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Can I tell linq 2 SQL not to update certain columns after attachment or use of UpdateModel?

Let's assume I have the following situation, the update method in my service accepts a model (the one that is going to be updated) as an input parameter. The model can be unattached (in which case attach method is called before submitting changes) or attached (in which case we just submit changes). Edit actions just call this update method in my service. Now let's assume I cannot change the code in those actions (the code that produces the model to be updated). Can I still somehow prevent certain columns from updating from within the update method. Note that I might want to set those columns using linq to SQL, but only during insert method.
I'm quite sure I'm trying something unconventional here, but it might help me write some easy to reuse code. If it cannot be done, then I'll solve it differently, but it never hurts to try something new.

The Attach method does provide an override to accept both a modified and original entity.
Attach Modified Entity on Data Context
When using this the internal change tracker will figure out which columns have been updated and will only update those ones on the datasource which have changed, rather than updating all columns.
Alternatively if you want more explicit control over which properties are updated, you can reattach your entity as unmodified in its original state:
Attach Modified/Unmodified Entity on Data Context
This will hook up the internal change tracker to the PropertyChanging events on the entity so it can be tracked. You would then simply change the values of the properties on that entity in the Update method on your Service.
void Update(MyModel model)
{
using (MyContext ctx = new MyContext())
{
ctx.DeferredLoadingEnabled = false;
ctx.MyEntities.Attach(model.OriginalEntity);
model.OriginalEntity.Value = model.ModifiedEntity.Value;
ctx.SubmitChanges();
}
}
The pitfall of these approaches means you must maintain both the original and modified entities in your model, but could be set when your entities are loaded - a simple shallow copy of the object should do the trick by deriving from ICloneable in a partial class for each entity.

Nhibernate GetEntry with un-saved entity object

Before I save an entity I want to check if its dirty.
So I have the entity object came back from the ajax. (with ID).
The object is not saved yet, and I want to get the entity entry with it:
// this will return null, I assume it is because the object it not persisted yet.
session.GetSessionImplementation().PersistenceContext.GetEntry(entity);
// this will return what I want, but merge will save the object...
session.GetSessionImplementation().PersistenceContext.GetEntry(session.Merge(entity));
Isnt this the entire idea behind dirty and un-dirty, is knowing them before the save action?
How can I get the entry of the object without having the persisted object yet?
Update
To give a real life use case, Lets say that I have the method NhiUtil.IsPropertyDirty that inside do the GetEntry from above.
if(NhiUtil.IsPropertyDirty("Password",userEntityFromClient, session)){
userEntityFromClient = Hush(userEntityFromClient.Password);
}
session.SaveOrUpdate(userEntityFromClient);
If I am forced inside IsPropertyDirty to merge/save userEntityFromClient in order to get GetEntry to work, I cant take business logic decisions before the actual save...
so the entire IsPropertyDirty is unusable...
Thanks

That's why we do have the NHiberante. If you go through your question and read about ORM, you must find that exactly that's why these tools are here. That's what they do for us.
Other words, do the standard steps:
1) Get the object from the Client (upper layers/tiers) via deserialization
optional get the object from session.Get(id) and bind it, skip the Merge later
optional do validation (business layer)
2) Merge the object, session.Merge() and call the SaveOrUpdate()
3) Flush the session
That's pretty it. 1) If we firstly get-by-id and bind the object from the recieved data (JSON) we already do have an object in the ISession. And the ISession is the one doing for us the dirty checking
In case 2) that we have detached object, use the session.Merge() which will again do all for us inside of the ISession.
Flush, only the dirty
The essence is in the 3) Flushing. It will be converted into the Update/Insert statement only if the object is dirty. If it is the same (unchanged) ... no call to DB engine
Some interesting sources:
Chapter 24. Best Practices
In a three tiered architecture, consider using SaveOrUpdate().
When using a distributed architecture, you could pass persistent objects loaded in the middle tier to and from the user interface tier. Use a new session to service each request. Use ISession.Update() or ISession.SaveOrUpdate() to update the persistent state of an object.
9.4.2. Updating detached objects
... SaveOrUpdate() ...
... using Merge(Object o). This method copies the state of the given object onto the persistent object with the same identifier. If there is no persistent instance currently associated with the session, it will be loaded. The method returns the persistent instance. If the given instance is unsaved or does not exist in the database, NHibernate will save it and return it as a newly persistent instance. Otherwise, the given instance does not become associated with the session. In most applications with detached objects, you need both methods, SaveOrUpdate() and Merge().
And also interesting: Ayende - The difference between Get, Load and querying by id
And the 12.1. Interceptors, which allows to hook on some events, e.g.:
public override bool OnFlushDirty(object entity,
object id,
object[] currentState,
object[] previousState,
string[] propertyNames,
IType[] types)

