Using Inner/Outer join to eager fetch belongs_to associations

rubyonrails-talk
1

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.

2

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

3

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

4

But is a plugin really required? Can't you feed something to find_by_sql that will get you both types of objects?

5

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

6

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. :wink:

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.