Configure One-to-Zero-or-One Relationship:

Here, we will configure One-to-Zero-or-One relationship between two entities, e.g. Entity1 can be associated with zero or only one instance of Entity2.

Take an example of the following Student and StudentAddress entities.

public class Student
{
    public Student() { }

    public int StudentId { get; set; }
    public string StudentName { get; set; }

    public virtual StudentAddress Address { get; set; }

}
     
public class StudentAddress 
{
    public int StudentAddressId { get; set; }
        
    public string Address1 { get; set; }
    public string Address2 { get; set; }
    public string City { get; set; }
    public int Zipcode { get; set; }
    public string State { get; set; }
    public string Country { get; set; }

    public virtual Student Student { get; set; }
}
        

Visit Entity Relationship section to understand how EF manages one-to-one, one-to-many, and many-to-many relationships.

Now, let's configure Student and StudentAddress entities for One-to-Zero-or-One relationship where Student can have zero or maximum one StudentAddress.

As you may know, a one-to-zero-or-one relationship happens when the primary key of one table becomes PK & FK in another table in relational database such as SQL Server. So, we need to configure above entities in such a way that EF creates Students and StudentAddresses table in the DB where it will make StudentId in Student table as PK and StudentAddressId column in StudentAddress table as PK and FK both.

Configure one-to-zero-or-one relationship using DataAnnotations:

Here, we will apply DataAnnotations attributes on Student and StudentAddress entities to establish one-to-zero-or-one relationship.

The Student entity follows the default code-first convention as it includes StudentId property which will be key property. So we don't need to apply any attributes on it because EF will create Students table and make StudentId as a primary key in the DB.

For the StudentAddress entity, we need to configure StudentAddressId as PK & FK both. StudentAddressId property follows the default convention for primary key. So we don't need to apply any attribute for PK. However, we also need to make it a foreign key which points to StudentId. So, apply [ForeignKey("Student")] on StudentAddressId property which will make it foreign key for Student entity as shown below.

public class Student
{
    public Student() { }

    public int StudentId { get; set; }
    public string StudentName { get; set; }

    public virtual StudentAddress Address { get; set; }

}
     
public class StudentAddress 
{
    [ForeignKey("Student")]
    public int StudentAddressId { get; set; }
        
    public string Address1 { get; set; }
    public string Address2 { get; set; }
    public string City { get; set; }
    public int Zipcode { get; set; }
    public string State { get; set; }
    public string Country { get; set; }

    public virtual Student Student { get; set; }
}
        

Thus, Student and StudentAddress entities has One-to-Zero-or-One relationship.

When StudentAddress entity does not follow conventions:

If, for example, StudentAddress entity does not follow the convention for PK i.e. different name for Id property then you need to configure it for PK as well. Consider the following StudentAddress entity which has property name StudentId instead of StudentAddressId.

public class Student
{
    public Student() { }

    public int StudentId { get; set; }
    public string StudentName { get; set; }

    public virtual StudentAddress Address { get; set; }

}
     
public class StudentAddress 
{
    [Key, ForeignKey("Student")]
    public int StudentId { get; set; }
        
    public string Address1 { get; set; }
    public string Address2 { get; set; }
    public string City { get; set; }
    public int Zipcode { get; set; }
    public string State { get; set; }
    public string Country { get; set; }

    public virtual Student Student { get; set; }
}
        

In the above example, we need to configure StudentId property as Key as well as ForeignKey. This will make StudentId property in StudentAddress entity as PK and FK both.

Note: Student include StudentAddress navigation property and StudentAddress includes Student navigation property. With one-to-zero-or-one relationship, Student can be saved without StudentAddress but StudentAddress entity cannot be saved without Student entity. EF will throw an exception if you try to save StudentAddress entity without Student entity.

Configure One-to-Zero-or-One relationship using Fluent API:

Here, we will use Fluent API to configure Student and StudentAddress entities. Please note that we will not apply any DataAnnotations attributes in Student and StudentAddress entities because we will use Fluent API to configure them.

