Entity Framework One-to-Many Relationships

In a one-to-many relationship, each row of data in one table is linked to one or more rows in the second table. It is the most common type of relationship.

• A one-to-many relationship happens when the primary key of one table becomes foreign keys in another table.
• The foreign key is defined in the table that represents the many end of the relationship.

Let's say we have Author and Book entities.

public class Author
{
    public int AuthorId { get; set; }
    public string Name { get; set; }
}

public class Book
{
    public int BookId { get; set; }
    public string Title { get; set; }
}

Configure One-to-Many Relationship

In EF6, most of the times you don't need to configure the one-to-many relationship because one-to-many relationship conventions cover all combinations. You can establish a one-to-many relationship by using any of the following code first conventions.

Code First Conventions

Add Reference Navigation Property

Include a reference navigation property of type Author in the Book entity class.

public class Author
{
    public int AuthorId { get; set; }
    public string Name { get; set; }
}

public class Book
{
    public int BookId { get; set; }
    public string Title { get; set; }
    public Author Author { get; set; }
}

Adding an Author navigation property will create a one-to-many relationship between the Authors and Books tables in the database by adding a foreign key Author_AuthorId to Books table.

Add Collection Navigation Property

You can achieve one-to-many relationship by adding the collection navigation property of Book entity in the Author entity class.

public class Author
{
    public int AuthorId { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Book> Books { get; set; }
}

public class Book
{
    public int BookId { get; set; }
    public string Title { get; set; }
}

Try it

Add Navigation Properties in Both Entities

Adding navigation properties at both entities will also result in a one-to-many relationship. For example, Author class contains a collection of Books while Book class contains a navigation property of type Author.

public class Author
{
    public int AuthorId { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Book> Books { get; set; }
}

public class Book
{
    public int BookId { get; set; }
    public string Title { get; set; }
    public Author Author { get; set; }
}

Try it

Fully Defined Relationship

A fully defined relationship at both ends will also create a one-to-many relationship. For example, the Book entity includes foreign key property AuthorId with its reference property Author and Author class contains a collection of Books.

public class Author
{
    public int AuthorId { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Book> Books { get; set; }
}

public class Book
{
    public int BookId { get; set; }
    public string Title { get; set; }
    public int AuthorId { get; set; }
    public Author Author { get; set; }
}

Try it

All these conventions produces the same result in the database.

Using Fluent API

You can also configure relationships using Fluent API to override the default conventions and make it more maintainable.

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
    // configures one-to-many relationship
    modelBuilder.Entity<Book>()
        .HasRequired<Author>(b => b.Author)
        .WithMany(a => a.Books)
        .HasForeignKey<int>(b => b.AuthorId);
}

Try it