Rails - May 2008 - Get parents via a condition on a child? (ActiveRecord)

I come from a PHP background where I worked without an ORM.  I''m having
some trouble converting what I know will work in raw SQL into
ActiveRecord models.

I have the following two models (not my actual models, just as a generic
example):

class Topic < ActiveRecord::Base
  has_many :posts
  ...
end

class Post < ActiveRecord::Base
  belongs_to :topic
  attr_accessible :body
  ...
end

I want to work with the collection of Topics that have Posts with a
certain value of :body.  That is, "All topics that contain posts whose
body is equal to ''example''"

If I was working with straight SQL I would do this

SELECT topics.* FROM
  topics JOIN posts ON topics.id = posts.topic_id
  WHERE
  posts.body = "example"

In ActiveRecord I can get the list of Posts without a problem.

Post.find(:all, :conditions => {:body => "example"})

I think this is equivalent to the above

Post.find_all_by_body("example")

What I can''t figure out is how to get the collection of Topics.

This doesn''t work because Topics don''t have :body''s

Topic.find(:all, :include => :posts, :conditions => {:body =>
"example"})

I''ve looked at the generated SQL and because the :posts table is
aliased
I don''t think I can come up with SQL fragment that would consistently
work.

I can see how I may be able to first get the collection of Posts and
then in another call to #find get the Topics I need, but that seems too
roundabout.

What''s the best way to do this with ActiveRecord?

Thanks!
-- 
Posted via http://www.ruby-forum.com/.

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In my limited understanding, I think if you managed to pull out the
collection of Post, you have access to all Topic that are linked to
that collection of Post.

I don''t know if this will work, I think the following example will
print out the Topic name of each of the post that has body = example.


  allpost = Post.find(:all, :conditions => {:body => "example"})

  allpost.each do | onepost |
      puts onepost.topic.name
  end



On May 14, 2:01 pm, Walter Wilfinger <rails-mailing-l...@andreas-
s.net> wrote:
> I come from a PHP background where I worked without an ORM.  I''m
having
> some trouble converting what I know will work in raw SQL into
> ActiveRecord models.
>
> I have the following two models (not my actual models, just as a generic
> example):
>
> class Topic < ActiveRecord::Base
>   has_many :posts
>   ...
> end
>
> class Post < ActiveRecord::Base
>   belongs_to :topic
>   attr_accessible :body
>   ...
> end
>
> I want to work with the collection of Topics that have Posts with a
> certain value of :body.  That is, "All topics that contain posts whose
> body is equal to ''example''"
>
> If I was working with straight SQL I would do this
>
> SELECT topics.* FROM
>   topics JOIN posts ON topics.id = posts.topic_id
>   WHERE
>   posts.body = "example"
>
> In ActiveRecord I can get the list of Posts without a problem.
>
> Post.find(:all, :conditions => {:body => "example"})
>
> I think this is equivalent to the above
>
> Post.find_all_by_body("example")
>
> What I can''t figure out is how to get the collection of Topics.
>
> This doesn''t work because Topics don''t have
:body''s
>
> Topic.find(:all, :include => :posts, :conditions => {:body =>
> "example"})
>
> I''ve looked at the generated SQL and because the :posts table is
aliased
> I don''t think I can come up with SQL fragment that would
consistently
> work.
>
> I can see how I may be able to first get the collection of Posts and
> then in another call to #find get the Topics I need, but that seems too
> roundabout.
>
> What''s the best way to do this with ActiveRecord?
>
> Thanks!
> --
> Posted viahttp://www.ruby-forum.com/.
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So you want to find all the topics where a post in that topic has the body
of "example"?

Topic.find(:all, :include => :posts, :conditions => ["posts.body
?",params[:query]])

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