When Student and StudentAddress follow the conventions:

Student and StudentAddress entities follow the default code-first convention for PrimaryKey. So, we don't need to configure them to define their PrimaryKeys. We only need to configure StudentAddress entity where StudentAddressId should be ForeignKey.

The following example sets one-to-zero or one relationship between Student and StudentAddress using Fluent API.

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    
    // Configure Student & StudentAddress entity
    modelBuilder.Entity<Student>()
                .HasOptional(s => s.Address) // Mark Address property optional in Student entity
                .WithRequired(ad => ad.Student); // mark Student property as required in StudentAddress entity. Cannot save StudentAddress without Student

}
        

In the above example, Student entity is configured using HasOptional() method which indicates that the StudentAddress navigation property in Student entity is an optional (not required when saving Student entity). Then, WithRequired() method configures StudentAddress entity and make Student navigation property of StudentAddress as required (required when saving StudentAddress entity. It will throw an exception when StudentAddress entity is saving without Student navigation property). This will make StudentAddressId as ForeignKey also.

Thus, you can configure One-to-Zero-or-one relationship between two entities where Student entity can be saved without attaching StudentAddress object to it but StudentAddress entity cannot be saved without attaching an object of Student entity. This makes one end required.

When StudentAddress entity do not follow conventions:

Now, let's take an example of StudentAddress entity where it does not follow primary key convention i.e. have different Id property name than <type name>Id. Consider the following Student and StudentAddress entities.

public class Student
{
    public Student() { }

    public int StudentId { get; set; }
    public string StudentName { get; set; }

    public virtual StudentAddress Address { get; set; }

}
     
public class StudentAddress 
{
    public int StudentId { get; set; }
        
    public string Address1 { get; set; }
    public string Address2 { get; set; }
    public string City { get; set; }
    public int Zipcode { get; set; }
    public string State { get; set; }
    public string Country { get; set; }

    public virtual Student Student { get; set; }
}
        

So now, we need to configure StudentId property of StudentAddress for PrimaryKey of StudentAddress as well as ForeignKey as shown below.

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    // Configure StudentId as PK for StudentAddress
    modelBuilder.Entity<StudentAddress>()
        .HasKey(e => e.StudentId);
        
    // Configure StudentId as FK for StudentAddress
    modelBuilder.Entity<Student>()
                .HasOptional(s => s.Address) 
                .WithRequired(ad => ad.StudentId); 

}
        

Configure One-to-One relationship using Fluent API:

We can configure One-to-One relationship between entities using Fluent API where both ends are required, meaning Student entity object must include StudentAddress entity object and StudentAddress entity must include Student entity object in order to save it.

Note:One-to-one relationship is technically not possible in MS SQL Server. It will always be one-to-zero or one. EF forms One-to-One relationships on entities not in DB.

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    // Configure StudentId as PK for StudentAddress
    modelBuilder.Entity<StudentAddress>()
        .HasKey(e => e.StudentId);
        
    // Configure StudentId as FK for StudentAddress
    modelBuilder.Entity<Student>()
                .HasRequired(s => s.Address) 
                .WithRequiredPrincipal(ad => ad.Student); 

}
        

In the above example, modelBuilder.Entity<Student>().HasRequired(s => s.Address) makes Address property of StudentAddress is required. .WithRequiredPrincipal(ad => ad.Student) makes Student property of StudentAddress entity as required. Thus it configures both ends required. So now, when you try to save Student entity without address or StudentAddress entity without Student, it will throw an exception.

Note: Here, Principal entity is Student and dependent entity is StudentAddress.

DataAnnotations and Fluent API example for one-to-zero or one relationship will create the following database:

one-to-one relationship in code first

You can check the relationship between Student and StudentAddress in the database, as shown below:

one-to-one relationship in code first

If you create an entity data model of a created database then it will appear like the diagram shown below:

one-to-one relationship in code first

Learn how to configure a one-to-many relationship in the next section.