Ruby: Include a dynamic module name

Question

I have a situation in my Rails application where I need to include arbitrary modules depending on the current runtime state. The module provides custom application code that is only needed when certain conditions are true. Basically, I'm pulling the name of a company from the current context and using that as the filename for the module and its definition:

p =  self.user.company.subdomain + ".rb"
if File.exists?(Rails.root + "lib/" + p)
include self.class.const_get(self.user.company.subdomain.capitalize.to_sym)
    self.custom_add_url
end

My test module looks like this:

module Companyx
    def custom_add_url
    puts "Calling custom_add_url"   
end 
end

Now in the console, this actually works fine. I can pull a user and include the module like so:

[1] pry(main)> c = Card.find_by_personal_url("username")
[2] pry(main)> include c.class.const_get(c.user.company.subdomain.capitalize)=> Object
[3] pry(main)> c.custom_add_url

Calling custom_add_url

If I try to run the include line from my model, I get

NoMethodError: undefined method `include' for #<Card:0x007f91f9094fb0>

Can anyone suggest why the include statement would work on the console, but not in my model code?

Community · Accepted Answer · 2017-05-23 12:34:14Z

16

I'm doing a similar thing. I found this answer useful: How to convert a string to a constant in Ruby?

Turns out I was looking for the constantize method. This is the line I'm using in my code:

include "ModuleName::#{var.attr}".constantize

Edit:

So ack, I ran into various problems with actually using that line myself. Partially because I was trying to call it inside a method in a class. But since I'm only calling one method in the class (which calls/runs everything else) the final working version I have now is

"ModuleName::#{var.attr}".constantize.new.methodname

Obviously methodname is an instance method, so you could get rid of the new if yours is a class method.

edited May 23, 2017 at 12:34

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answered Oct 26, 2012 at 16:18

undefinedvariable

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Comments

Dave Newton · Accepted Answer · 2020-03-17 13:46:09Z

5

Include is a method on a class.

If you want to call it inside a model, you need to execute the code in the context of its singleton class.

p =  self.user.company.subdomain + ".rb"
if File.exists?(Rails.root + "lib/" + p)
myself = self
class_eval do
  include self.const_get(myself.user.company.subdomain.capitalize.to_sym)
end
self.custom_add_url

EDIT:

class << self doesn't accept a block; class_eval does, hence it preserves the state of local variables. I've modified my solution to use it.

edited Mar 17, 2020 at 13:46

Dave Newton

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answered Apr 27, 2012 at 13:49

Joe Pym

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5 Comments

Aaron Vegh Over a year ago

Changing the context seems to add a complication: the self object ceases to exist (or more accurately, its association with the user object does)? I have tried various methods to work around this and read this article to understand what's going on here (yehudakatz.com/2009/11/15/…) without success.

Joe Pym Over a year ago

Oh, my bad. Basically, self in the new context is the class. It's similar to doing a class eval. I did something like this as an experiment on a personal project. The top of this file should help: github.com/Hitonagashi/UndergroundFootball/blob/master/app/…

Aaron Vegh Over a year ago

Not quite sure I understand: I think you're talking about the initialize method in your code, where you're adding all possible skills as class methods dynamically? I don't see how that applies to my problem, as I can't add an associated object to the class. Is there some way to access the instance's relations to code inside the class block?

Joe Pym Over a year ago

I've updated my solution with a class_eval approach(and the reasoning for it). Let me know how that works for you!

Aaron Vegh Over a year ago

Yes! I just figured out the same thing myself while you were writing. Took another look at your sample code and it got through my head what you were talking about. My ultimate solution is even simpler: self.class_eval "include #{self.user.company.subdomain.capitalize}"

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