## Primary app
class Job < ActiveRecord::Base
  include JobStuff

  belongs_to :user

  def foo
    "bar"
  end
end

## Plugin
module JobStuff
  validates_presence_of :bar
end
#=> NameError: undefined local variable or method `validates_presence_of' for JobStuff:Module

#or

class Job
  def bar
    "foo"
  end
end
# Job.new.bar => NoMethodError: undefined method `bar' for #<Job:0x20f7448>