Userモデル
- job_id: references
Jobモデル
- klass: string

を考える

class User < ApplicationRecord
  belongs_to :job
end

class Job < ApplicationRecord
  has_many :users

  def method_missing(name, *args)
    fail if klass.blank? || self.class.included_modules.include?("Modules::#{self.klass.camelize}".constantize)
    extend("Modules::#{self.klass.camelize}".constantize)
    send(name, *args) and return if respond_to?(name)
    fail
  end
end

module Modules
  module Engineer
    def bark
      p 'hoge'
    end
  end
end

engineer = Job.create(klass: 'engineer')
user = User.create(job: engineer)
user.job.bark # => hoge

やや冗長だけどやや安全版↓
（decoratorパターンになってしまった）

class User < ApplicationRecord
  belongs_to :job
end

class Job < ApplicationRecord
  has_many :users

  def bark
    fail
  end

  def mixined
    klass.present? ? extend("Modules::#{self.klass.camelize}".constantize) : self
  end
end

module Modules
  module Engineer
    def bark
      p 'hoge'
    end
  end
end

engineer = Job.create(klass: 'engineer')
user = User.create(job: engineer)
user.job.mixined.bark # => hoge

完

というのは嘘で素直にSTIしたほうが良いのではないでしょうか。