Prevent EF from trying to update/insert child objects

Question

I have an Address object that has a City property. When creating a brand new Address to be inserted via EF6, I fill all required basic Address properties (address line 1, postal code, etc), but I don't need a fully hydrated City instance so long as it has the ID like so:

address.City = new City { Id = 1 };

When I attempt to insert my Address, it also attempts to do validation on the City's properties, but I don't want to do any CRUD on City since all I need is its ID.

I found the below question that introduced me to detaching entries from the DbContext so that EF does not attempt to do CRUD on said objects:

How do I stop Entity Framework from trying to save/insert child objects?

What seems to be happening when I detach the City instance is that it also nulls it out, so my Address has a null City property. This is a problem because City is also required, so a DbEntityValidationException is thrown saying "The City field is required".

I am new to EF, so perhaps the way I am going about all of this is just wrong to begin with.

Edit By request, here's all my code:

Building my Address entity in my client before passing it to WebApi endpoint:

var user = new AppUser { Id = 1 };
var address = new Address
{
    City = new City { Id = 277 },
    Line1 = "123 whatever ln",
    PostalCode = "01233",
    CreatedBy = user,
    ModifiedBy = user,
    CreatedOn = DateTime.Today,
    ModifiedOn = DateTime.Today
};

In my ASP.NET app, I create an array of instances I want to detach from the context:

Detached = new object[] {
    value.Principle.ModifiedBy,
    value.Principle.CreatedBy,
    value.Principle.City
};

Just before the save, I detach all instances in the array:

foreach (var d in DetachedObjects)
{
    dbContext.Entry(d).State = EntityState.Detached;
}
dbContext.SaveChanges();

What I thought was happening with detaching properties was that it was simply telling EF not to do any CRUD on them, but I didn't want it to null them out because I want the parent/principle entity to have the ID for its FK.

Here are my Address and City classes:

[DebuggerDisplay("{Line1}")]
public class Address : CisEntity
{
    [MaxLength(200)]
    [Required]
    public string Line1 { get; set; }

    [MaxLength(200)]
    public string Line2 { get; set; }

    [Required]
    public City City { get; set; }

    [MaxLength(25)]
    public string PostalCode { get; set; }
}

[DebuggerDisplay("{Name}, {Province.Name}, {Province.Country.Name}")]
public class City : CisEntity, IEntityName
{
    [Required]
    public Province Province { get; set; }

    [MaxLength(100)]
    public string Name { get; set; }
}

Answer 1

If you don't want the City to be required when performing CRUD, remove the required attribute. If you truly need a City for your Address in your program, but not in the database, then do as you are and null out the City piece before performing CRUD on Address. Otherwise it will be inserted. You should go check out what your tables look like in your database. EF will be tracking these in separate tables, and the City column on Address will be a foreign key. If you decorate your City property on Address with the required attribute, it means that the column is non-nullable. This means that in the database this column must contain a foreign key to a record in the City table, hence a City must exist.

Answer 2

I believe I understand your question and I'm answering 3 years later. LOL.

For the main entity, context.Addresses.Add(address);

for the related or child entities, context.Entry(address.City).State = EntityState.Modified;

Do this for every other related entity.