Gorm When using embedded structure query, the result is not consistent with the expectation

Your Question

I have two model structs. When I combine these two structs for querying, a problem occurs.

The document you expected this should be explained

type User struct {
    ID   uuid.UUID `gorm:"primarykey;column:id"`
    Name string    `gorm:"column:name"`
}

func (User) TableName() string {
    return "users"
}

type Order struct {
    ID     uuid.UUID `gorm:"primarykey;column:id"`
    UserID uuid.UUID `gorm:"column:user_id"`
    Name   string    `gorm:"column:name"`
}

func (Order) TableName() string {
    return "orders"
}

It is correct when I use the following query. When the order is not matched, nil is returned. Like this user: {31e8f5fe-9848-4088-b786-ff75f1ab7563 user 1} order: <nil>

user, order := &User{}, &Order{}
userTable, orderTable := user.TableName(), order.TableName()

var uo []struct {
    User
    *Order
}
err = db.Model(user).Select(
    userTable+".*, "+orderTable+".*",
).Joins(
    "LEFT JOIN "+orderTable+" ON "+userTable+".id = "+orderTable+".user_id",
).Where(userTable+".id = ?", "31e8f5fe-9848-4088-b786-ff75f1ab7563").Scan(&uo)

But when I use the following order struct. When the order is not matched, nil is not returned. Like this user: {31e8f5fe-9848-4088-b786-ff75f1ab7563 user 1} order: &{00000000-0000-0000-0000-000000000000 00000000-0000-0000-0000-000000000000 <nil>}

type Order struct {
    ID     uuid.UUID `gorm:"primarykey;column:id"`
    UserID uuid.UUID `gorm:"column:user_id"`
    Name   *string   `gorm:"column:name"`
}

But uuid.UUID will still return nil. If it is string or *int etc., it will not return nil.

Expected answer

I want to know why this is happening. It makes my logic for judging whether the order matches inconsistent.