WithRequiredPrincipal, why still have to define Required or ForeignKey to be able to compile? - entity-framework

Question

public class Admin : EntityTypeConfiguration<Admin>
{
//[ForeignKey("Blog")] -- If I enable this, it compiles
public int AdminId { get; set; }
public string AdminName { get; set; }
public string AdminPicture { get; set; }
//[Required] -- Or If I enable this, it compiles
public virtual Blog Blog { get; set; }
}
public class Blog : EntityTypeConfiguration<Blog>
{
public int BlogId { get; set; }
public string BlogName { get; set; }
public string BlogUrl { get; set; }
public virtual Admin Admin { get; set; }
public Blog()
{
HasRequired(a => a.Admin).WithRequiredPrincipal(b=>b.Blog);
}
}
As long as I am defining HasRequired and WithRequiredPrincipal keys, why VS still creates below error.
Unable to determine the principal end of an association between the types 'Dummy.Models.Blog' and 'Dummy.Models.Admin'. The principal end of this association must be explicitly configured using either the relationship fluent API or data annotations.
Second thing is, even I enable [Required] or [ForeingKey] attr., in edmx designer, I only see 1 - 0..1 But I must see 1 - 1 (both end required)

Answer 1

1-1 relationship is not possible at database level, because you can't insert two rows at the same time. 1-1 is only possible at class validation level.
To make a 1-1 relationship, the primary key of the dependant entity must be the foreign key of the principal entity; that's the only way to make a 1-1 relationship. So, you have to make the following changes (considering that you are using EF Code First):
public class Admin
{
public int AdminId { get; set; }
public string AdminName { get; set; }
public string AdminPicture { get; set; }
public virtual Blog Blog { get; set; }
}
Blog should not have its own BlogId, because a blog belongs to an admin, and the admin can have only one blog (1-1 relationship). If you create a BlogId, with an AdminId FK, you would be making a 1-n relationship. Furthermore, do not mix the entity class with the mapping class, they should be different things. See the example below:
public class Blog
{
public int AdminId { get; set; } //PK AND FK
public string BlogName { get; set; }
public string BlogUrl { get; set; }
public virtual Admin Admin { get; set; }
}
Creating the relationship with a mapping class:
public class BlogMapping : EntityTypeConfiguration<Blog>
{
public BlogMapping()
{
HasKey(i => i.AdminId);
HasRequired(a => a.Admin)
.WithRequiredDependent(i => i.Blog);
}
}
Register the mapping inside the dbContext class:
public class MyDbContext : DbContext
{
public DbSet<Admin> Admins { get; set; }
public DbSet<Blog> Blogs { get; set; }
protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
modelBuilder.Configurations.Add(new BlogMapping());
}
}
This will generate the following migration:
CreateTable(
"dbo.Admins",
c => new
{
AdminId = c.Int(nullable: false, identity: true),
AdminName = c.String(),
AdminPicture = c.String(),
})
.PrimaryKey(t => t.AdminId);
CreateTable(
"dbo.Blogs",
c => new
{
AdminId = c.Int(nullable: false),
BlogName = c.String(),
BlogUrl = c.String(),
})
.PrimaryKey(t => t.AdminId)
.ForeignKey("dbo.Admins", t => t.AdminId)
.Index(t => t.AdminId);
Hope this helps!