golua - declaring lua class with defined methods

I'm trying to use golua package to build an extension API for my application written in Go. My idea is to have several classes exposed to lua VM, for example Book class:

<!-- language: lua -->

local book = Book.Create("Le Petit Prince")
print(book)
book:save()

What I'm able to do now is just basic:

<!-- language: golang -->

type Book struct {
    Id int64
    Title string
}

func BookCreate(L *lua.State) int {
    title := L.ToString(1)
    p := &Book{Id: 1, Title: title}
    L.PushGoStruct(p)
    return 1
}

func BookToString(L *lua.State) int {
    book, _ := L.ToGoStruct(1).(*Book)
    L.PushString(fmt.Sprintf("Book(Id=%d, Title=\"%s\")", book.Id, book.Title))
    return 1
}

L := lua.NewState()
defer L.Close()
L.OpenLibs()

L.NewMetaTable("Book")
L.SetMetaMethod("Create", BookCreate)
L.SetMetaMethod("tostring", BookToString)
L.SetGlobal("Book")

Which allows me to do this:

<!-- language: lua -->

local book = Book.Create("Le Petit Prince")
print(Book.tostring(book))

But not this:

local book = Book.Create("Le Petit Prince")
print(book:tostring())

// reflect: call of reflect.Value.Type on zero Value

My questions

1. How to create lua class equivalent to one described in A Simplified Way to Declare Lua Classes?
2. How to add "magic methods" to class like __tostring here

I have not used go, but it looks like you never set Book as a metatable for the new book you create. Im fairly sure this does not happen automatically.

See an example I found here https://github.com/stevedonovan/luar/blob/master/luar.go#L52

What is important here is that when a userdata or any object is made (your book), you need to get the global metatable and then set it as metatable with L.SetMetaTable(-2)

After @Rochet2's answer and after reading Chapter 28. User-Defined Types in C of Programming in Lua I've came up with working solution. I know nothing about Lua and just a little about Go so I might be wrong in my conclusions.

> This edition of Programming in Lua was written for Lua 5.0 - golua utilizes Lua 5.1 (for now).

BookCreate function

We create new userdata instead of pushing plain Go struct to stack. Then we set metatable of newly created book to "Book".

L.NewUserdata returns unsafe.Pointer so we cast it to Book.

<!-- language: golang -->

func BookCreate(L *lua.State) int {
    title := L.ToString(1)
    book := (*Book)(L.NewUserdata(uintptr(unsafe.Sizeof(Book{}))))

    L.LGetMetaTable("Book")
    L.SetMetaTable(-2)

    book.Id = 1;
    book.Title = title;

    return 1
}

BookToString function

Here we just take userdata from stack and cast it to Book.

> Be aware that what we pop out of stack might not be pointer to Book struct so if we want to prevent nil pointers / other Go errors some basic type check might be required.

func BookToString(L *lua.State) int {
    book := (*Book)(L.ToUserdata(1))
    L.PushString(fmt.Sprintf("Book(Id=%d, Title=\"%s\")", book.Id, book.Title))

    return 1
}

main function

We initialize new "Book" metatable and make itself its own metatble. We add two methods Create and __tostring and make "Book" a global.

L := lua.NewState()
defer L.Close()
L.OpenLibs()

L.NewMetaTable("Book")
L.PushString("__index")
L.PushValue(-2)
L.SetTable(-3)
L.SetMetaMethod("Create", BookCreate)
L.SetMetaMethod("__tostring", BookToString)
L.SetGlobal("Book")

This is what we can do in Lua now:

local book = Book.Create('Le Petit Prince')
print(book)
print(book:__tostring())
print(Book.__tostring(book))

--out:
Book(Id=1, Title="Le Petit Prince")
Book(Id=1, Title="Le Petit Prince")
Book(Id=1, Title="Le Petit Prince")

I hope someone will find it useful. Full code here.