Problem with m2m relation

[denudge]

I can't get an m2m relation to work, though it is nearly the same as the example one.
It is firing somewhere in the ParkStruct() function in the hydration phase during the join read.
"Order" is "App" here, and "Item" is "Group" here, and one subtle difference is that each group must belong to an app,
so even before the join, every group already has an app_id.

After inserting a simple INSERT INTO apps_default_groups (app_id, group_id) VALUES (1, 1); into the relation table in Postgres, the error when fetching an app with ID 1 and .WithRelation("DefaultGroups") is:
bun: m2m relation=DefaultGroups does not have base model=App with key=['\x01' '\x01'] (check join conditions)

Could somebody please give me some hint what I have to fix?

My models are roughly:

type App struct {
	bun.BaseModel `bun:"table:apps"`
	Id            int64     `bun:"id,pk,autoincrement"`
	Name          string    `bun:"name"`
	Groups        []Group   `bun:"rel:has-many,join:id=app_id"`
	DefaultGroups []Group   `bun:"m2m:apps_default_groups,join:App=Group"` // I don't understand this join syntax, but If I change something here, bun complains that it cannot find the field in the relation model
}

type Group struct {
	bun.BaseModel `bun:"table:groups"`
	Id            int64     `bun:"id,pk,autoincrement"`
	AppId         int64     `bun:"app_id"`
	App           *App      `bun:"rel:belongs-to,join:app_id=id"`
	Name          string    `bun:"name"`
}

type AppToDefaultGroup struct {
	bun.BaseModel `bun:"table:apps_default_groups"` // without this, registerModel() cannot find a mapping to the table name
	AppId         int64  `bun:"app_id,pk"`
	App           *App   `bun:"rel:belongs-to,join:app_id=id"`
	GroupId       int64  `bun:"group_id,pk"`
	Group         *Group `bun:"rel:belongs-to,join:group_id=id"`
}