How to Declare One to One Relationship Using Entity Framework 4 Code First (Poco)

How to declare one to one relationship using Entity Framework 4 Code First (POCO)

Are you just looking for something like this?

public class User
{
    public int Id { get; set; }
    public string Username { get; set; }
    public Profile Profile { get; set; }
    public int ProfileId { get; set; }
}

public class Profile
{
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string PostalCode { get; set; }
    // etc...
}

public class UserMapping : EntityConfiguration<User>
{
    public UserMapping()
    {
        this.HasKey(u => u.Id);
        this.Property(u => u.Username).HasMaxLength(32);

        // User has ONE profile.
        this.HasRequired(u => u.Profile);
    }
}

public class ProfileMapping : EntityConfiguration<Profile>
{
    public ProfileMapping()
    {
        this.HasKey(p => p.Id);
        this.Property(p => p.FirstName).HasMaxLength(32);
        this.Property(p => p.LastName).HasMaxLength(32);
        this.Property(p => p.PostalCode).HasMaxLength(6);
    }
}

EDIT: Yeah I didn't have VS in front of me but you need to add the following line in the UserMapping instead of the current HasRequired and also add a ProfileId property (instead of Profile_Id that you added):

this.HasRequired(u => u.Profile).HasConstraint((u, p) => u.ProfileId == p.Id);

I currently don't think there's a way around this, but I'm sure it'll change since we're only in CTP4. It'd be nice if I could say:

this.HasRequired(u => u.Profile).WithSingle().Map(
    new StoreForeignKeyName("ProfileId"));

This way I wouldn't have to include a ProfileId property. Maybe there is a way around this currently and it's still to early in the morning for me to think :).

Also remember to call .Include("Profile") if you want to include a "navigational property".

How to use Entity Framework 4.1 Code First to force a one to one relationship for a one to many relationship in the database

Thanks to Jakub Konecki for the link to the article, it did not actually contain the answer I was looking for, but it did link to an earlier post in the series where I found the answer.

The way to force this one to one association is as follows:

public class ClassAMapping : EntityTypeConfiguration<ClassA>
{
    public ClassA()
    {
        HasKey(x => x.Id);

        HasOptional<ClassB>(x => x.ClassB)
                .WithRequired()
                .Map(x => x.MapKey("ClassBId"));
    }
}

This mapping reads as:

"The ClassA entity has an optional association with one ClassB entity, but this association is required for the ClassB entity."

Please note that this solution is uni-directional and will not allow the following:

ClassB b = new ClassB();
string test = b.ClassA.SomeString;

If a bi-directional association is required check out the link that was found which elaborates further.

The article linked by Jakub is part of a series of posts which are a good read if you are trying to sort out your EF4.1 associations.

Entity Framework Code First One to One relationship on composite key

You just need to define that the primary keys are composite...

modelBuilder.Entity<Parent>().HasKey(p => new { p.UniqueID, p.OwnerID });
modelBuilder.Entity<Child1>().HasKey(c => new { c.UniqueID, c.OwnerID });
modelBuilder.Entity<Child2>().HasKey(c => new { c.UniqueID, c.OwnerID });

...and then define the two one-to-one relationships:

modelBuilder.Entity<Parent>()
    .HasOptional(p => p.Child1)
    .WithRequired(); // or .WithRequired(c => c.Parent)

modelBuilder.Entity<Parent>()
    .HasOptional(p => p.Child2)
    .WithRequired(); // or .WithRequired(c => c.Parent)

You cannot define a constraint though (except probably by defining a trigger in the database) that would ensure that a given parent may only refer to one of the two children, but not to both. You must handle this restriction in your application's business logic.

EF 4.1 Code First - Mapping a one to one relationship with non standard column names

I think when you define this mapping...

modelBuilder.Entity<Contact>()
    .HasOptional(b => b.WebsiteMembership)
    .WithRequired(b => b.Contact)
    .Map(b => b.MapKey("WebsiteMembershipId"));

...you implicitely define what's the principal and what's the dependent of the relationship. Obviously Contact is the principal because having the WebsiteMembership property set is optional for the Contact, it can be stored without the WebsiteMembership. The WebsiteMembership entity on the other side has a required reference to a Contact which means it depends on the Contact.

