Relationship 1 to 1 with Entity Framework

Question

I have 2 entities: Equipamento and Databook. A Equipamento can only have one Databook and a Databook is only for a Equipamento. How can I make this relationship with Entity Framework?

Follows the Classes:

public class Equipamento
{
    public int EquipamentoID { get; set; }

    public string Nome { get; set; }
    public string Sigla { get; set; }
    public Databook databook { get; set; }
}

public class Databook
{
    public int DatabookID { get; set; }

    public string Indentificacao { get; set; }   
    public Equipamento equipamento { get; set; }
}

Answer 1

A one to one mapping is done through the relationship where the key of one is also key of the other. So we will have the key field in the class Todo being the key to the class parte.

public class Todo{
    public virtual Parte ObjParte { get; set; }
}

public class Parte{
    [Key]
    public int TodoID { get; set; }
    public virtual Todo ObjTodo { get; set; }
}

In class Contexto we can make this mapping also with Fluent Api:

public class Contexto {
    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Parte>()
            .HasKey(x => x.TodoID)
            .HasRequired(x => x.ObjTodo)
            .WithRequiredDependent(p => p.ObjParte);
    }
}

Answer 2

Generally, the relationship is made through the information of Foreign Keys (as Foreign Keys).

For annotations with attributes, it would look like this in Equipamento:

public class Equipamento
{
    [Key]
    public int EquipamentoID { get; set; }

    public string Nome { get; set; }
    public string Sigla { get; set; }

    // Conforme comentário do AP, essa propriedade não pode existir, ou o EF se perde.
    //public virtual Databook databook { get; set; } 
}

Note the use of virtual. It is recommended especially when using the Entity Framework with proxies class usage setting. Here, the type property Databook becomes only a navigation property.

A browsing property only signals to the Entitu Framework that a related record can be found in the specified entity. In this case, we are indicating that a Equipamento may (or may not) have a Databook.

And so in Databook:

public class Databook
{       
    [Key]
    public int DatabookID { get; set; }

    [ForeignKey("equipamento")]
    public int EquipamentoID { get; set; }   
    public virtual Equipamento equipamento { get; set; }

    // ou assim:
    // sendo esse caso o necessário para quando se trata de entidades 
    // com chaves compostas.
    //
    //[Key]
    //public int EquipamentoID { get; set; }   
    //[ForeignKey("EequipamentoId")]
    //public virtual Equipamento equipamento { get; set; }

    public string Indentificacao { get; set; }
}