How to use the Foreign Key annotation in C#?

Question

I am layman and would love to learn about.

I will use in the example Model A and B not to get confused:

I saw about the Model A take note Foreign Key in prop virtual Model B virModelB referencing a prop int ModelBID to store the data.

And in the Model B have a prop virtual ICollection<ModelA> virModelA, but I don’t understand how one Model is "seeing" the other... There is some difference in making a Model for each relationship (one-to-one, one-to-Many, Many-to-Many)?

In short:

• Build each Model
  • Foreign Key Annotation
  • Icollection<>
• Relationships
  • One-to-one
  • One-to-Many
  • Many-to-Many

Answer 1

Basically the DataAnnotations [ForeignKey] is only to clarify the foreign key field in relationships between two entities, for example:

public class Livro
{
    public int Id { get; set; }
    public string Titulo { get; set; }
    public int AutorId { get; set; }

    [ForeignKey("AutorId")]
    public virtual Autor { get; set; }
}

public class Autor
{
    public int Id { get; set; }
    public string Nome { get; set; }
    public virtual ICollection<Livro> Livros { get; set; }
}

In the example quoted above we have a Book class and an Author class, this is a One-To-Many relationship of Autor for Livro, because an author can have several books, plus one book has only one author. We use the annotation [ForeignKey("AutorId")] to explain to Efcore that our foreign key that references the book to the author is AutorId. We could also do the relationship without explaining to Efcore which field will be the key, causing EF Core itself to create an automatically generated field to perform the key, in this case we would not even need to create field AutorIdin the class Book as below:

public class Livro
{
    public int Id { get; set; }
    public string Titulo { get; set; }
    public virtual Autor { get; set; }
}

public class Autor
{
    public int Id { get; set; }
    public string Nome { get; set; }
    public virtual ICollection<Livro> Livros { get; set; }
}

The two examples above depict the same class structure, the difference being that in the second example who will generate the foreign key is EF Core itself. Which will generate the following table structure (I will use SQL as the basis):

CREATE TABLE Livro(
   Id INT NOT NULL,
   Nome NVARCHAR,
   PRIMARY KEY(Id)
);

CREATE TABLE Autor(
   Id INT NOT NULL,
   Titulo NVARCHAR,
   AutorId INT NOT NULL,
   FOREIGN KEY (AutorId) REFERENCES Livro (Id)
   PRIMARY KEY(Id)
);

OBS.: It may be that the structure of the tables has some difference as to this, more must approach to the described above.

Efcore promotes the use of fluent mapping without the use of Dataannotations, so in the official documentation of Microsoft or of Efcore, probably you will not find reference to the use of Dataannotations. However you will find more details of the use of Dataannotations in Entityframework 6 Code First documentation.