
Does anyone have a good solution for this? Some explanation would be great aswell.
Thanks a lot.

Answer 1

The first problem is here:
for i := range ints {
vals[i] = &ints[i] // Copy references into the slice
}
This is you setting values that are meant to be RawBytes as pointers to interfaces.
Before I explain what that's meant to be doing I'll see if I can explain what the general idea is here.
So normally when getting the response from SQL in Go you'd have a slice with each column and type (id int, name string, ...) so you can then read each SQL record into this slice and each column will be mapped to the value of the same type.
For cases like yours where you will have more variety in the response from SQL and need Go to handle it, you'd do this:
for i := range ints {
ints[i] = &vals[i] // Copy references into the slice
}
What this is saying is each of your interface values holds a pointer to the vals array that will hold the response from SQL. (In my examples I use [][]byte instead of RawBytes so value in vals would be a slice of byte values from SQL.)
You'd then do:
err := rows.Scan(ints...)
Since interface can evaluate to any type, when the ints array becomes populated it will accept any value then update the position in vals array based on the pointer with the value from SQL as a RawBytes type.
HTH