LINQ 查询为实体数据库中的嵌套对象返回 null

Question

I have a database I created using entity code first

In that DB I have a structure similar to the following

---

class ClassA
{
  public virtual int ID {get;set}
  public virtual string some_text {get;set}
  public virtual ClassB B {get;set}
  public virtual ClassC C {get;set}
  ...
}

class ClassB
{
  public virtual int ID {get;set}
  public virtual string some_text {get;set}
  public virtual string some_values {get;set}
  ...
}

class ClassC
{
  public virtual int ID {get;set}
  public virtual string some_text {get;set}
  public virtual string some_values {get;set}
  ...
}
....

---

Finally I have a context for those objects with all of the interface to query the DB

---

public class ClassADb : DBContext, IClassADataSource
{
  public DBSet<ClassA> As {get;set}
  public DBSet<ClassB> Bs {get;set}
  public DBSet<ClassC> Cs {get;set}
  ...
}

When I create the DB and explore it I can see that it was created what seems to be correctly:

In the ClassA_Table I see foreign keys for ClassB_ID, ClassC_ID, etc as well as all of the primitive types encapsulated in ClassA (ints, strings, bools, dates, etc)

Also when performing something along the lines of:

ClassB MyB = new ClassB();
//some code to initialize B
...
Bs.Add(MyB)

ClassC MyC = new ClassC();
//some code to initialize C
Cs.Add(MyC);

ClassA MyA = new ClassA();

A.B = MyB;
A.C = MyC;
...

db.SaveChanges();

I again explore the DB and see in Table_A a new row with references to those B and C objects (row id's corresponding to those objects in the B_table , C_table)

The Problem I am having is that when I do a select from As container , I can retrieve the A object but the nested B and C objects are null

The primitive types are OK (not empty)

Some fixes I tried

The virtual keyword is lazy , so in the constructor of the class accessing the database i did

db.Configuration.ProxyCreationEnabled = false;

But still when doing something along the lines of

A myA = db.As.Find(1);

A.some_text ; // not null
A.B ; //NULL!!!!
A.C ; // NULL

What is causing the entity framework not to fetch the A and B object?

Answer 1

You have to explicitly load related entities explicitly using Include() .

db.As.Include("B").Include("C").Where(a => [some condition]);

In newer version of the Entity Framework there is also a wrapper method around this method accepting a lambda expression avoiding the strings.

db.As.Include(a => a.B).Include(a => a.C).Where(a => [some condition]);