The principal (Contact) is the entity with the primary key, the dependent (WebsiteMembership) the entity with the foreign key. So, when you use MapKey to set the name WebsiteMembershipId of the foreign key column you define a column for the table WebsiteMembership, not for Contact. But WebsiteMembership already has a property called WebsiteMembershipId. (I believe that's the point where EF complains that a "Property name 'WebsiteMembershipId' was already defined.) In a one-to-many relationship the foreign key would be the ContactId as a separate property/column and you would have to use HasForeignKey instead of MapKey. But in a one-to-one relationship it is clear that the foreign key is the primary key at the same time, so you don't need to define the foreign key at all.

To cut a long story short: Just remove MapKey:

modelBuilder.Entity<Contact>()
    .HasOptional(b => b.WebsiteMembership)
    .WithRequired(b => b.Contact);

(But now I am curious if you can define a relationship between an int and a decimal property or if you get the next error.)

Entity Framework 4.1 Code-First and Inserting New One-to-Many Relationships

After hours of troubleshooting/trial and error, I've solved my problem.
My POCO classes are also used in a disconnected environment, where
the objects are detached from the context, modified, and then re-attached.

In order to determine which navigation property collection items were affected, I overrode
the Equals and GetHashCode methods in the Address class to determine equality. Apparently this affects the ability for EF 4.1 to insert a complete collection of navigation property objects???

Here are the original equality methods which caused the issue:

public override bool Equals(object obj)
{
    Address address = obj as Address;
    if (address == null) return false;
    return address.AddressId == this.AddressId;
}

public override int GetHashCode()
{
    return this.AddressId.GetHashCode();
}

In order to correct the problem, I created a custom equality comparer
for the navigation object rather than including it directly in the address class.

public class AddressEqualityComparer : IEqualityComparer<Address>
{
    public bool Equals(Address address1, Address address2)
    {
        if (address1.AddressId == address2.AddressId)
            return true;
        else
            return false;
    }

    public int GetHashCode(Address address)
    {
        return address.AddressId.GetHashCode();
    }
}

My context.People.Add method call worked as expected after I made this change.

If anyone knows why overriding the equality methods in the class causes
EF 4.1 to only insert the first item in the collection, that would be
great information.

How to create One-to-many relationship in Entity Framework 4.1 using Code First and Data Annotations?

But that is not one-to-many relationship. That is one-to-one relation ship which says that each book has exactly one page. The error says that it cannot determine if the book or the page is principal in the one-to-one relation.

You must modify your entities like this:

public class Book
{
    public int ID { get; set; }
    public string Author { get; set; }
    public virtual ICollection<Page> Pages { get; set; }
}

public class Page
{   
    public int ID { get; set; }
    public int BookID { get; set; }
    public virtual Book Book { get; set; }
    public string OtherField { get; set; }
}

EF: one-to-one relationship

This works for me

namespace Ef1to1
{
    public class TestContext : DbContext
    {

        public TestContext()
            : base("Data Source=127.0.0.1;database=Junk;Integrated Security=SSPI;")
        {

        }
        public DbSet<Person> Persons { get; set; }
        public DbSet<PhoneNumber> PhoneNumbers { get; set; }
    }
    [Table("Person")]
    public class Person 
    {
        [Key, Column("PersonId")]
        public long Id { get; set; }

        public virtual PhoneNumber phoneNumber { get; set; }
    }

    [Table("PhoneNumber")]
    public class PhoneNumber
    {

        [Key, Column("PhoneNumberId"), ForeignKey("person")]
        public long Id { get; set; }

        public virtual Person person { get; set; }
    } 

   public class Program
    {

        static void Main(string[] args)
        {
            var realNumber = new PhoneNumber();
            var person = new Person() { phoneNumber = realNumber }; 
            var context = new TestContext();

            context.Persons.Add(person) ;
            context.SaveChanges();
            ;

        }
    }
}

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