Bdtjtk

I am using EF Core 2.1

How to map one-to-one relationship in EF Core. I have Customer & Course domain entity where one customer will have one Course.

This is my how my Customer & CoursePOCO looks like.

public class Customer

{

    public int Id { get; set; }

    public string Name { get; set; }



}



public class Course

{

    public int Id { get; set; }

    public string CouseName { get; set; }

    public virtual Customer Customer { get; set; }

}

I am using FluentAPI.

 public class CourseConfiguration : IEntityTypeConfiguration<Course>

{

    public void Configure(EntityTypeBuilder<Parent> builder)

    {

        builder.HasKey(x => x.Customer.Id) //not allowing -> throws error

        //The properties expression 'x => Convert(x.Customer.Id, Object)' is not valid. 

        // The expression should represent a simple property access: 't => t.MyProperty'. 

        // When specifying multiple properties use an anonymous type: 't => new { t.MyProperty1, t.MyProperty2 }'. 

        // Parameter name: propertyAccessExpression

    }

}

Since it's one to one relation, I don't want to create an extra key in Contact (FK -CustomerId),

Primary requirement:-
I want to define Id (in Course) as PK + FK & in this relation Customer is parent entity.

Like if I was Configuration based Migration, I would do as follows:-

public class Course

{

    [Key]

    [ForeignKey("Customer")]

    public int Id { get; set; }

    public string Name { get; set; }

    public virtual Customer Customer { get; set; }

 }

same thing I would like to do using Fluent API in EF Core??

Thanks!!

As other answers point out, the key point is to use use the HasForeignKey<>() method to configure a foreign key.

But be careful that the foreign key should be set on the dependent entity, instead of the principal entity.

How to in details:

Add a navigation property to Course for Customer

public class Customer

{

    public int Id { get; set; }

    public string Name { get; set; }

    public Course Course {get;set;} 

}

and now set the Course.Id as a FK that references the Customer.Id

public class AppDbContext : DbContext

{

    public AppDbContext (DbContextOptions<AppDbContext> options)

        : base(options)

    {

    }

    protected override void OnModelCreating(ModelBuilder builder)

    {

        base.OnModelCreating(builder);



        builder.Entity<Customer>(entity=>

        {

            entity.HasOne(customer=>customer.Course)

                .WithOne(course=> course.Customer)

                .HasForeignKey<Course>(course=>course.Id); 

        });

    }



    public DbSet<App.Models.Customer> Customer { get; set; }

    public DbSet<App.Models.Course> Courses{ get; set; }

}

the generated sql scripts is :

CREATE TABLE [Customer] (

    [Id] int NOT NULL IDENTITY,

    [Name] nvarchar(max) NULL,

    CONSTRAINT [PK_Customer] PRIMARY KEY ([Id])

);



GO



CREATE TABLE [Courses] (

    [Id] int NOT NULL,

    [CouseName] nvarchar(max) NULL,

    CONSTRAINT [PK_Courses] PRIMARY KEY ([Id]),

    CONSTRAINT [FK_Courses_Customer_Id] FOREIGN KEY ([Id]) REFERENCES [Customer] ([Id]) ON DELETE CASCADE

);



GO

Something similar to below should help. HasOne method is chained with WithOne would help to establish one-to-one relationship :

public class Customer

{

    public int Id { get; set; }

    public string Name { get; set; }



}



public class Course

{

    public int Id { get; set; }

    public string CourseName { get; set; }

    public int CustomerId {get;set;}

    public virtual Customer Customer { get; set; }

}



protected override void OnModelCreating(ModelBuilder modelBuilder)

{

    modelBuilder.Entity<Course>()

        .HasOne(a => a.Customer)

        .WithOne(b => b.Course)

        .HasForeignKey<Course>(b => b.CustomerId);

}