Question

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Ruby on Rails: How can i join with a derived table?

posts table

                                   Table "public.posts"
   Column    |          Type          |                     Modifiers                      
-------------+------------------------+----------------------------------------------------
 id          | integer                | not null default nextval('posts_id_seq'::regclass)
 title       | character varying(100) | not null
 content     | character varying(500) | not null
 created_at  | date                   | 
 updated_at  | date                   | 
 tags        | character varying(55)  | not null default '50'::character varying
 category_id | integer                | not null default 1
Indexes:
    "posts_pkey" PRIMARY KEY, btree (id)

comments table

                                   Table "public.comments"
   Column   |          Type          |                       Modifiers                       
------------+------------------------+-------------------------------------------------------
 id         | integer                | not null default nextval('comments_id_seq'::regclass)
 post_id    | integer                | not null
 name       | character varying(255) | not null
 email      | character varying(255) | not null
 content    | character varying(500) | not null
 created_at | date                   | 
 updated_at | date                   | 
Indexes:
    "comments_pkey" PRIMARY KEY, btree (id)

I’m trying to make a equivalent rails 3 sql statement of this.

select posts.id, posts.title 
from posts 
inner join (select distinct post_id,created_at  
            from comments 
            order by created_at DESC limit 5
            ) as foo 
        on posts.id=foo.post_id 
order by foo.created_at DESC; 

It’s to get recent commented-post (limit 5). It’s a complex sql query. Need to select posts.title from posts table joining comments table.

Answer 1

The Rails3 way to do a query like that is find_by_sql:

posts = Post.find_by_sql(%q{
    select posts.id, posts.title 
    from posts 
    inner join (select distinct post_id,created_at  
                from comments 
                order by created_at DESC limit 5
                ) as foo 
            on posts.id=foo.post_id 
    order by foo.created_at DESC
})

You could use joins if you wanted but, IMHO, that would just obfuscate things:

Post.select('posts.id, posts.title').
     joins('join (select distinct post_id, created_at from comments order by created_at desc limit 5) as foo on posts.id = foo.post_id').
     order('foo.created_at desc')

Or perhaps something even more complicated like this:

join_to = Comment.select('distinct post_id, created_at').
                  order('created_at desc').
                  limit(5).
                  to_sql

posts = Post.select('posts.id, posts.title').
             joins("join (#{join_to}) as foo on posts.id = foo.post_id").
             order('foo.created_at desc')

Some things are just easier and clearer in SQL than AREL. The find_by_sql method exists for a reason:

find_by_sql provides you with a simple way of making custom calls to the database and retrieving instantiated objects.

and this sort of thing is exactly what find_by_sql is for. You’ll have to pull out find_by_sql for a lot of advanced SQL features (such as CTEs) and there’s no reason to be afraid of it or avoid it: ActiveRecord is a tool, not a dogmatic way of life. find_by_sql does have downsides (such as not playing that nicely with scopes) but so does the standard AREL stuff (such as not playing nicely with non-trivial SQL).