EF 6 fluent api IsRequired attribute

问题

I have two classes: address and city. I want city property to be required in address class, but when I add property(p => p.City).IsRequired() to fluent api I get error that City must non-nullable value type, but when I decorate City property with [Required] annotation everything works.

So how to do it with fluent api and why property(p => p.Street).IsRequired() works for string - string isn'e non-nullable value type

public class Address
    {
        public int AddressId { get; private set; }
        public string Street { get; internal set; }
        [Required]
        public City City { get; internal set; }            
    }

public class CIty
{
        public int CityId {get; private set; }
        public string Name {get; internal set;}
}

回答1:

In order to specify the cardinality of a relationship, you need to use the HasRequired method instead -- the Property method is only used for scalar properties.

modelBuilder.Entity<Address>().HasRequired(a => a.City);

来源：https://stackoverflow.com/questions/19772327/ef-6-fluent-api-isrequired-attribute