Insert new object with existing object

Question

I am new to EF 4 and this is what I have done so far:

Create an edmx file based on my database
Create a code generation for my objects (POCO). Now I have a model1.tt, when expanded I see al my classes
Create a repository for each class, based on IRepository

Now, I am working with two objects, A and B. Object A has a property of type B. In my winform I have a combo filled with objects of type B. When the save button is pressed, a new instance of class A is created and all the properties are set. The object B property is set as follows:

objectA.myObjectB = (objectB)cmbBObjects.selectedItem;

Then I create a repository for objectA and call the save method. In this save method I have this code±

public bool Save(ObjectA obj)
{
  using(MyContext context = new MyContext())
  {
    context.objectAs.AddObject(obj);
    context.SaveChanges();
  }
}

This code, does save a new entry to the database, but it is also creating a new record for object B! I don't want this, because object B already exists in the database! (I have selected this one from the combobox).

This is how I fill my combobox:

In the objectB repository:

public IList<ObjectB> GetAll()
{
    using(MyContext context = new MyContext())
    {
        IList<ObjectB> objects = context.objectBs.ToList();
        return objects;
    }
}

In my form:

ObjectBRepository rep = new ObjectBRepository();
IList<ObjectB> objects = rep.GetAll;

cmbBObjects.Datasource = objects;
// etc..

So my question is, what do I have to do to save object A without creating a new record for objectB?

score 19 · Accepted Answer · edited Jun 24 '17 at 13:35

19

If you don't want insert objectB you must inform EF about it. When you call context.objectAs.AddObject(obj) for objectA you are saying: I want to insert objectA and all its dependencies. But obviously you don't want to save dependecies so you must either:

Load objectB from DB before adding it to objectA. In such case EF will know that objectB is existing object and it will not insert it again.
Attach objectB to context before adding it to objectA. ObjectB will be handled as existing but unchanged.
Set the state of objectB after inserting objectA. You can do that by calling: context.ObjectStateManager.ChangeObjectState(objectB, EntityState.Unchanged)

Example of the fist suggestion:

var id = objectB.Id;
objectA.myObjectB = context.ObjectBs.SingleOrDefault(o => o.Id == id);

Example of the second suggestion:

context.ObjectBs.Attach(objectB);
objectA.myObjectB = objectB;

Example of the third suggestion:

objectA.myObjectB = objectB;
context.ObjectAs.AddObject(objectA);
context.ObjectStateManager.ChangeObjectState(objectB, EntityState.Unchanged);

edited Jun 24 '17 at 13:35

Luke

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answered Mar 24 '11 at 08:25

Ladislav Mrnka

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Thank you. I don't understand suggestion 1. The second suggestion: In the repository of objectA, do something like: `var obj = context.ObjectB.where(b=> b.Id == objectA.myObjectB.Id).firstOrDefault(); context.ObjectA.Attach(obj); ` Is this correct? – Martijn Mar 24 '11 at 08:31
@LadislavMrnka: Exactly, all possible options, and i suppose second option best one for above example. +1 – Andrew Orsich Mar 24 '11 at 08:45
I've tried your second solution, but slightly different. In my Save method of my ObjectA repository I had this: `context.objectBs.Attach(objectA.myObjectB); context.objectAs.AddObject(objectA); context.SaveChanges();` But here I got a key alreaydy exists exception at line `context.objectAs.AddObject(objectA);` Why was that? – Martijn Mar 24 '11 at 08:45
You must `Attach` `objectB` before you assing it to `objectA` otherwise you will attach both objects and you will not be able to add `objectA`. – Ladislav Mrnka Mar 24 '11 at 08:47
@Martijn: You should attach objectB to context after you get it from dataContext and only objectB not objectA. – Andrew Orsich Mar 24 '11 at 08:48
So, when I had done it this way: `var objB = objectA.objectB; context.objectBs.Attach(objB); //etc..` It had worked? – Martijn Mar 24 '11 at 08:50
The reason I am asking is because my Save method of objectA has one argument: Objecta: `public bool Save(objectA)` Otherwise I have to create a Save method with an argument for each object and I don't think this is the way to go.. – Martijn Mar 24 '11 at 08:52
@Martijn: no, *var b = (objectB)cmbBObjects.selectedItem;* than *context.objectBs.Attach(b);* than assign to objectA and call *Save* method. – Andrew Orsich Mar 24 '11 at 08:53
In case of using repositorie the best option is the first one. Otherwise you will have to modify your repositories to support these scenarios. – Ladislav Mrnka Mar 24 '11 at 08:55
Thnx. But is it desireable to have the context in the UI layer? I don't think so, because the UI layer is then directly talking to the data layer. So I have to solve this in my repository, right? – Martijn Mar 24 '11 at 09:00
Sure you will ask repositoryB to get the object and assign it to `objectA`. But be aware that you must share context among repositories. As you can see repository generally doesn't make things easier. – Ladislav Mrnka Mar 24 '11 at 09:08
And what is the best way to share the context between repositories? And if the repositories are shared, where is the context created for the first time? Who/what is responsible for that? – Martijn Mar 24 '11 at 09:11
Welcome to the word of architecture. The answers you are looking for are: unit of work and dependency injection / inversion of control. – Ladislav Mrnka Mar 24 '11 at 09:13
Hehe, thnx. Do you know a website that can point me at the right direction to start with? – Martijn Mar 24 '11 at 09:18
Actually no. I'm web / wcf developer so I don't know how this is usually handled in WinForms / WPF (ok I have an idea but it is just a theory). But you can ask another question about it and hopefully somebody else will help you. – Ladislav Mrnka Mar 24 '11 at 09:20

score 1 · Answer 2 · answered Mar 24 '11 at 08:21

1

I suppose that problem in following row:

objectA.myObjectB = (objectB)cmbBObjects.selectedItem;

Because of result (objectB)cmbBObjects.selectedItem detached from datacontext entity framework create new instance. Instead this you can:

1.Assign objectB id to objectA

var b = (objectB)cmbBObjects.selectedItem;
objectA.myObjectBId = b.Id;

2.Or load objectB from dataContext and than assign to objectA:

var b = (objectB)cmbBObjects.selectedItem;
var dcB = context.ObjectBs.Single(x=> x.Id == b.Id);
objectA.myObjectB = dcB;

Just try my suggestions and come back with results, because i don't know exactly.

Hope this help.

answered Mar 24 '11 at 08:21

Andrew Orsich

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objectA does not have a objectB id directly, this must be set as follows (I suppose) `objectA..myObjectB.Id = b.Id` But, I think this will throw an argumentNull exception, because the `.myObjectB` property is not set. The second solution: I don't think I want my context in the winform, I'd like to keep that object in my repositories. – Martijn Mar 24 '11 at 08:26
@Martijn: Okay, than you should go second way suggested by me(load objectB from dataContext and assign to objectA). – Andrew Orsich Mar 24 '11 at 08:36

Insert new object with existing object

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