Rails - Jul 2008 - Simple activerecord relationship question

Hey guys, I have a really simple question about activerecord

Lets say I have a Post model which has a has_many relationship with a
comments model. (a post is a blog post with many comments)

So, when I execute this

@posts = Post.find(:all)

does every post object in @posts contain all the comment objects for
that post?

I know if I do a post.comments I will find them there... but I just
want to know what goes on when I bring all the posts or a single post,
if the post object will be created with every single related comment
object that will take a lot of memory every time I instance one

or are the comment objects instanced when I do a .comments and not
before?


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They''re only instanced, when you access them with
@post.comments

But you can eager load them with
@posts = Post.find(:all, :include => [:comments])
for better performance, if you know you''ll use them anyway

In this case it would get them with an INNER JOIN

btw: You can see the generated SQL in development.log
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thanks for your reply thorsten,

So I don''t have to worry whenever I find a post that it will bring
along with it hundreds of comment objects? only whenever I try to
access them they are queried and instanced?
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Yep,
take te most simple case
@post = Post.find(123)
your SQL for the normal find will look like:
SELECT ALL * FROM posts WHERE id=123;
(in principle, the ALL * is a bit more complicated for Rails
internals, but not making it slower)
In this case a
@posts.comments.each do |comment|
   ...do something with them here...
end

will give you another query like
SELECT ALL * FROM comments WHERE post_id=123;

But only once i think, if you access comments later in code again,
Rails knows, that it''s already in memory

If you use include
@post = Post.find(123, :include => [:comments])

the SQL would look like
SELECT ALL * FROM posts INNER JOIN comments ON
post.id=comments.post_id
which is of course more efficient if you will use them anyway

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Thanks! I had my activerecord basics all wrong, this really helps a
lot

On Jul 16, 11:47 am, Thorsten Müller
<thors...-1oxKqHKwyltBDgjK7y7TUQ@public.gmane.org>
wrote:
> Yep,
> take te most simple case
> @post = Post.find(123)
> your SQL for the normal find will look like:
> SELECT ALL * FROM posts WHERE id=123;
> (in principle, the ALL * is a bit more complicated for Rails
> internals, but not making it slower)
> In this case a
> @posts.comments.each do |comment|
>    ...do something with them here...
> end
>
> will give you another query like
> SELECT ALL * FROM comments WHERE post_id=123;
>
> But only once i think, if you access comments later in code again,
> Rails knows, that it''s already in memory
>
> If you use include
> @post = Post.find(123, :include => [:comments])
>
> the SQL would look like
> SELECT ALL * FROM posts INNER JOIN comments ON
> post.id=comments.post_id
> which is of course more efficient if you will use them anyway
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