Incompatibility with Gorm

[tobiassjosten]

I'm having some trouble with getting this library playing together with Gorm and I'm not entirely sure where the fault is at. Maybe someone here could help point me in a direction to figure out where (and how) to solve this?

The problem is that Gorm doesn't seem to understand when ulid.ULIDs are empty, so when I have such a field it tried writing to it, which causes foreign key failure.

I've seen that this library implements a SQL Valuer and it looks good but, still, it doesn't work for me.

Any ideas what could be going wrong?

Here's an example:

package main

import (
	"math/rand"
	"os"
	"time"

	"github.com/oklog/ulid/v2"
	"gorm.io/driver/mysql"
	"gorm.io/gorm"
)

type One struct {
	ID    ulid.ULID `gorm:"primaryKey;type:varbinary(255)"`
	Name  string
	Two   *Two
	TwoID ulid.ULID
}

type Two struct {
	ID ulid.ULID `gorm:"primaryKey;type:varbinary(255)"`
}

func main() {
	db, _ := gorm.Open(mysql.Open(os.Getenv("DB_DSN")), &gorm.Config{})

	db.AutoMigrate(One{})
	db.AutoMigrate(Two{})

	now := time.Now()
	entropy := ulid.Monotonic(rand.New(rand.NewSource(now.UnixNano())), 0)

	one := One{ID: ulid.MustNew(ulid.Timestamp(now), entropy), Name: "Asdf"}

	db.Create(one)
}

[giautm]

What is the error? Please copy and paste to here.

[tobiassjosten]

Thanks for the quick reply, @giautm! Here's the error I'm seeing:

2020/09/08 10:45:14 /…/main_test.go:24 FOREIGN KEY constraint failed
[0.487ms] [rows:0] INSERT INTO `ones` (`id`,`name`,`two_id`) VALUES ("<binary>","Asdf","<binary>")

[giautm]

I don't see relation between One and Two model. Do you add relation by hand? plz provide more detail about database schema.

• The error said, you must insert the Two with that ID first then insert the One. Or you must set NULLABLE for that column (two_id).

[tobiassjosten]

Gorm infers the relation – you'll see that One has a property referencing Two and that's the relation. That is further encoded in the TwoID property, which is saved to the database.

I just heard back from the creator of Gorm and it seems that ULIDs' Value always return true, which tells Gorm that there's a value even for an emptyULID. That sounds like it could be the culprit here.

go-gorm/gorm#3427

So if my One has a null property for Two, Gorm will see that as an actual reference to a Two that doesn't exist. Hence the foreign key failure.

Does that make sense? Would that be fixable here?

[peterbourgon]

Well, a zero-value ULID is a valid ULID, in the same way that 0 is a valid int. How would you solve this problem for ints?

edit: Possible paths forward, as I see them:

• Change the Value method to report zero-value ULIDs as invalid — I don't think this is correct, but happy to discuss
• Provide a wrapper type w/ Value method that special-cases zero-value ULIDs as invalid — but you could do this yourself?
• Use *ulid.ULID instead of ulid.ULID in your model

Any others?

[tobiassjosten]

I agree about your first option and also think your second option sounds like the best way forward. I managed to solve it in another manner (see below) but yours is more generally applicable, so that's probably what most users should use.

You have already done a lot of good work here but can I still just suggest adding a line or two about this to the README? I would have found that immensely useful. :)

Either way, thank you so much for the quick help!

(In case anyone else comes looking months from now, my own solution was centralizing Gorm interactions through a set of functions where I can do null/empty-checks and use Gorm's Omit()´/Select()` to control what's being written.)