Monarch's server side model achieves parity with Arel on the SQL generation front.

gistfile1.ru

specify "joins to groupings" do
  blog_post_counts =
    Blog.left_join_to(BlogPost).
      group_by(Blog[:id]).
      project(Blog[:id].as(:blog_id), BlogPost[:id].count.as(:num_posts))
  Blog.join_to(blog_post_counts).project(:title, :num_posts).to_sql.should be_like(%{
    select
      blogs.title,
      t1.num_posts
    from
      blogs, (
        select
          blogs.id as blog_id,
          count(blog_posts.id) as num_posts
        from
          blogs
          left outer join blog_posts
            on blogs.id = blog_posts.blog_id
        group by blogs.id
      ) as t1
    where
      blogs.id = t1.blog_id
  })

  Blog.join_to(blog_post_counts).to_update_sql(:title => blog_post_counts[:num_posts]).should be_like(%{
    update
      blogs, (
        select
          blogs.id as blog_id,
          count(blog_posts.id) as num_posts
        from
          blogs
          left outer join blog_posts
            on blogs.id = blog_posts.blog_id
        group by blogs.id
      ) as t1
    set
      blogs.title = t1.num_posts
    where
      blogs.id = t1.blog_id
  })
end