EF4 “Code First” enables you to optionally override its default database persistence mapping rules, and configure alternative ways to map your classes to a database.

There are a few ways to enable this.  One of the easiest approaches is to override the “OnModelCreating” method defined on the DbContext base class:

The OnModelCreating method above will be called the first time our NerdDinners class is used within a running application, and it is passed a “ModelBuilder” object as an argument.  The ModelBuilder object can be used to customize the database persistence mapping rules of our model objects.  We’ll look at some examples of how to do this below.

EF only calls the “OnModelCreating” method once within a running application – and then automatically caches the ModelBuilder results.  This avoids the performance hit of model creation each time a NerdDinners class is instantiated, and means that you don’t have to write any custom caching logic to get great performance within your applications.