I just recently built a rake task in my project where I wanted to loop through a list of Users, queue up some data for each one, then loop back through them again to process the data that had been queued. I didn’t need the data to live for very long, but I needed it for 2 separate requests.
The problem with this is that in Ruby on Rails, instance variables are not persistent across requests. This means their scope is limited to one usage of an object and the next time you call for that object, the variable is gone.
Unfortunately, the only solution I could imagine for my problem without persistent instance variables was to create a massive join table to connect my generated data between Users. Not only would this have been a lot of work, it would have been a big waste considering the data would have only lived for about a minute as the rake task was executed.
I wanted to improve/increase the scope of instance variables, and so my solution was to take advantage of Class Variables.
At the bottom of my User class, I now have the following:
protected def late_employees @@late_employees ||= {} @@late_employees[self] ||= {} @@late_employees[self] end def late_team_members @@late_team_members ||= {} @@late_team_members[self] ||= {} @@late_team_members[self] end def clear_late_emails @@late_employees[self] = {} @@late_team_members[self] = {} end
As you can see, each element of the Class Variable hashes are referenced by the current instance. So when I call u.late_employees, it’s always the same across requests until either I call u.clear_late_emails or the server dies.
To use them, just treat them like any normal instance method/variable. Here is an example of how you can do that:
def employee_late(user, days) if user.manager == self late_team_members[user] = days else late_employees[user] = days end end def has_late_employees? !(late_employees.empty? and late_team_members.empty?) end def send_late_employee_emails if has_late_employees? EvaluationNotifier.deliver_late_employee_notification(self) end length = late_employees.length + late_team_members.length clear_late_emails length end
Voila, instance variables that are persistent across requests!
9 comments ↓
Thank you, however, How do you use the variable ?, I meant how do you store data into and get data out of those variables? could you give an example of that?
Certainly! I’ll append it to the blog itself so that it’s easier to follow. Thank you for the question.
What do your deployments look like? Does this work if Mongrel#1 handles the first request, and Mongrel#2 handles the second one?
Gwyn, great question! I tested locally starting up two environments to be sure and found that they do not share instance variables across servers. I’m pretty sure that instance/class variables are stored in memory, meaning they will only be available to the local environment.
The catch is that grabbing the same model on each environment will give you two separate instances, and thus two separate sets of instance variables.
If you need data to extend to everywhere, including across your separate Mongrel’s, you’ll need to store it in the db.
Its good
How i take data from static variable which i put in model,can we do caching statically
Hi, I am also facing the same problem in one of my projects, but with only one variable and I am not using a list. How about then.. how can persist the values..
@Sam: If it’s only one variable, just set it in a class method:
This should be accessible from anywhere by calling
ClassName.project_name
Does that help?
Hi,
i’m trying to use class variable but i’m not able to do that.. can anyone explain me how can i set and retrieve the value of the variable?
Thanks!