如何删除jinzhu / gorm中的“属于”关联

Can anyone help me, how to remove a belongs to association in go-gorm?

Here are my simple models:

type User struct {
   gorm.Model
   Name      string
   FirstName string
}

type Customer struct {
   Notes []Note `gorm:"polymorphic:Owner;"` // other models can have notes as well
}

type Note struct {
    gorm.Model
    User      User `json:"-"`
    UserID    uint
    OwnerID   uint
    OwnerType string
}

For "Has many" and "Many to many" associations I can remove the relations. In this example, this works:

db.Model(&customer).Association("Notes").Delete(&note)

However, if I try following:

db.Model(&note).Association("User").Delete(&user)

I get a pointer error:

panic: runtime error: index out of range
... src/github.com/jinzhu/gorm/association.go:242 +0x21ce

The ''user'' and ''note'' objects exist and the relation is there (UserID is set to 1 in the database).