Models / Entities vs complex types

Question

I have a problem understanding complex data types vs entities in ASP MVC and C#. I started by creating the following type, which is not meant to be an entity (DB table):

public class Period : IPeriod
{
    public DateTime From { get; set; }
    public DateTime To { get; set; }
}

I added this type to a model:

public class Event
{
    public int ID { get; set; }
    public string Title { get; set; }
    public Period EventTime { get; set; }
}

This worked fine. It seemed to understand that Period is a complex type, but not an entity. It just added the columns EventTime_From and EventTime_To to the Event table.

However, then I added some complexity to the Period type by removing the interface, moving the class to a different namespace and adding methods to the class. Suddenly, when trying to rebuild the database, it was suddenly interpreted as an entitytype, thus throwing an Exception for missing a primary key.

What is the system here? When are classes seen as complex types, and when are they seen as entities?

Answer 1

It is hard to say what exactly is going on without knowing all changes you made but you can always tell EF explicitly that Period is a complex type by either marking it with ComplexType attribute or by using fluent mapping:

modelBuilder.ComplexType<Period>();