Rails Caching DB Queries and Best Practices

Question

The DB load on my site is getting really high so it is time for me to cache common queries that are being called 1000s of times an hour where the results are not changing. So for instance on my city model I do the following:

def self.fetch(id)   
  Rails.cache.fetch("city_#{id}") { City.find(id) }   
end 

def after_save
  Rails.cache.delete("city_#{self.id}")
end

def after_destroy
  Rails.cache.delete("city_#{self.id}")
end

So now when I can City.find(1) the first time I hit the DB but the next 1000 times I get the result from memory. Great. But most of the calls to city are not City.find(1) but @user.city.name where Rails does not use the fetch but queries the DB again... which makes sense but not exactly what I want it to do.

I can do City.find(@user.city_id) but that is ugly.

So my question to you guys. What are the smart people doing? What is the right way to do this?

Answer 1

With respect to the caching, a couple of minor points:

It's worth using slash for separation of object type and id, which is rails convention. Even better, ActiveRecord models provide the cacke_key instance method which will provide a unique identifier of table name and id, "cities/13" etc.

One minor correction to your after_save filter. Since you have the data on hand, you might as well write it back to the cache as opposed to delete it. That's saving you a single trip to the database ;)

def after_save
  Rails.cache.write(cache_key,self)
end

As to the root of the question, if you're continuously pulling @user.city.name, there are two real choices:

• Denormalize the user's city name to the user row. @user.city_name (keep the city_id foreign key). This value should be written to at save time.

-or-

• Implement your User.fetch method to eager load the city. Only do this if the contents of the city row never change (i.e. name etc.), otherwise you can potentially open up a can of worms with respect to cache invalidation.

Personal opinion: Implement basic id based fetch methods (or use a plugin) to integrate with memcached, and denormalize the city name to the user's row.

I'm personally not a huge fan of cached model style plugins, I've never seen one that's saved a significant amount of development time that I haven't grown out of in a hurry.

If you're getting way too many database queries it's definitely worth checking out eager loading (through :include) if you haven't already. That should be the first step for reducing the quantity of database queries.