Forum: Ruby on Rails Using Inner/Outer join to eager fetch belongs_to associations

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Justin H. (Guest)
on 2008-11-04 08:45
(Received via mailing list)
I was wondering if it was possible to have rails use an inner/outer
join to eager fetch a belongs_to association, rather than having it
generate multiple SQL queries?

In this case, here are my model objects with the assoications:

class Restaurant < ActiveRecord::Base
  belongs_to :state
end

class State < ActiveRecord::Base
  has_many :restaurants
end


In my controller action, I am currently doing...
  @restaurant = Restaurant.find(params[:id], :include => [:state])

... which is causing multiple SQL queries to be generated, one to
select the restaurant info, and the other to select the state info. Is
there a way to easily override this default behavior and have rails
generate a single query with an INNER or OUTER join while still
populating the state info for the restaurant?

I did a bit of research on the changes to ActiveRecord between 2.0 and
2.1, and I understand why the default behavior is to generate multiple
SQL queries, since in the case where you have a relationship such as
has_many that is referenced in the :include, if ActiveRecord were to
use a single SQL query, AR might end up having to parse a number of
records much larger than the number of distinct rows you will be
displaying.

I am also aware that if you invoke a condition on a row not in the
table corresponding to the model object in the call to find(), that
ActiveRecord will fall back on the old method of generating a single
query with joins. However, I would prefer to not add any conditions,
as that seems like kind of a hack. If absolutely necessary, I suppose
I a condition like "states.id = restaurants.state_id" could be added,
but that just seems altogether ugly.

However, in some cases, such as the one I mention above, I think it
might be desirable to be able to ask ActiveRecord to generate a single
query, since the result of the SQL would be at most a single row
anyhow.
Justin H. (Guest)
on 2008-11-04 09:01
(Received via mailing list)
I did a bit more looking in to using a :conditions hash to force
ActiveRecord to fall back on the Rails 2.0 method of eager fetching
associations, and this is absolutely not what I am looking for. For
one, the Rails 2.0 eager fetch strategy always uses OUTER JOINs, and
it also includes a join for all associations specified as an :include,
rather than just those that are specified in a condition.

I am looking for some way to essentially force ActiveRecord to eagerly
fetch specific associations/:include's using an INNER JOIN in the
query that is also being used to fetch the data for the base object.
The more I think about it, the more I think that this functionality
does not really exist in ActiveRecord, and it would have to be written
as an extension/plugin.

However, since I'm pretty new to Rails, I wanted to put it to all the
experts on this forum before throwing in the towel.

- Justin
Frederick C. (Guest)
on 2008-11-04 11:32
(Received via mailing list)
On Nov 4, 7:00 am, Justin H. <removed_email_address@domain.invalid> wrote:
> The more I think about it, the more I think that this functionality
> does not really exist in ActiveRecord, and it would have to be written
> as an extension/plugin.
>
> However, since I'm pretty new to Rails, I wanted to put it to all the
> experts on this forum before throwing in the towel.

Yup you've got it pretty much all figured out. For what it's worth,
when it was just a belongs_to/has_one, the overhead of generating the
crazy query with the joins, the database running the query, rails
doing its fancy parse thingy on the results meant that

Foo.find 123456, :include => :bar

was often no faster than
f = Foo.find 123456
Bar.find 456798

Fred
Roy P. (Guest)
on 2008-11-04 18:56
(Received via mailing list)
But is a plugin really required?  Can't you feed something to
find_by_sql that will get you both types of objects?
Justin H. (Guest)
on 2008-11-05 06:48
(Received via mailing list)
I'm sure you could write a SQL query to fetch everything with a single
query. However, one of the things I like about ORM, be it
ActiveRecord, Hibernate, etc, is not writing SQL, and being able to do
write the data access code in the native language (Ruby, Java, C#,
etc.). Of course, it is comforting to know that you can always fall
back on SQL when you need it, but most of the time, I would prefer to
let the ORM generate the SQL.

I was simply curious to see if it was possible to get ActiveRecord to
fetch data for the base object plus some of it's associations in a
single query using INNER JOINs. It appears this is not possible
without either resorting to SQL queries (find_by_sql), or creating
some kind of extension for ActiveRecord, in the case where you don't
want to use SQL. If it ever becomes a major issue for me, I guess I'll
just have to pick one of the two methods, unless of course, by that
time someone has already written a plugin.

Thanks for the responses.

- Justin
Roy P. (Guest)
on 2008-11-05 21:24
(Received via mailing list)
Fetch, for sure--I'm just unsure whether AR will use the extra data to
actually populate the child objects.  If only I wasn't so lazy, I'd
consult the docs and/or try it out. ;-)

Definitely take your point about taking advantage of the ORM.  But it
seems to me that some people get fetishistic about avoiding SQL--are
willing to torture AR into generating the exact SQL they want.  That's
fun for a bit, but there's definitely a point where AR leaves you
hanging--which is why find_by_sql exists.
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