One often overlooked feature of NHibernate is the use of formula properties. Formula properties are properties that do not map to columns in the database, but instead are mapped using raw SQL queries.
The formula is mapped in the NHibernate mapping file using the same property element that normal properties use, just replacing the column keyword for the formula keyword.
<property name="FormulaPropertyName" formula="SQL STATEMENT" />
The basic idea is that when NHibernate loads your object it will issue your custom SQL at the same time, allowing for complex read-only properties to offload their work to the database.
For example, assume you have an class representing an order on an e-commerce website. An order is typically composed of line items, and a common question to ask the order as a whole is “what is your subtotal?”. Without weighing the pros and cons of such a decision assume that you do not want to store the subtotal as a field on the order table.
A perfectly acceptable solution would be to have a read-only property as such:
public decimal SubTotal{ get{ decimal subTotal = 0; foreach(LineItem li in this.LineItems){ subTotal += li.ExtendedPrice; //i.e., units ordered times unit price. } return subTotal; } }
), I have been able to contribute to my open source project again.