Re: Questions on using orm

[Thomas Gazagnaire <thomas.gazagnaire@xxxxxxxxx>]

Hi,

Two answers to your questions:

=== Answer 1 (the direct answer) ===

> Question 1: is there any way to actually use foreign keys to query?
> 
> I would like to do basically:
> 
> SELECT c.* from Chapter c, Book b where b.book_name = "My book";

You can use custom functions:

let get_chapters_by_book_name db book_name =
  chapter_get db ~custom:(fun c -> c.chapter_fk_book.book_name = book_name)

> ---
> Question 2: is there any way to do cascading deletes?
> 
> chapter_delete ~recursive:true ~db:chapter_db some_chapter
> 
> This does something I cannot really envision ever needing...It allows me to 
> delete the chapter _and_ the book (doesn't really seem like something too 
> natural), where as I would like the reverse to actually happen:
> 
> book_delete ~recursive:true ~db:book_db some_book
> 
> I would expect this to delete the book and everything that actually keeps 
> foreign keys to the book item, much like cascading deletes in SQL.
> 
> Instead, at the moment, I have to query for the book object, then use it to 
> query for all chapters underneath, use each to call chapter_delete 
> individually, and afterwards delete the book. There is something here that 
> feels it won't just be optimized away for me :)

This is not possible (as there is no notion of foreign key in ORM). For a 
better solution, see answer 2.

== Answer 2 (the indirect one) ==

> type book = {
>   book_name : string;
> } and chapter = {
>   chapter_fk_book : book;
>   chapter_name : string;
> } with orm

This is not the most natural way to define book/chapters if you would do it 
without persistence in mind. For instance, I would do:

type book = {
  book_name     : string;
  book_chapters: chapter list;
} and chapter = {
  chapter_name = string;
} with orm

let get_chapters_by_book_name db book_name =
  match get_book db ~book_name:(`Eq book_name) with
  | []   -> []
  | [b] -> b.chapters
 | _    -> failwith "too many books with the same name"

and then recursive deletion works as expected.

Hope this helps,
Thomas