Occasionally we need to do something more fancy. These instances usually come up when we’re splitting one query into two because rails doesn’t support :force_index or :adapter_specific_find_options on ActiveRecord::Base.find. We understand this motivation but really hate ActiveRecord::Base.connection#find_by_sql or ActiveRecord::Base.connection#execute. These are not rational hatreds.

So when we get into a situation where we’re going to be caching manually it’s usually in the controller and we almost always end up with a pattern of:

@object = Rails.cache.read('really/complicated/and/stinky/key')
if @object.nil?
@object = what_should_my_object_be?
end

That’s fine in a contrived example but we were doing this in about 10 different places and it looked like a good candidate for drying up.

Here’s the solution we use:

module ActiveSupport
module Cache
class Store
def read_and_write_if_nil(key, options = {})
object = read(key)
if object.nil?
object = yield
write(key, object, options)
end
object
end
end
end
end

And the production example looks like this:

account_ids = Rails.cache.read_and_write_if_nil("member_ids_for_clan_#{@clan.id}", :expires_in => 5.minutes) do
@clan.members.find(:all, :order => 'groupies DESC', :select => 'accounts.id').collect(&:id)
end