Gorm Automigrate cannot work properly when unique constraint changed.

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Description

I basically use a migrate function to update my database(mysql) schema. But when I changed my field attribute and run the migration function again. The unique constraint of user_name was not removed.

//migration function
func migration() {
    DB.Set(`gorm:table_options`, "charset=utf8mb4").
        AutoMigrate(&model.Task{})
}

//Task Struct Before
type Task struct {
    gorm.Model
    Uid      string
    UserName string `gorm:"unique"`
    Title    string
    Content  string `gorm:"type:longtext"`
}

//Task Struct Now
type Task struct {
    gorm.Model
    Uid      string
    UserName string
    Title    string `gorm:"unique"`
    Content  string `gorm:"type:longtext"`
}

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