# config/routes.rb

map.resources :companies, :has_many => :branches, :collection => {:all => :get}

# app/controllers/companies_controller.rb

# this assumes you have a boolean column defining main in the branches table
def show
  @company.find(params[:id])
  @branch = @company.branches.scoped_by_main(true)
end

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