Rails 2.0 is getting its first bunch of new features. One of the first additions was a new set of clothes for the query cache. David started this feature quite a while ago but never finished it and it wasn’t activated.

Here is the catalyst which prompted the development:

 Blog.find(1).articles.each { |a| puts "#{a.blog.title} #{a.title}" }
   => Blog Load (0.000461)   SELECT * FROM blogs WHERE (blogs.id = 1) 
   => Article Load (0.000521)   SELECT * FROM articles WHERE (articles.blog_id = 1)
   => Blog Load (0.000461)   SELECT * FROM blogs WHERE (blogs.id = 1) 
   => Blog Load (0.000460)   SELECT * FROM blogs WHERE (blogs.id = 1) 
   => Blog Load (0.000469)   SELECT * FROM blogs WHERE (blogs.id = 1) 
   => Blog Load (0.000460)   SELECT * FROM blogs WHERE (blogs.id = 1) 
   => Blog Load (0.000462)   SELECT * FROM blogs WHERE (blogs.id = 1) 
   [..]

Painful stuff. This is because each belongs_to has its own cache. So for each article the blog object has to be loaded again even though we just received a perfectly capable set of data from the database a second earlier.

With a recent change you can now do the following:

Blog.cache do 
   Blog.find(1).articles.each { |a| puts "#{a.blog.title} #{a.title}" }
   => Blog Load (0.000461)   SELECT * FROM blogs WHERE (blogs.id = 1) 
   => Article Load (0.000521)   SELECT * FROM articles WHERE (articles.blog_id = 1)
end

For the duration of the cache block the same query will not be run twice ( unless you run any INSERTS or UPDATES, in which case all the cache is flushed to disk )

For Rails 2.0 we hope to cultivate these humble beginnings into an automatic cache so that all read queries are only run once per rails request if possible.