Entity Splitting in Entity Framework

Entity Splitting in Entity Framework

One of the mapping scenarios
that I talked about in the
session at WDC this week
but I didn’t show an example
is entity splitting in Entity
Framework. This post will explain
what is entity splitting and how to implement this mapping scenario
in Entity Framework.

What is Entity Splitting?

Entity splitting is a scenario that happen when our entity is constructed
from many tables in the database. This happens for example when we don’t
want duplications in our database and use lookup tables instead which our
table has a reference key to those tables. More scenarios can be that we
decide to split our entity representation in the database.
When we have such scenarios the wizard of Entity Framework don’t know
that we split our entity and we have to map the entity to more then
one table.

Splitting to Two Tables Example

In the example I have a database that looks like this:

As you can see I split the employee entity to two tables: Employees and
Address. The two tables have the same primary key which is EmployeeID.
When I use the Entity Framework wizard and construct my model
the result will be:

I want to have a single employee entity which contains its address.
How can I do that?
The answer is easy. I’ll map Employee entity to two tables instead of
one and drop the Address entity.

Step 1

Cut and paste the address details (City, Street and ZipCode) to the
Employee entity.

Step 2

Add a second mapping to the Employee entity. The mapping should be
to the Address table. When you do that Entity Framework is smart enough
to map the properties to their database fields.
The result should look like:

Step 3

Remove the Address entity from the model.

Step 4

Test the result.
I use the following code to test the model:

using (EntityFrameworkExampleEntities context = new EntityFrameworkExampleEntities())

{

    var employee = new Employee

    {

        EmployeeType = "Developer",

        City = "Tel Aviv",

        EmployeeFirstName = "Dan",

        EmployeeLastName = "Ronen"

    };

    context.AddToEmployeeSet(employee);

    context.SaveChanges();

    var newEmployee = context.EmployeeSet.First(e => e.City.Equals("Tel Aviv"));

    Console.WriteLine("{0} {1}", newEmployee.EmployeeFirstName, newEmployee.EmployeeLastName);

}

Pay Attention!
In situations such as more then one table entity, we have table joins
whenever we retrieve the data and multiple updates whenever we use
CUD operations. If in your situation you don’t need the address properties
every time you retrieve an employee you should use other mapping scenarios
such as the navigation property that was constructed in the model in the
first place.

Summary

Lets sum up, in the post I showed how to map a split entity into a
single Entity Framework entity. Also you should always consider whether
to use entity splitting or to use other mapping scenarios because of
the performance impact of multiple joins.