Configure One-to-One Relationships using Fluent API in Entity Framework Core

Here you will learn how to configure one-to-one relationships between two entities using Fluent API, if they do not follow EF Core conventions.

Generally, you don't need to configure one-to-one relationships manually because EF Core includes Conventions for One-to-One Relationships. However, if the key or foreign key properties do not follow the convention, then you can use data annotation attributes or Fluent API to configure a one-to-one relationship between the two entities.

Let's configure a one-to-one relationship between the following Student and StudentAddress entities, which do not follow the foreign key convention.

public class Student
{
    public int Id { get; set; }
    public string Name { get; set; }
       
    public StudentAddress Address { get; set; }
}

public class StudentAddress
{
    public int StudentAddressId { get; set; }
    public string Address { get; set; }
    public string City { get; set; }
    public string State { get; set; }
    public string Country { get; set; }

    public int AddressOfStudentId { get; set; }
    public Student Student { get; set; }
}

To configure a one-to-one relationship using Fluent API in EF Core, use the HasOne, WithOne and HasForeignKey methods, as shown below.

public class SchoolContext : DbContext
{
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer("Server=.\\SQLEXPRESS;Database=EFCore-SchoolDB;Trusted_Connection=True");
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Student>()
            .HasOne<StudentAddress>(s => s.Address)
            .WithOne(ad => ad.Student)
            .HasForeignKey<StudentAddress>(ad => ad.AddressOfStudentId);
    }

    public DbSet<Student> Students { get; set; }
    public DbSet<StudentAddress> StudentAddresses { get; set; }
}

In the above example, the following code snippet configures the one-to-one relationship.

modelBuilder.Entity<Student>()
    .HasOne<StudentAddress>(s => s.Address)
    .WithOne(ad => ad.Student)
    .HasForeignKey<StudentAddress>(ad => ad.AddressOfStudentId);

Let's understand it step by step.

• modelBuilder.Entity<Student>() starts configuring the Student entity.
• The .HasOne<StudentAddress>(s => s.Address) method specifies that the Student entity includes one StudentAddress reference property using a lambda expression.
• .WithOne(ad => ad.Student) configures the other end of the relationship, the StudentAddress entity. It specifies that the StudentAddress entity includes a reference navigation property of Student type.
• .HasForeignKey<StudentAddress>(ad => ad.AddressOfStudentId) specifies the foreign key property name.

Now, to reflect this in the database, execute migration commands, add-migration <name> and update-database. The database will include two tables with one-to-one relationship as shown below.

The following figure illustrates the Fluent API configuration for a one-to-one relationship.

You can start configuring with the StudentAddress entity in the same way, as below.

modelBuilder.Entity<StudentAddress>()
    .HasOne<Student>(ad => ad.Student)
    .WithOne(s => s.Address)
    .HasForeignKey<StudentAddress>(ad => ad.AddressOfStudentId);

Thus, you can configure a one-to-one relationship in entity framework core.