I have run into the same problem.  I'll give you two ways (that I can 
think of) to implement this.  There may be better ways, but I used the 
first option.

OPTION #1

Create a new model.
app/models/common_model.rb
class CommonModel < ActiveRecord::Base
  # class method you want to share
  def self.some_method
    # do something
  end
 
  # instance method you want to share
  def some_other_method
    # do something
  end
end

Then you can have all of your other models inherit from CommonModel

app/models/existing_model.rb
require_dependency 'common_model'
class ExistingModel < CommonModel
  # model specific code goes here
end

OPTION #2
Another alternative would be to use mixins as you suggested, possibly 
like this.

app/models/common_methods.rb
module CommonMethods
  def some_method
  end
end

app/models/existing_model.rb
require_dependency 'common_methods'
class ExistingModel < ActiveRecord::Base
  # mixin the module to get instance methods
  include CommonMethods
end

Either of these implementations would make the common methods accessible 
to your models, and therefore accessible to any controllers that can 
access those models.

Hope this helps,
Pete

Bob Aman wrote:

>Ok, so, I have a method that I need to use in several models and two
>controllers.  I don't want to duplicate code, and initially, rail's
>concept of a helper seemed like a good fit, but it looks like maybe it
>only works with controllers, so maybe not.  I'm a pretty much a nuby,
>but I think maybe I want to do a mixin?  If so, where would the module
>with my method go within the rails application folder structure?  And
>how do I do a mixin?
>  
>