Virtual Fields

This wiki article is a explanation of Virtual Fields for bizobjs. Virtual Fields are seen and accessed just like regular data fields. The only exception is they are not stored in the database. These fields are compiled at runtime. A good example of a virtual field is a subtotal for an order item.

We have a table that keeps track of items added to an order. The table has several variables:

• auto-incrementing ID fields
• ID for order
• ID for item
• Quantity
• Price

This is all good and dandy, except for we want an easy way to calculate the sub-total. We don't store this information in the database because it's redundant and we can get it from the quantity and price. So, we use a virtual fields to calculate the subtotal for a row dynamically. Take a look at the following code for the Bizobj:

class OrderItemsBizobj(dabo.biz.dBizobj):
	def afterInit(self):
                self.addFrom("orderItems")
		self.addField("ID")
		self.addField("OrderID")
		self.addField("ItemID")
		self.addField("Quantity")
		self.addField("Price")
		
		self.DataSource = "orderItems"
		self.KeyField = "ID"
		self.NonUpdateFields=["ID"]
	
	        self.VirtualFields = {"SubTotal":self.getSubTotal}
	
	def getSubTotal(self):
		return self.Record.Price * self.Record.Quantity

Now, this seems pretty straight forward right. The VirtualFields property is just a dictionary of key/value pairs where the key is the name of the field and the value is the function to call to get the value. You can now use the field "SubTotal?" as if it were a regular bizobj field. Every time we need to get the value of SubTotal? for a row, the getSubTotal function is called.