_form.haml

= form_for([:admin, @stock]) do |f|
  - if @stock.errors.any?
    #error_explanation
      %h2
        = pluralize(@stock.errors.count, "error")
        prohibited this stock from being saved:
      %ul
        - @stock.errors.full_messages.each do |msg|
          %li= msg
  .field
    = f.label :value, "Værdi"
    %br/
    = f.number_field :value
  .field
    = f.label :month, "Måned"
    %br/
    = f.number_field :month
  .field
    = f.label :year, "År"
    %br/
    = f.number_field :year
  
  .field
    = f.fields_for([:admin, Investor]) do |fi|
      = fi.collection_select(:investor, :id, Investor.all, :id, :name, {:prompt => 'Vælg investor'})

    %br/
  .actions
    = f.submit "Gem"

stocks_controller.rb

def create
    @stock = Stock.new(params[:stock])
    @is = InvestorStock.new(params[:stock][:investor])

    respond_to do |format|
      if @stock.save
        @is.stock_id = @stock.id
        @is.save
        format.html { redirect_to admin_stock_url(@stock), notice: 'Stock was successfully created.' }
        format.json { render json: @stock, status: :created, location: @stock }
      else
        format.html { render action: "new" }
        format.json { render json: @stock.errors, status: :unprocessable_entity }
      end
    end
  end