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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