The Entity Framework Core Fluent API HasOne method is used to configure the one side of a one to many relationship, or one end of a one to one relationship.
In order to fully configure a valid relationship, it is necessary to follow the Has/With pattern and pair the use of HasOne with either the WithOne or WithMany method, depending on whether the relationship being configured is a one-to-one relationship or a one-to-many relationship.
One To One Relationship
The following example illustrates the use of HasOne with WithOne to configure a one-to-one relationship:
public class SampleContext : DbContext
{
public DbSet<Author> Authors { get; set; }
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<Author>()
.HasOne(a => a.Biography)
.WithOne(b => b.Author);
}
}
public class Author
{
public int AuthorId { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public AuthorBiography Biography { get; set; }
}
public class AuthorBiography
{
public int AuthorBiographyId { get; set; }
public string Biography { get; set; }
public int AuthorId { get; set; }
public Author Author { get; set; }
}
One To Many Relationship
This example illustrates the use of the HasOne method together with the WithMany method to configure a one-to-many relationship:
public class SampleContext : DbContext
{
public DbSet<Employee> Employees { get; set; }
public DbSet<Company> Companies { get; set; }
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<Employee>()
.HasOne(e => e.Company)
.WithMany(c => c.Employees);
}
public class Company
{
public int Id { get; set; }
public string Name { get; set; }
public ICollection<Employee> Employees { get; set; }
}
public class Employee
{
public int Id { get; set; }
public string Name { get; set; }
public Company Company { get; set; }
}
Data Annotations
Relationships can be defined with the ForeignKey and InverseProperty attributes.
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