Entity Framework Core One To One Relationships Conventions

Entity Framework Core will create a one to one relationship when both entities involved in the relationship contain a navigation property to the other, and the dependent entity includes a foreign key property for the principal entity. The following example illustrates a one to one relationship between Author and AuthorBiography:

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 DateTime DateOfBirth { get; set; }
    public string PlaceOfBirth { get; set; }
    public string Nationality { get; set; }
    public int AuthorId { get; set; }
    public Author Author { get; set; }

EF Core will generate a unique index on the AuthorId foreign key column in the AuthorBiographies table to ensure that only one biography can be associated with any particular author.

Further Reading

Configuring one to one relationships