Question

class Wall < ActiveRecord::Base
  has_many :shelves
end

class Shelf < ActiveRecord::Base
  has_many :book
  belongs_to :wall
end

class Book < ActiveRecord::Base
  belongs_to :shelf
end


CREATE TABLE `walls` (
 `id` INT(11) NOT NULL AUTO_INCREMENT,
 `created_at` DATETIME NOT NULL,
 `updated_at` DATETIME NOT NULL,
 PRIMARY KEY (`id`)
) 
CREATE TABLE `shelves` (
    `id` INT(11) NOT NULL AUTO_INCREMENT,
    `wall_id` INT(11) NULL DEFAULT NULL,
    `created_at` DATETIME NOT NULL,
    `updated_at` DATETIME NOT NULL,
    PRIMARY KEY (`id`)
)
CREATE TABLE `books` (
    `id` INT(11) NOT NULL AUTO_INCREMENT,
    `shelf_id` INT(11) NULL DEFAULT NULL,
    `created_at` DATETIME NOT NULL,
    `updated_at` DATETIME NOT NULL,
    PRIMARY KEY (`id`)
)

I want to move a book from one shelf to another, so I write…

  book.shelf = new_shelf

but this doesn’t taken care of all the associations automatically.
When I query the old shelf for books, it returns the book I supposedly moved to another shelf.

what am I missing?

Answer 1

Call save after doing book.shelf = new_shelf

You also have to call reload on the former shelf if it was pulled from the db before the update takes place.