No reply...
Is it impossible to avoid many select statements for many-to-many
relation?
The following ruby code:
Book.all.each do |book|
book.authors.each {|author| p author }
end
generats a lot of query such as:
select * from books;
select * from authors ... where writings.book_id = 1
select * from authors ... where writings.book_id = 2
select * from authors ... where writings.book_id = 3
Is there any way to solve this problem?
--
regards,
makoto kuwata
On 2008-12-04, 10:06 pm, makoto kuwata <[EMAIL PROTECTED]> wrote:
> Hi,
>
> Is eager loding available for N : N relation?
> Or is it possible to emurate eager loading manually?
>
> For example:
> * Book, Author, and Writing models classes exist
> * Book : Author = N : N (through Writing)
>
> In this case, the following code generates a lot
> of 'select * from authors ...' statements.
>
> Book.all.each do |book|
> book.authors.each {|author| p author }
> end
> #=> select * from books;
> #=> select * from authors ... where writings.book_id = 1
> #=> select * from authors ... where writings.book_id = 2
> #=> select * from authors ... where writings.book_id = 3
> #=> ...
>
> So I tried the following code:
>
> module Enumerable
> def index_by
> hash = {}
> self.each do |item|
> hash[yield(item)] = item
> end
> hash
> end
> end
>
> ## get all books and authors
> books_by_id = Book.all.index_by {|book| book.id }
> authors_by_id = Author.all.index_by {|author| author.id }
> ## group them
> books = {}
> authors = {}
> Writing.all.each do |writing|
> book = books_by_id[writing.book_id]
> author = authors_by_id[writing.author_id]
> (books[author.id] ||= []) << book
> (authors[book.id] ||= []) << author
> end
> ## set association manually
> all_books = Book.all
> all_books.each {|book| book.authors = authors[book.id] }
> all_authors = authors.all
> all_authors.each {|author| author.books = books[author.id] }
>
> But I got the following error:
>
> DataMapper::Associations::ImmutableAssociationError: You can not
> modify this association
>
> Could you give me any adivces?
>
> --
> regards,
> makoto kuwata
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