Adding Custom Property to an Entity in Entity Framework

Let’s assume we have the following scenario.  We have a Person entity in the Entity Framework Model and it a relationship to a Pet entity. Assume the Pet entity has a property called PetName that we want to show on the Person detail, edit, grids, etc. The easiest way is to add a property to the Person entity called PetName. The trick is how to populate it and how to save any changes that users make to it. You can use this same technique (described below) regardless of whether the data you are pulling into the Person table is in your model, entity, etc.

Yes, I understand this example is not really a great example. The model really isn’t that great, but I hope you get the idea.

The first thing to know is that for any entity in your Entity Framework Model you can extend that object by creating a partial class that is in that entities same namespace and classname. Here is our new partial class.

namespace
{
public partial class Person
{
private string _PetName;
public string PetName
{
get
{
string val = sting.Empty;
if (!string.IsNullOrEmpty(_PetName)) val = _PetName;
else
{
if (this.Pet != null)
{
val = this.Pet.PetName;
}
_PetName = val;
}
return val;
}

set { _PetName = value; }
}

 

Notice that we populate the PetName property in a lazy load fashion. Meaning, we don’t go get the data until it is requested. If you use Include(“Pet”) when you get the Person object then the Pet object will be in memory already and not a separate round-trip to the database for each person that is accessed. This query would typically be in your Domain Service and might be called something like GetPersons() or GetPeople() if you renamed it.

This great for displaying information. Note that you don’t need the Set portion of the PetName property if you are just displaying the information and not editing it.

Adding Editing Functionality

To handle the editing we need to go to the Domain Service. Here you should have a UpdatePerson() and InsertPerson() methods. In each of these methods we want to do something to the Pet entity or even call a stored procedure, etc. In our case the the Pet entity is what we want to update so I’ll show you how to do that. Be sure to do the update AFTER the AttachAsModified call otherwise the Load() method call will fail because the currentPerson is not in the ObjectContext.

public void Person(Person currentPerson)
{
this.ObjectContext.People.AttachAsModified(currentPerson, this.ChangeSet.GetOriginal(currentPerson));
if (!currentPerson.PetReference.IsLoaded) currentPerson.PetReference.Load();
if (currentPerson.Pet != null)
{
currentPerson.Pet.PetName = currentPerson.PetName;
}
}

 

Limitations

This is actually pretty easy as you can see. One thing to note about adding properties in general is that you will get a runtime error if you try to use property we added in a Linq To Entities query. This means that filters and sorting by clicking on column headers on the GridView generally won’t work without some extra works that changes the property to the path it actually maps to. In this case it is possible, but in other cases such as stored procs being called, this mapping would not be possible.