gistfile1.rb

# InventoryAdjustmentTransaction

  def validate
    return unless @item_id || !inventory_adjustment_entry.item
    
    item = (self.inventory_adjustment_entry && self.inventory_adjustment_entry.item) || Item.find(@item_id)
    if item.date_qty_falls_below_zero(inventory_adjustment_entry)
      errors.add('item_qty', 'Cannot cause inventory qty to fall below zero.')
    end
  end

#Item

  def date_qty_falls_below_zero(modified_entry, time = :forever)
    time = time.end_of_day if time.is_a?(Time)

    # Collect the item's inventory action entries and include the modified entry.
    entries = (time != :forever) ? inventory_adjustment_entries.find_before(time) : inventory_adjustment_entries.find_all
    
    current_qty = entries.inject('0'.to_d) do |qty, entry|
      if entry.debit_credit_flag == 'Debit'
        qty += entry.item_qty
      else
        qty -= entry.tem_qty
      end
    end
    
    return modified_entry.debit_credit_flag == 'Debit' ? (current_qty + modified_entry.item_qty < 0) : (current_qty - modified_entry.item_qty < 0)
  end