When creating one to many relationships one might bump into the following problem:

Consider the following database design:

Tables:

Customers
-Id
-FirstName
-LastName
-…snip…
-City

Cities
-Id
-Name
-Zip

With a many to one relation from Customers.City to Cities.Id

While this is easy to do in SQL, the LINQ to Entities syntax might seem a bit different.

Consider the following piece of code:

City c = (from city in ctx.CitySet
where city.Name == cityName && city.Zip == cityZip
select city).FirstOrDefault();

This selects the city (if it exists) or null.

Now the problem is: IF the city exists, how do I point the current customer’s city to that particular city.

I tried this:

//customer is a customer object
customer.City = c;

But that resulted in duplicate cities in the database.

The solution was actually quite easy, I browsed through all the properties (Intellisense!) and found this:

customer.CityReference.EntityKey = c.EntityKey;

This resulted in the one to many relationship I had in mind. No duplicate cities in my database.

I love database normalization!