Source Code

Entity Framework 5 Code First Relationships

One to One or Many

The day that Edward understands the Library Card.

Edward knocked on Professor Relvar’s door and waited fairly patiently for him to answer. Once inside, they sat and Edward immediately started talking.

“Professor, here’s the sandwich you asked for. My mom says she sprinkled cinnamon on it, and she’s betting you like it. And my dad wants to know if you’re going to tell me about--” Edward screwed up his eyes in thought, “being fluent and happy.”

The Professor smiled and said, “I suspect you mean the Fluent API. Yes, yes, your father always loved using that, and he’s not alone. Since you clearly want to get started right away, I’ll have to nibble on this delectable sandwich while explaining.”

We can describe, [he continued], our relationships to the library two ways. One is declaratively, using attributes on the class and its procedures. The other is overriding the OnModelCreating method in our data context and using the Fluent API. You’ll hear a lot of noise over this, but I assure you one way is not better than the other. They are different, that’s all. I myself have used both, but I confess to preferring attributes.

Many like the fluent style because they can keep their classes pristine. POCO is the humorous acronym people use--plain old CLR objects. With this approach, the classes aren’t explicitly database oriented.
I, however, like decorating my classes as much as practical. I already know they’ll be used to interact with the database, and even if they aren’t it’s valuable to understand things such as which fields are required. Even decorating with attributes doesn’t tie you to the Framework. In short, a class doesn’t have to know it’s part of the Framework, regardless of using the Fluent API or attributes.

Now, let me see your library card. Hmm. Yes. Nice, but I don’t care for the new typeface. Now, did you know that each person is assigned a library card at birth? It’s one of the secrets to our little success here.  Get people started right, Socrates always said. Or was that Mr. Schultz?

Well, you can have as many library cards as you want. You might travel to Salmon Island and visit their lovely undersea facility. But you must have at least one card. This is a One to One or Many relationship. A person must have at least one card, and a card can belong to just one person.

We start modeling that relationship like so:

LearningEFSchema.cs

public class Person
{
    //Primary Key
    public int PersonId { get; set; }
    [Required]
    public string Name { get; set; }

    //Navigation
    public virtual ICollection<LibraryCard> LibraryCards { get; set; }

    public Person()
    {
        LibraryCards = new HashSet<LibraryCard>();
    }
}

public class LibraryCard
{
    //Primary Key
    public int LibraryCardId { get; set; }
    [Required]
    public int Number { get; set; }
    //Foreign Key column
    public int PersonId { get; set; }

    //Navigation
    public virtual Person Person { get; set; }
}

LearningEFDb.cs

public class LearningEFDb: DbContext
{
    public DbSet<LibraryCard> LibraryCards { get; set; }
    public DbSet<Person> Persons { get; set; }

Notice in Person we’ve added a navigation property to the person’s cards. We use ICollection because it’s the simplest collection that allows insertion via the Add method. IEnumerable, as you should recall from your schooling, does not. We’re using interfaces to help those who enjoy writing unit tests.

In the Person constructor, I initialize the collection using HashSet. Why don’t I initialize using a Collection? Simple convenience. The Collection class is in the System.Collections.ObjectModel namespace, which we aren’t referencing, while HashSet is already available in System.Collections.Generic. A HashSet is the simplest collection that, again, allows adding. You could certainly use a List, and many people do, but there is some extra memory overhead.

We declare our navigation properties as virtual so that the Framework can implement lazy loading. I’m not lazy by nature, perhaps you are. Yet I think we can all agree there are some times it’s good to be lazy.
In the LibraryCard class, I include a navigation property back to the Person.

Now, there are some who say I’m too particular about my database tables, and that may be. But I’m old and allowed to be set in my ways. The Framework, left to its own, would name the Person foreign key Person_PersonId, which I find distasteful and difficult to type. “But, Professor, why do you care?” they ask. “You won’t be using the database directly, anyway.” I assure you, each person who says this comes to me a year later saying, “We’re constantly querying our database directly. If only we had done as you do, we would save typing.”

By explicitly defining a PersonId property, the Framework will use it as the foreign key column name. It does this because of our naming convention. If we had named the virtual person property Learners and wanted a foreign key named StudentId, then we would use an attribute to tell the Framework about the relationship, like this:

//Foreign Key column
public int StudentId { get; set; }

//Navigation
[ForeignKey("StudentId")]
public virtual Person Learners { get; set; }

If your mother were here, she would--rather stridently, I’m afraid, in her youth--say, “Professor, this is not a One to One or Many. This is a One to Zero or Many.” And she would be right. We are not, at this point, requiring a person to have a library card.

What’s that you say? How are we doing the reverse and requiring a card to have a person? My dear boy, it’s because we’ve declared PersonId as a non-nullable int. May I continue? To require a person to have a card, we need to introduce some validation.

public class Person : IValidatableObject
    {
        //Primary Key
        public int PersonId { get; set; }
        [Required]
        public string Name { get; set; }

        //Navigation
        public virtual ICollection<LibraryCard> LibraryCards { get; set; }

        public Person()
        {
            LibraryCards = new HashSet<LibraryCard>();
        }

        public IEnumerable Validate(ValidationContext validationContext)
        {
            if (LibraryCards.Count() < 1)
            {
                yield return new ValidationResult("A Person must have at least one Library Card");
            }
        }

Do you see? When the Framework attempts to SaveChanges, it calls Validate if a class inherits from IValidatableObject. We can put all sorts of interesting validations in our class. You should note something however. The geniuses--you might detect some sarcasm--who managed this part of the Framework decided that if a class failed its property attribute validations, such as [Required], it would not bother running the Validate method. Only if the properties are valid will it run Validate. I don’t know why. Wouldn’t you rather know all of your mistakes up front? Still, it remains very useful. If we run the program with no changes, we get an error.

Program.cs

Create Person without a name.
Saving changes.
  ERROR: The Name field is required.
Add the required name and save.
Saving changes.
  ERROR: A Person must have at least one Library Card

Let’s add to our program.

edward.Name = "Edward Farley";
edward.LibraryCards.Add(new LibraryCard() { Number = 123 });
SaveChanges();
Console.WriteLine("Get the record we just saved, and display.");
edward = _db.Persons.First();
Console.WriteLine("Person: " + edward.Name 
    + " has card: " + edward.LibraryCards.First().Number);

And run it.

Create Person without a name.
Saving changes.
  ERROR: The Name field is required.
Add the required name and Library Card and save.
Saving changes.
Successful save.
Get the record we just saved, and display.
Person: Edward Farley has card: 123
Finished

The resulting LibraryCards table script is like this.

T-SQL

create table LibraryCards (
  LibraryCardId int not null identity primary key,
  Number int not null,
  PersonId int not null foreign key references Persons(PersonId)
)

And that is quite enough. Look, I’ve barely taken a bite. But please, tell your mother it was delicious, which I’m sure it will be!

ICollection vs List
POCO
Charles M Schultz (To my knowledge neither Mr. Schultz nor Socrates is quoted saying“get people started right.”)