add class variable to all models using concern to extend AR?

rubyonrails-talk
1

Hi All,

I've previously used concerns to extend my models without problems. I'm now trying to extend all my models in one swell-foop by extending AR with a concern. In brief, I want to add a class variable to each individual model using a concern that extends AR. Specifically - I need to find the where (and how) to define a class variable scoped to a specific model in my AR extending concern. Is this sane? Possible?

Thanks,

Matt

2

Maybe? This seems like requiring the children to know a specific implementation detail of a parent class via the concern mixin. Instead, I’d provide accessor methods.

3

Hi All,

I’ve previously used concerns to extend my models without problems. I’m

now trying to extend all my models in one swell-foop by extending AR

with a concern. In brief, I want to add a class variable to each

individual model using a concern that extends AR. Specifically - I

need to find the where (and how) to define a class variable scoped to a

specific model in my AR extending concern. Is this sane? Possible?

You don’t want a class variable for this - they are shared between all the subclasses. A sample IRB trace:

irb: class Foo

end

===> nil

irb: module Bar

def wat=(value)

  @@wat = value

end

def wat

  @@wat

  end

end

===> nil

irb: Foo.send(:include, Bar)

===> Foo

irb: f = Foo.new

===> #Foo:0x00000100b5a008

irb: f.wat = ‘huh’

===> “huh”

irb: f.wat

===> “huh”

irb: class Baz < Foo

end

===> nil

irb: class Baz2 < Foo

end

===> nil

irb: b = Baz.new

===> #Baz:0x00000100b7ac68

irb: b.wat

===> “huh”

irb: b2 = Baz2.new

===> #Baz2:0x00000100b82580

irb: b2.wat = ‘nope’

===> “nope”

irb: f.wat

===> “nope”

You likely want a class instance variable instead, to avoid the sharing. More details here:

http://www.railstips.org/blog/archives/2006/11/18/class-and-instance-variables-in-ruby/

–Matt Jones

4

Thanks Matt,

Your observations are exactly what I was running into. Good to know I should focus on class instance variables. Inheritance is a bit of mixed bag- I want it for all subclasses of my models whose superclass is not AR::Base.

The link is very helpful, wish I would have seen it earlier. A couple followups- can you comment on whether some of the approach therein has been replaced with the use of 'class_attribute'? Also, do you forsee any issues with using a Concern rather than the extend approach described there?

At present I've fallen back to just extending individual models (this in retrospect might be safer in the long run, albeit not as magikal).

Thanks again for your help, Matt