JSON fields from sqlite3

Question

I have a json field in a sqlite3 collection. My schema looks like:

CREATE Table Animals(
  id int,
  sounds json,
  name string
)

I understand the go-sqlite interface does not support the json datatype explicitly. However, my json is quite simple, as all fields are json arrays, eg;

["bark", "woof", "growl"]
["meow", "hiss", "growl"]

So a full record might be:

id   sounds                      name
1    ["bark", "woof", "growl"]   Fido
2    ["meow", "hiss", "growl"]   Rufus

Using the package:

_ "github.com/mattn/go-sqlite3"

I am able to extract my json field with

var id sql.NullInt64
var name sql.NullString
var sounds []uint8

err := db.QueryRow("SELECT id,name,sounds FROM Animals WHERE id = ?;", 1).Scan(&id, &name, &sounds)

fmt.Println(strconv.Itoa(id) + "|" + name + "|" + strings.Join(sounds, "+"))

// does print correctly:
1|Fido|bark+wood+growl

That is, it seems the sqlite3 json gets stored in a unicode string(?) as a series of...bytes?...that I can convert to string with the String module. I'm additionally interested in the "+" join operation so I can make a query+string+looking+thing out of this for another application downstream.

However, I'd really like to bundle this all up in JSON, and take advantage of JSON unmarshalling/parsing rather than my ad hoc custom prints. When I try:

type Animal struct {
    id int                   `json:"id"`
    name sql.NullString      `json:"name"`
    sounds []uint8           `json:"sounds"`
}

var a Animal

err := db.QueryRow("SELECT id,name,sounds FROM Animals WHERE id = ?;", 1).Scan(&a.id, &a.name, &a.sounds
)

It prints a bona fide array of integers. How can I embed the strings.Join(sounds []uint8) declaration + function transformation combo in my json-enabled type definition?

Additionally, it's not clear to me how to use the []uint8 string in the event the json is a nulled [] or true NULL and further make it robust against these.

Some refs:

Are you married to that database schema? SQLite doesn't have any native support for JSON so anything you do will be a mess of kludges. You'd probably have an easier time using a separate animal_sounds table then collecting everything together with JOINs and a bit of Go to build the Animal structs they way you want them. BTW, that column that you're creating as json is actually just text. — mu is too short
– mu is too short, Commented Jan 31, 2018 at 6:32
I think private field like id int in struct can't be pick up by golang json encoder. — Morty Choi
– Morty Choi, Commented Jan 31, 2018 at 7:19
If you really want to store JSON documents, you should use a document store such as MongoDB, CouchDB, or similar. Don't try to force a round peg into a square hole, when round holes exist, too. — Jonathan Hall
– Jonathan Hall, Commented Jan 31, 2018 at 7:39
sqlite has supported json fields for years. Mongo isn't the only game in document storage town. Also, if you go to the sql message boards, they'd instead be saying the same problem should be /even less/ json, in that the sounds field just should have been a join to another table. ;) There are a lot of ways to skin a cat. — Mittenchops
– Mittenchops, Commented Feb 1, 2018 at 1:47

Jonathan Hall · Accepted Answer · 2021-03-08 07:39:56Z

Your question brings up several topics. But the easiest answer to all of them is probably:

Don't use a relational database.

You seem to want to fetch objects/documents, so using a storage mechanism that natively supports this will prevent the need for kludges everywhere. MongoDB, CouchDB, or some other NoSQL solution is probably the right fit for your desires.

But having said that, there are answers to your specific questions. Put together, they arguably make for something complex and ugly, though.

Your sounds type.

Create a custom type, which implements the sql.Scanner interface, and unmarshals the JSON value for you:

    type Sounds []string

    func (s *Sounds) Scan(src interface{}) error {
        switch t := src.(type) {
        case []byte:
            return json.Unmarshal(t, &s)
        default:
            return errors.New("Invalid type")
        }
    }

Scanning into a struct

Use sqlx for this. It allows you to scan your entire row into a struct much more easily than the standard library. It can use the db tag to match rows to the struct field.

A single struct for DB and JSON

You can have multiple tags in your struct:

    type Animal struct {
        id int                   `db:"id" json:"id"`
        name sql.NullString      `db:"name" json:"name"`
        sounds []uint8           `db:"sounds" json:"sounds"`
    }

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