Entity Framework Code First One to One Relationship

Question

I have the following two models

public class Account
{
     public int Id { get; set; }
     public string Name { get; set; }
     public int CurrentPeriodId { get; set; }

     [ForeignKey("CurrentPeriodId")]
     public virtual Period CurrentPeriod { get; set; }

     public virtual ICollection<Period> Periods { get; set; }
}

public class Period
{
     public int Id { get; set; }
     public int AccountId { get; set; }
     public DateTime StartDate { get; set; }
     public DateTime EndDate { get; set; }

     public virtual Account Account { get; set; }
}

And I am trying to run the following query:

from p in context.Periods
where p.AccountId == accountId &&
      p.Id == p.Account.CurrentPeriodId
select p.StartDate

And I get a sql exception saying "Invalid column name Account_AccountId".

I know I could approach this from the Account side and do something like

from a in context.Accounts
where a.Id == id
select a.CurrentPeriod.StartDate

But I would like to know how to setup the relationship to get the other query to work. What am I missing?

Answer 1

I think your implementation is wrong. AFAIK, You cannot define one-to-one relationship using this approach in EF.
Take a look at your generated database, you have an Account_Id column defined in your Periods table. Here's what you have to define:

public class Account
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int CurrentPeriodId { get; set; }
    public virtual Period CurrentPeriod { get; set; }
    public virtual ICollection<Period> Periods { get; set; }
}

public class Period
{
    public int Id { get; set; }
    public DateTime StartDate { get; set; }
    public DateTime EndDate { get; set; }
}

Then, context and initializer:

public class TestContext : DbContext
{
    public DbSet<Account> Accounts { get; set; }
    public DbSet<Period> Periods { get; set; }

static TestContext()
{ 
    Database.SetInitializer(new DbInitializer());
}

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Account>().HasRequired(a => a.CurrentPeriod).WithMany().HasForeignKey(a => a.CurrentPeriodId);
    }
}

class DbInitializer : DropCreateDatabaseAlways<TestContext>
{
    protected override void Seed(TestContext context)
    {
        context.Database.ExecuteSqlCommand("ALTER TABLE Accounts ADD CONSTRAINT uc_Period UNIQUE(CurrentPeriodId)");
    }
}

For more information about One-To-One relationship, read Mr. Manavi's blog series at this address.
Update:
Based on your comment, if you want to have a reference to Account from Period, You can put a ForeignKey attribute on your account's primary key.

A good sample is located at this address (the part with title "Map a One to Zero or One Relationship")