问题
I added a counter cache but can't get it to update. But I can update the parent - the Blog Post model by adding a new blog post - and I can update the child - Comments model - by adding a new comment. The counter cache is supposed to keep track of the total number of comments per blog post by auto-updating the blog_posts.comments_count field. I'll outline some of the steps I went through and hopefully someone will notice something I did wrong. The schema dump is at the end.
I have a Blog Post model:
class Post < ActiveRecord::Base
set_table_name("blog_posts")
belongs_to :author, :class_name => "User", :foreign_key => 'author_id'
has_many :comments, :class_name => "Comment",
:foreign_key => 'post_id', :order => "created_at desc", :dependent => :destroy
has_many :categorizations
has_many :categories, :through => :categorizations
named_scope :recent, :order => "created_at desc", :limit => 5
end
and a Comments model with a counter_cache set to the post model:
class Comment < ActiveRecord::Base
belongs_to :post, :class_name => "Post", :foreign_key => "post_id", :counter_cache => true
belongs_to :author, :class_name => "User", :foreign_key => "author_id"
end
I created a migration to add the counter_cache column to the blog_posts table:
class AddCommentCounter < ActiveRecord::Migration
def self.up
add_column :blog_posts, :comments_count, :integer, :limit => 4, :default => 0, :null => false
Post.find(:all).each do |post|
current_count = post.comments.size
post.update_attribute(:comments_count, current_count)
end
end
def self.down
remove_column :blog_posts, :comments_count
end
end
But the migration fails to update the blog posts with the current_count. It's always zero.
I opened up the Rails console to try to update_attribute manually:
Loading development environment (Rails 2.3.2)
>> p = Post.find 1
p = Post.find 1
=> #<Post id: 1, title: "test", content: "test", author_id: 1, status: "ok", created_at: "2009-05-21 19:27:14", updated_at: "2009-05-24 07:02:35", comments_count: 0>
>> p.comments
p.comments
=> [#<Comment id: 5, post_id: 1, author_id: 1, content: "Fifth Comment", status: "ok", created_at: "2009-05-24 07:08:56", updated_at: "2009-05-24 07:08:56">, #<Comment id: 4, post_id: 1, author_id: 1, content: "Fourth Comment", status: "ok", created_at: "2009-05-24 07:05:32", updated_at: "2009-05-24 07:05:32">, #<Comment id: 3, post_id: 1, author_id: 1, content: "Third Comment", status: "ok", created_at: "2009-05-24 06:34:59", updated_at: "2009-05-24 06:34:59">, #<Comment id: 2, post_id: 1, author_id: 1, content: "Second Comment", status: "ok", created_at: "2009-05-24 05:20:43", updated_at: "2009-05-24 05:20:43">, #<Comment id: 1, post_id: 1, author_id: 1, content: "First Comment", status: "ok", created_at: "2009-05-21 19:27:14", updated_at: "2009-05-21 19:27:14">]
>> p.comments.size
p.comments.size
=> 5
>> p.comments_count
p.comments_count
=> 0
>> p.update_attribute(:comments_count, 5)
p.update_attribute(:comments_count, 5)
=> true
>> p.comments_count
p.comments_count
=> 5
>> p.save
p.save
=> true
But when I look in the database comments_count = 0.
Any ideas would be more than joyously appreciated.
My schema.db looks like this:
ActiveRecord::Schema.define(:version => 20090524055907) do
create_table "blog_posts", :force => true do |t|
t.string "title", :limit => 100, :default => "", :null => false
t.text "content", :null => false
t.integer "author_id", :default => 0, :null => false
t.string "status", :limit => 20, :default => "", :null => false
t.datetime "created_at"
t.datetime "updated_at"
t.integer "comments_count", :default => 0, :null => false
end
add_index "blog_posts", ["author_id"], :name => "index_blog_posts_on_author_id"
create_table "categories", :force => true do |t|
t.string "name", :limit => 50, :default => "", :null => false
t.string "short_name", :limit => 30, :default => "", :null => false
t.string "description", :default => "", :null => false
t.datetime "created_at"
t.datetime "updated_at"
end
create_table "categories_posts", :force => true do |t|
t.integer "category_id", :null => false
t.integer "post_id", :null => false
end
add_index "categories_posts", ["category_id"], :name => "index_categories_posts_on_category_id"
add_index "categories_posts", ["post_id"], :name => "index_categories_posts_on_post_id"
create_table "comments", :force => true do |t|
t.integer "post_id", :default => 0, :null => false
t.integer "author_id", :default => 0, :null => false
t.text "content", :null => false
t.string "status", :limit => 25, :default => "", :null => false
t.datetime "created_at"
t.datetime "updated_at"
end
create_table "sessions", :force => true do |t|
t.string "session_id", :default => "", :null => false
t.text "data"
t.datetime "created_at"
t.datetime "updated_at"
end
add_index "sessions", ["session_id"], :name => "index_sessions_on_session_id"
add_index "sessions", ["updated_at"], :name => "index_sessions_on_updated_at"
create_table "users", :force => true do |t|
t.string "username", :limit => 25, :default => "", :null => false
t.string "hashed_password", :limit => 40, :default => "", :null => false
t.string "first_name", :limit => 25, :default => "", :null => false
t.string "last_name", :limit => 40, :default => "", :null => false
t.string "email", :limit => 50, :default => "", :null => false
t.string "display_name", :limit => 25, :default => "", :null => false
t.integer "user_level", :limit => 3, :default => 0, :null => false
t.datetime "created_at"
t.datetime "updated_at"
end
end
回答1:
Take a look at the updated code from railscasts episode 23 on counter_cache.
The counter attributes are attr_readonly. Perhaps update-counters is what you need to use in place of update attributes in your migration?
来源：https://stackoverflow.com/questions/903294/counter-cache-doesnt-update-but-i-can-save-to-the-parent-and-the-child