default_scope and associations

问题:

Suppose I have a Post model, and a Comment model. Using a common pattern, Post has_many Comments.

If Comment has a default_scope set:

default_scope where("deleted_at IS NULL")

How do I easily retrieve ALL comments on a post, regardless of scope? This produces invalid results:

Post.first.comments.unscoped

Which generates the following queries:

SELECT * FROM posts LIMIT 1;
SELECT * FROM comments;

Instead of:

SELECT * FROM posts LIMIT 1;
SELECT * FROM comments WHERE post_id = 1;

Running:

Post.first.comments

Produces:

SELECT * FROM posts LIMIT 1;
SELECT * FROM comments WHERE deleted_at IS NULL AND post_id = 1;

I understand the basic principle of unscoped removing all existing scopes, but shouldn't it be aware and to keep the association scope?

What is the best way to pull ALL comments?

回答1:

For some strange reasons,

Comment.unscoped { Post.last.comments }

includes the default_scope of Comment,

however,

Comment.unscoped { Post.last.comments.to_a }
Comment.unscoped { Post.last.comments.order }

do not include the default_scope of Comment.

I experienced this in a rails console session with Rails 3.2.3.

回答2:

with_exlusive_scope is deprecated as of Rails 3. See this commit.

Before (Rails 2):

Comment.with_exclusive_scope { Post.find(post_id).comments }

After (Rails 3):

Comment.unscoped { Post.find(post_id).comments }

回答3:

Rails 4.1.1

Comment.unscope(where: :deleted_at) { Post.first.comments }

Or

Comment.unscoped { Post.first.comments.scope }

Note that I added .scope, it seems like this block should return kind of ActiveRecord_AssociationRelation (what .scope does) not ActiveRecord_Associations_CollectionProxy (without a .scope)

回答4:

This is indeed a very frustrating problem which violates the principle of least surprise.

For now, you can just write:

Comment.unscoped.where(post_id: Post.first)

This is the most elegant/simple solution IMO.

Or:

Post.first.comments.scoped.tap { |rel| rel.default_scoped = false }

The advantage of the latter:

class Comment < ActiveRecord::Base
  # ...

  def self.with_deleted
    scoped.tap { |rel| rel.default_scoped = false }
  end
end

Then you can make fun things:

Post.first.comments.with_deleted.order('created_at DESC')

Since Rails 4, Model.all returns an ActiveRecord::Relation , rather than an array of records. So you can (and should) use all instead of scoped:

Post.first.comments.all.tap { |rel| rel.default_scoped = false }

回答5:

class Comment
  def post_comments(post_id)
    with_exclusive_scope { find(all, :conditions => {:post_id => post_id}) }
  end
end

Comment.post_comments(Post.first.id)