Entity Framework 4.1 – One To One Mapping

In the snippet below you can see that a customer has an address property.

    public class Customer
    {
        public Customer()
        {
           Address = new Address();
        }
 
        public Guid Id { get; set; }
        public string Name { get; set; }
        public Address Address { get; set; }
        public Guid AddressId { get; set; }
    }
 
    public class Address
    {
        public Guid Id { get; set; }
        public string City { get; set; }
        public string Country { get; set; }
        public string Street { get; set; }
    }

The database that gets generated though defines this as a one to many relationship.

Which is not what you’re expecting, or is it? I haven’t defined any restrictions on the address class so indeed an address could be shared between multiple customers. Let’s change that.

    public class Customer
    {
        public Customer()
        {
           Address = new Address();
        }
 
        public Guid Id { get; set; }
        public string Name { get; set; }
        public Address Address { get; set; }
 
    }
 
    public class Address
    {
        public Guid Id { get; set; }
        public string City { get; set; }
        public string Country { get; set; }
        public string Street { get; set; }
        [Required]
        public Customer Customer { get; set; }
    }

I’ve added a navigation property from Address to Customer and marked the Customer property as required. This basically is telling EF that there is a one to one mapping between the two and the primary key of the customer should be used in the relationship. The primary key of the customer will also become the primary key of the address. If you did not add the required attribute you’ll get an invalidoperationexception saying that it’s unable to determine the principal end of an association.

You can also use the fluent api.

    public  class  CustomerContext
        : DbContext
    {
        public IDbSet<Customer> Customers { get; set; }
 
        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Address>().HasRequired(x => x.Customer);
            base.OnModelCreating(modelBuilder);
        }
 
    }

They both result in the same database structure.

What if both ends are required? A customer always has an address and an address always has a customer. Marking both ends as required will again result in the same exception as before.

In order to model this you have to use the fluent api and there are two ways to achieve the desired result.

    public  class  CustomerContext
        : DbContext
    {
        public IDbSet<Customer> Customers { get; set; }
 
        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Address>()
                .HasRequired(x => x.Customer)
                .WithRequiredDependent(x => x.Address);
            base.OnModelCreating(modelBuilder);
        }
    }

I’m saying here that the address entity has a required customer property and that the customer class is actually in charge, MSDN reference.

Resulting database structure:

Another way to achieve the same result:

    public  class  CustomerContext
        : DbContext
    {
        public IDbSet<Customer> Customers { get; set; }
 
        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Address>()
                .HasRequired(x => x.Customer)
                .WithRequiredDependent();
            modelBuilder.Entity<Customer>()
                .HasRequired(x => x.Address)
                .WithRequiredPrincipal();
            base.OnModelCreating(modelBuilder);
        }
    }

MSDN link on WithRequiredPrincipal.

You saw that the WithRequiredPrincipal and Depedant actually have one taking a lambda and one with no arguments. This allows you to exclude a navigation property and still get a proper one to one mapping. Which brings me back to my starting point.

public class Customer
    {
        public Customer()
        {
           Address = new Address();
        }
 
        public Guid Id { get; set; }
        public string Name { get; set; }
        public Address Address { get; set; }
    }
 
    public class Address
    {
        public Guid Id { get; set; }
        public string City { get; set; }
        public string Country { get; set; }
        public string Street { get; set; }
 
 
    }
 
    public  class  CustomerContext
        : DbContext
    {
        public IDbSet<Customer> Customers { get; set; }
 
        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Entity<Customer>()
                .HasRequired(x => x.Address)
                .WithRequiredPrincipal();
            base.OnModelCreating(modelBuilder);
        }
    }

20 Replies to “Entity Framework 4.1 – One To One Mapping”

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Sergii Lazurenko says:

09/08/2011 at 12:13

Good post. Thank you very much, Benny!

yuriylsh says:

26/08/2011 at 20:23

Thank you, Benny, your post helped a lot to resolve one problem I had in my EF code. Appreciate it!

Shimmy says:

15/05/2012 at 05:31

Is there a way to use data annotations instead of the fluent API in this scanario? (0/1-1)

Kevin Bolander says:

22/10/2012 at 06:01

thank you. nice decription much appreciated. tyty

User123 says:

12/12/2012 at 06:07

How to display on the web i.e i mean calling the function in the Controller action explain that also..given a Customer Id it should display the both related rows from Customer and Address

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BillJam says:

14/03/2013 at 21:14

That’s fine except Address never gets saved to the database since there is no DBSet for it in the Context (first of the both ends required scenarios, I never tried the other).

Jake says:

02/06/2013 at 08:59

Hi, i`v used this approeach but when i see in database there is not a column created customer for Address.
in fact when i want to retrive data it says ” null reference exception”.
may you help me?

Loukouzou says:

12/07/2013 at 04:39

Clean concise explanations.. Thanks

Memet Olsen says:

07/10/2013 at 10:50

This helped me, thanks!

Marco Alves says:

13/12/2013 at 20:24

Thank you very much!

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Andrea says:

26/11/2014 at 14:10

Thanks. And if i need to build one-to-one realationship between 2 tables with 2 keys ?
For Example
Customers (Id1 (key),id2(key),Name)
Addresses (Id1 (key),id2(key),City,Country,Street)

Süleyman Vurucu says:

12/01/2015 at 16:59

What about that case if I need two Adress field for the customer.
Customer
—> PrivateAdress
—> FirmAdress

??

an phu says:

03/02/2015 at 19:25

What if I don’t want bidirection navigation e.g. Customer is my aggregate root nor foreign key properties like AddressId?

1. an phu says:
  
  03/02/2015 at 19:34
  
  What if the Address navigation property on Customer class is optional? Is there a way to do this using attributes?
  
Amit says:

15/06/2015 at 21:03

Thank you Benny.

Marcel says:

23/07/2016 at 20:16

Fucking Awesome man… Thanks a lot…

meysam says:

07/08/2016 at 10:21

hi .
i have a problem. please help me .
how can i have a one-to-one relationship between a primarykey from a product table (for example) and non-primarykey from information (for example) in code first ?

20 Replies to “Entity Framework 4.1 – One To One Mapping”

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