company.rb

class Company < ActiveRecord::Base
  set_primary_key :nip
  attr_accessible :address_number, :address_postalcode, :address_street,
                  :application_type, :company_url, :name, :nip, :trade,
                  :admin_id

  has_many :jobs
  has_many :employments
  has_many :employers, through: :employments

  validates :nip,
    nip: true,
    presence: true,
    uniqueness: true
  validates :name,
    presence: true
  validates :admin_id,
    presence: true

  def admin
    employers.find(admin_id)
  end
end

# == Schema Information
#
# Table name: companies
#
#  address_number     :string(255)
#  address_postalcode :string(255)
#  address_street     :string(255)
#  admin_id           :integer
#  application_type   :string(255)
#  company_url        :string(255)
#  created_at         :datetime         not null
#  name               :string(255)
#  nip                :integer          primary key
#  updated_at         :datetime         not null
#

employer.rb

class Employer < ActiveRecord::Base
  attr_accessible :email, :first_name, :last_name, :phone_number

  has_many :employments
  has_many :companies, through: :employments
  has_many :jobs

  validates :first_name,
    presence: true
  validates :email,
    presence: true
end

# == Schema Information
#
# Table name: employers
#
#  created_at   :datetime         not null
#  email        :string(255)
#  first_name   :string(255)
#  id           :integer          not null, primary key
#  last_name    :string(255)
#  phone_number :string(255)
#  updated_at   :datetime         not null
#

employment.rb

class Employment < ActiveRecord::Base
  attr_accessible :company_id, :created_at, :employer_id
  belongs_to :company
  belongs_to :employer
end

# == Schema Information
#
# Table name: employments
#
#  company_id  :integer
#  created_at  :datetime         not null
#  employer_id :integer
#  id          :integer          not null, primary key
#  updated_at  :datetime         not null
#

job.rb

class Job < ActiveRecord::Base
  attr_accessible :additional_description, :address_city, :address_number, :address_postalcode,
                  :address_street, :application_address, :application_date, :employer_id,
                  :position, :requirements, :job_description, :trade_id, :company_id

  attr_accessible :company_attributes, :employer_attributes

  belongs_to :company
  belongs_to :employer
  belongs_to :trade

  accepts_nested_attributes_for :company, :employer

  validates :position,
    presence: true
  validates :trade_id,
    presence: true

  def self.create_with_company(job_params)
    company_attributes = job_params.delete(:company_attributes)
    employer_attributes = job_params.delete(:employer_attributes)

    new(job_params) do |job|
      job.employer = Employer.find_or_create_by_email(employer_attributes)
      company_attributes[:admin_id] = job.employer.id if Company.find_by_nip(company_attributes[:nip]).nil?
      job.company = Company.find_or_create_by_nip(company_attributes)
      Employment.create(employer_id: job.employer.id, company_id: job.company.id)
    end
  end
end

# == Schema Information
#
# Table name: jobs
#
#  additional_description :text
#  application_date       :date
#  company_id             :integer
#  created_at             :datetime         not null
#  employer_id            :integer
#  id                     :integer          not null, primary key
#  job_description        :text
#  position               :string(255)
#  requirements           :text
#  trade_id               :integer
#  updated_at             :datetime         not null
#

jobs_controller.rb

class JobsController < ApplicationController
  def index
    @jobs = Job.all
  end

  def new
    @job = Job.new
    @job.company = Company.new
    @job.employer = Employer.new
  end

  def create
    @job = Job.create_with_company(params[:job])

    if @job.save
      redirect_to jobs_path,
        notice: t('activerecord.successful.messages.created', model: @job.class.model_name.human)
    else
      render 'new'
    end
  end

  def edit
    @job = Job.find(params[:id])
  end

  def update
    @job = Job.find(params[:id])

    if @job.update_attributes(params[:job])
      redirect_to jobs_path,
        notice: t('activerecord.successful.messages.created', model: @job.class.model_name.human)
    else
      render 'edit'
    end
  end
end

new.html.haml

= simple_form_for @job, html: { multipart: true, class: 'form-horizontal' } do |f|
  = f.simple_fields_for :company do |c|
    = c.input :nip
    = c.input :name
    = c.input :address_street
  = f.input :trade_id, :collection => Trade.all
  = f.input :job_description
  = f.input :position
  = f.input :requirements
  = f.simple_fields_for :employer do |e|
    = e.input :first_name
    = e.input :last_name
    = e.input :email
  = f.submit