Without going through the code magic, it doesn't do one single select
nor a join, but groups the selects per commentable_type per page,
which would still be a saving in my opinion compared to one select
call per comment record.
>> Comment.paginate(:all, :include => :commentable, :page => 1, :per_page => 5)
Comment Load (0.6ms) SELECT * FROM "comments" LIMIT 5 OFFSET 0
Article Load (0.8ms) SELECT * FROM "articles" WHERE
("articles"."id" IN (1,2))
Photo Load (0.2ms) SELECT * FROM "photos" WHERE ("photos"."id" =
1)
On Oct 5, 8:10 am, Bruno Bornsztein <[email protected]>
wrote:
> I'm pretty confused by that. How does will_paginate know which tables to
> join on? Every row in the comments table could be associated with a
> different kind of commentable. Unless I'm missing something, there's no way
> that select can be done in one query. Example:
> Comments Table
> ID commentable_type
> 1 Post
> 2 User
> 3 Clipping
> 4 User
> 5 Post
>
> How can you eagerly load the commentable association? You'd have to do a
> join on posts, users, and clippings, without knowing in advance which tables
> you need to join on.
>
> The only thing I can think of is that will_paginate grabs the result set and
> loops through it and loads the association for each row. Anybody else know
> how this is working?
>
> On Mon, Oct 5, 2009 at 10:04 AM, moritz <[email protected]> wrote:
>
> > will_paginate seems to be handling it:
>
> > >> Comment.paginate(:all, :include => :commentable, :page => 1, :per_page
> > => 2)
> > Comment Load (0.4ms) SELECT * FROM "comments" LIMIT 2 OFFSET 0
> > Article Load (0.2ms) SELECT * FROM "articles" WHERE
> > ("articles"."id" = 1)
> > Photo Load (0.3ms) SELECT * FROM "photos" WHERE ("photos"."id" =
> > 1)
>
> > >> Comment.paginate(:all, :include => :commentable, :page => 2, :per_page
> > => 2)
> > Comment Load (0.4ms) SELECT * FROM "comments" LIMIT 2 OFFSET 2
> > Article Load (0.8ms) SELECT * FROM "articles" WHERE
> > ("articles"."id" IN (1,2))
>
> > compared with:
> > >> Comment.paginate(:all, :page => 1, :per_page => 2)
> > Comment Load (0.4ms) SELECT * FROM "comments" LIMIT 2 OFFSET 0
> > SQL (0.2ms) SELECT count(*) AS count_all FROM "comments"
>
> > This definitely reduces the amount of database queries issued in a
> > longer list.
>
> > On Oct 5, 7:50 am, Bruno Bornsztein <[email protected]>
> > wrote:
> > > Hmmm.... strange that will_paginate doesn't raise an error. It should
> > *not* be
> > > able to eager load a polymorphic association like commentable. The
> > problem
> > > is that with a polymorphic association, the commentable could be from any
> > > number of tables (it could be a User, a Post, a Clipping, etc.), so eager
> > > loading won't work because Rails doesn't know which tables to join on
> > ahead
> > > of time.
> > > I suspect something else is going on here, like maybe will_paginate just
> > > disregards the :include parameter when it's polymorphic (or something).
>
> > > I'm not strongly opposed to using will_paginate if the integration is
> > clean
> > > and upgrading is smooth, but I don't see any great benefit in moving to
> > it
> > > (maybe someone can convince me).
>
> > > Thanks,
> > > Bruno
>
> > > On Sun, Oct 4, 2009 at 10:35 PM, moritz <[email protected]> wrote:
>
> > > > I'm getting an ActiveRecord::EagerLoadPolymorphicError with
> > > > paginating_find when I include an :include => :commentable parameter
> > > > in the call. This doesn't happen when I use will_paginate instead.
>
> > > > After testing and confirming this outside of CE, I am wondering if
> > > > it's possible and of interest to the community to replace
> > > > paginating_find with will_paginate within CE. What would be the
> > > > pitfalls? Any feedback is very welcome.
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