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

We can configure Entity Framework Core One-to-One Relationship by the use of Fluent API. The One-to-One Relationship is established by using the HasOne – WithOne pattern. Note that we can also create this relationship by using EF Core Conventions.

This tutorial is a part of Entity Framework Core series.

• 1. Introduction to Entity Framework Core
• 2. Installation of Entity Framework Core
• 3. Database-First approach in Entity Framework Core
• 4. DbContext Class in Entity Framework Core
• 5. Code-First Approach in Entity Framework Core
• 6. Migrations in Entity Framework Core
• 7. Insert Records in Entity Framework Core
• 8. Read Records in Entity Framework Core
• 9. Update Records in Entity Framework Core
• 10. Delete Records in Entity Framework Core
• 11. Conventions in Entity Framework Core
• 12. Configurations in Entity Framework Core
• 13. Fluent API in Entity Framework Core
• 14. Configure One-to-Many relationship using Fluent API in Entity Framework Core
• 15. Configure One-to-One relationship using Fluent API in Entity Framework Core
• 16. Configure Many-to-Many relationship using Fluent API in Entity Framework Core
• 17. Execute Raw SQL Queries using FromSqlRaw() method in Entity Framework Core
• 18. Execute SQL Stored Procedures using FromSqlRaw() & ExecuteSqlRawAsync() methods in Entity Framework Core

Let’s create Entity Framework Core One-to-One relationship between City and CityInformation entities using Fluent API.

The 2 entity classes are:

public class City
{
    public int Id { get; set; }
 
    public string Name { get; set; }
 
    public int CityInformationId { get; set; }
 
    public CityInformation CityInformation { get; set; }
}
 
public class CityInformation
{
    public int Id { get; set; }
 
    public int Population { get; set; }
 
    public string OtherName { get; set; }
 
    public string MayorName { get; set; }
 
    public City City { get; set; }
} 

The City entity has a Reference Navigation Property called CityInformation.

public CityInformation CityInformation { get; set; }

Similary the CityInformation entity has a Reference Navigation Property called City

public City City { get; set; }

Next configure the OnModelCreating method in the DB Context Class of Entity Framework Core as shown below.

public class CountryContext: DbContext
{
    public DbSet<City> City { get; set; }
    public DbSet<CityInformation> CityInformation { get; set; }

    public CountryContext(DbContextOptions<CountryContext> options) : base(options)
    {
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        //Write Fluent API configurations here

        modelBuilder.Entity<City>()
                    .HasOne(e => e.CityInformation)
                    .WithOne(e => e.City)
                    .HasForeignKey<City>(e => e.CityInformationId);
    }
}

In the above code we have used the HasOne – WithOne pattern to create One-to-One Relationship between City & CityInformation entities.

You can also create the same relationship the other way around like what we did in the below code.

modelBuilder.Entity<CityInformation>()
            .HasOne(e => e.City)
            .WithOne(e => e.CityInformation)
            .HasForeignKey<City>(e => e.CityInformationId);

On doing the EF Core Migrations the foreign key i.e. one-to-one relationship is created which is shown in the below image:

Understanding in Details

Let’s understand it in step by step.

• modelBuilder.Entity() starts configuring the City entity.
• The HasOne(e => e.City) method specifies that the City entity includes one CityInformation reference navigation property.
• The .WithOne(e => e.CityInformation) configures the other end of the relationship, i.e. the CityInformation entity. It specifies that the ‘CityInformation’ entity includes a reference navigation property of ‘City’ type.
• The .HasForeignKey(e => e.CityInformationId) specifies the foreign key property name which is the CityInformationId column.

Download the source code:

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