实体框架急切加载新对象与现有对象

Question

Let's assume that I have two tables (sql first EF pattern)

create table test1 (
    id int identity (1, 1),
    name1 varchar(64),
    fk_table2 int
);

create table test2 (
    id int identity (1, 1), 
    name2 varchar(64)
);

Let's assume that we start with nothing in table test1 and one record in test2 with id = 1 and name2 = 'hello' . The code snippet below fails due to a null reference as the link is not "loaded":

        using (var conn = new DatabaseObjects())
        {
            var t1 = new test1()
            {
                fk_test_2 = 1,
                name = "foo"
            };

            conn.test1.AddOrUpdate(t1);
            conn.SaveChanges();

            Console.WriteLine(t1.test2.name2);
        }

I understand why this is happening. My question is how best do I now populate this "link" for it to be used? A simple but incorrect (overly inefficient) way is by adding a call to conn.test2.Load() before the Console.WriteLine call. A related question is, if the test1 table is queried with an id of a previously created row as opposed to creating a new record, the link to table2 will not be null (it is "lazy loaded"?). I don't understand how to reconcile this inconsistent behavior. Basically, I would like the behavior of querying an existing record where the link to the test2 table exists after creating new records.

---

Edit to address comments below. Interestingly, when re-querying with the same conn object instance, the navigation properties are still null. But, if a new DatabaseObjects instance is created, the query returns an object with the navigation properties populated. Using the same example

using (var conn = new DatabaseObjects())
{
    var t1 = new test1()
    {
        fk_test_2 = 1,
        name = "foo"
    };

    conn.test1.AddOrUpdate(t1);
    conn.SaveChanges();

    var t2 = (from e in conn.test1 where e.id == t1.id select e).First(); 
    // t2 navigation properties are STILL null

    var conn2 = new DatabaseObjects();
    var t3 = (from e in conn2.test1 where e.id == t1.id select e).First();
    // t3 navigation properties are NOT null
}

Answer 1

You expect a normal C# object to perform lazy loading. That doesn't happen. You always need a proxy object for that.

Ef will create a proxy if you use

var t1 = conn.test1.Create();

Now you'll see that even without calling SaveChanges , test2 will be loaded only by accessing it. That is, if lazy loading is enabled on the context (which it is by default) and property test2 is defined as virtual .

Answer 2

Basically you just have to set the object yourself like this.

t1.test2 = conn.test2s.FirstOrDefault(t => t.ID == t1.fk_test_2)

I'm not aware of cleaner way to do this.