Validating data of a model from another controller in Rails

def create
  @client = Client.new(params[:client])

  respond_to do |format|
    if @client.save
      # Add purchase
      @sell = Purchase.new
      @sell.client_id = @client.id
      @sell.date = params[:date]
      # Fill another fields
      if @sell.save

        # Do another stuff...

      else
        format.html { render :action => "new" }
        format.xml  { render :xml => @client.errors, :status => :unprocessable_entity }
      end

      flash[:notice] = 'You have a new client!'
      format.html { redirect_to(:action => :show, :id => @evento.id) }
      format.xml  { render :xml => @client, :status => :created, :location => @client }
    else
      format.html { render :action => "new" }
      format.xml  { render :xml => @evento.client, :status => :unprocessable_entity }
    end
  end
end

belongs_to :client
validates_format_of :date, :with => /^20[0-9]{2}[-][0-9]{2}[-][0-9]{2}$/, :message => 'not valid'
validates_presence_of :date

Client.purchases.empty?
Client.purchases.size,
Client.purchases
Client.purchases<<(purchase)
Client.purchases.delete(purchase)
Client.purchases.find(purchases_id)
Client.purchases.find_all(conditions)
Client.purchases.build
Client.purchases.create