#models

class Branch < ActiveRecord::Base
belongs_to :company
end

class Company < ActiveRecord::Base
has_many :branches

def self.new_with_main_branch
  @company = new
  @branch = @company.branches.build(:main => true)
end
end

#controllers

class CompaniesController < ApplicationController
def new
 @company = Company.new_with_main_branch
end

def edit
 @company = Company.find(params[:id])
end

def create
  @company = Company.create(params[:company])
  if @company.save
    flash[:notice] = "Company successfully created"
    redirect_to companies_url(@company)
  else
    flash[:warning] = "Company creation failed"
    render :action => "new"
  end
end

end