Tag Archives: Entity Framework

In the past, I have worked with some ORM software such as X-tensive DataObject .NET, which generated the database schema based on the Entity models. This type of ORM is called Table-per-Type Inheritance, i.e. each table is mapped to single Entity in the model (*note 1).

However, this doesn’t always work if we have an existing database or we want to maximize the performance of the relational queries or we want to save space in database. In many cases, we don’t want the relational database to look like the Entity model.

Entity Framework supports Table-per-Hierarchy Inheritance, i.e.  different types in the Entity model that inherits from a base class and all mapped to a single table (*note 1).

This is an extension to the articles I found on the Internet:

• Inheritance and Associations with Entity Framework (Moses’s Blog)
• Single Table Inheritance in Entity Framework (Zeeshan Hirani)

(*note 1 were quotes from Moses’s Blog)

School Example

Let’s look at a very traditional “Person”, “Student” and “Instructor” example,  I have created a simple database. With “PersonTypeID” defined as a foreign key in a separated table and this is the major difference from the 2 links above.

In Visual Studio 2010,  I used the “Generate Model from Database” to create the initial EDMX file for my web application.  It helped 50% of the work and I strongly recommended.  Yet, there are  some manual work to do because Visual Studio doesn’t know what the conceptual level would look like.

At this point, it is pure relational-to-object. If you start coding in LINQ, it’s like LINQ-to-SQL.   However, our objective is to make it object oriented.  So, let’s add 2 new Entities in the design tool, namely “Student” and “Instructor” which inherits from base class “Person“.  Also, we have to delete the “HireDate” and “EnrollmentDate” from PERSON entity and reproduce the 2 properties.

If you compile right away, you will get the error:

Error 3005: Problem in mapping fragments starting at line xxx:Must specify mapping for all types in…. An Entity with Key (PK) will not round-trip when: Entity is type [xxxxxxxxx.yyyyyyyyy]

Conditional Mapping

To fix this, we have to add conditional mapping for the 2 Entities (Student and Instructor). This tells the engine to save the value 1 (PersonTypeID) to database (Person table) when we create a Person object.

Mapping Fragment Error

We now have the conditional mapping. Let’s compile again, but you will get this error:

Error 3032: Problem in mapping fragments starting at line xxx:Condition member ‘Person.PersonTypeID’ with a condition other than ‘IsNull=False’ is mapped. Either remove the condition on Person.PersonTypeID or remove it from the mapping.

In conceptual world, we don’t actually need to use “PersonTypeID” since we have already used conditional mapping to define the type of the person. So, we need to delete the “PersonalTypeID” from “Person” Entity.  Then, we need to delete the whole “PersonType” Entity too!  Here is the final model should look like:

If you are not using the “PeronalType” table to store the value, i.e. you are using integer or string, probably you may need to set the “Person” table to “Abstract”, otherwise you will get another annoying error:

Error 3023: Problem in Mapping Fragments starting at line xx,xx,xx : Column xxxx has no default value and is not nullable. A column value is required to store entity data.

Create object and Save to DB

You should now be able to compile without any error.  In the following sample ASPX code, we have created “Student” object and assigned some properties to it. Note that we have NEVER specified the “PersonalTypeID”.

using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Transactions;
using System.Data.Objects;
 
namespace EntityTest
{
    public partial class Test : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            SchoolEntities dataContext = new SchoolEntities();
 
            using (TransactionScope transactionScope = new TransactionScope())
            {
                Student student = new Student();
 
                student.EnrollmentDate = DateTime.Today;
                student.FirstName = "John";
                student.LastName = "Smith";
 
                dataContext.People.AddObject(student);
 
                dataContext.SaveChanges(SaveOptions.AcceptAllChangesAfterSave);
                transactionScope.Complete();
                dataContext.AcceptAllChanges();
            }
 
        }
    }
}

After running the above code, the “Student” is automatically saved with “PersonTypeID” assigned to “2″. It worked!