Entity Framework: Adding items to a collection wont persist in database

I created a project with a model first approach, in my model I declared a many to many relationship between the entities, Musician and Session (representing some kind of rehearsal or performance). As expected, this generated a pivot table in my database to handle the relationship, but no C# model class for the intermediate table. So, on my Session object I have a collection of Musicians declared as an ICollection:
public virtual ICollection<Musician> Musicians { get; set; }
Which is initialised as a HashSet in the Session constructor:
public Session()
{
this.Musicians = new HashSet<Musician>();
}
Now, somewhere else in my solution (in an MVC controller, if it matters), I want to assign an existing Musician to a given Session, so I have this code:
Musician musician = db.Musicians.First(m => m.Id == selectedMusician);
session.Musicians.Add(musician);
db.SaveChanges();
Just after session.Musicians.Add(musician) is called, I've inspected that the musician has been added to the collection, but calling db.SaveChanges doesn't seem to persist the change to the database.
By the way, db is my repository containing all my models, for example, this code successfully adds a new Session to the database:
db.Sessions.Add(session);
db.SaveChanges();
If somebody could point me in the right direction on how to add an object to a collection in this circumstance that would be a great help.
Cheers
EDIT: Turns out the problem had nothing to do with the fact that it was a collection. Like I mentioned in my post, I was trying to make the changes/saving from a controller class in an ASP MVC project. The session object was being passed in from the view, but I discovered any changes I made to this object wouldn't persist in the database (doh, silly me, should of checked this straight away). Looks like the object passed in by the view was just a copy as it wasn't coming directly from the context (in this case, my db object).
To fix it, I just queried the context for a Session object with the same Id in order to have access to the object that my changes would actually persist.

Using Attach with Linq To Sql and Stored Procs

I am trying to use the attach method to update an entity that was retrieve via a stored proc.
The stored proc is set up to return a specific instance, which is present in my dbml. The retrieval works as expected and returns a fully populated object. The reason I need to use a stored proc is that I need to update a property on that entity at the same time that it is retrieved.
After I have retrieved this entity, I am mapping it using AutoMapper to another model which is used in another tier of the app. This tier performs a few operations, and makes a change to the entity, and passes it back to the repository for updating.
The repository converts this business model back into a database model, and attempts to attach it to the datacontext in order to take advantage of the automagic updating.
No matter what combination of Attach(entity, true) Attach(entity) etc, and it gives me messages like "Row not found or changed" or "Unable to add an entity with the same primary key".
Does anyone have any experience with the Attach method and how it can be used to update entities that did not necessarily come from the data context using query syntax (ie in this case a stored proc)?
Thanks alot

First, if you are creating a copy of the object, making changes and then trying to attach the copied object to the same DataContext as the one with the original object in it, then this would probably result in the "Unable to add an entity with the same primary key" message. One way to handle this is:
1. Get object from DataContext
2. Make changes and map object (or vice versa - whatever order)
3. Update the original object with the new values made in the other tier
4. SubmitChanges on the DataContext containing the original object
or
Get the object from a DataContext and close the DataContext
Make your changes and do your mapping
Retrieve the object from the DataContext to which you want to save
Update that object with the values from your mapped object
SubmitChanges
Alternately, when you say you are using the proc because you need to update a property at the same time that you retrieve it, I'd need to see the proc, but if you are somehow committing this update after retrieving the information, then indeed the message "row not found or changed" is correct. This would be hard to do, but you could do it if you're loading the data into a temp table, doing the update, and then using a select from the temp table to populate the object. One thing you could try is setting that property, in the L2S designer, to AutoUpdate = Never and see if that makes the problem go away. If so, this is your problem.

1: is it the same data-context, and
2: is it the same entity instance (or one that looks like it)
This would only happen for the same data-context, I suspect. If it is the same entity, then it is already there; just call SumbitChanges. Otherwise, either use a second data-context or detach the original entity.

So if I retrieved the entity via a stored proc, is it being tracked by the datacontext?
The thing is.. I'm going from the data model, to a another model that is used by another component, and then back. Its not.. really the same instance, but it does have all the same properties.
IE
public Models.Tag GetEntity()
{
var dbTag = db.PROJ_GetEntity((int)EntityStatuses.Created, (int)EntityStatuses.CreatingInApi).SingleOrDefault();
return FromDb Entity(dbEntity);
}
var appModel = GetEntity(); // gets an Entity from a stored proc (NOT GetEntity_RESULT)
appModel.MakeSomeChanges();
_Repo.Persist(appModel);
public void Persist(Models.AppModel model)
{
var dbEntity = Mapper.Map(model);
db.Attach(dbEntity);
db.SubmitChanges();
}
This is somewhat pseudo code like.. but it demostrates pretty much exactly what I am doing.
Thanks

I'm upvoting weenet's answer because he's right - you can't use Attach to apply the changes.
Unlike Entity Framework, you can only attach an L2S object to a datacontext if it has never been attached before - i.e. it's a newed entity that you want to Insert into a table.
This does cause numerous problems in multi-layered environments - however I've been able to get around many of the issues by creating a generic entity synchronisation system, which uses reflection and expression trees.
After an object has been modified, I run the dynamic delegate against a new object from the DC and the modified object, so that only the differences are tracked in the DC before generating the Update statement. Does get a bit tricky with related entities, though.

How do you save a Linq object if you don't have its data context?

I have a Linq object, and I want to make changes to it and save it, like so:
public void DoSomething(MyClass obj) {
obj.MyProperty = "Changed!";
MyDataContext dc = new MyDataContext();
dc.GetTable<MyClass>().Attach(dc, true); // throws exception
dc.SubmitChanges();
}
The exception is:
System.InvalidOperationException: An entity can only be attached as modified without original state if it declares a version member or does not have an update check policy.
It looks like I have a few choices:
put a version member on every one of my Linq classes & tables (100+) that I need to use in this way.
find the data context that originally created the object and use that to submit changes.
implement OnLoaded in every class and save a copy of this object that I can pass to Attach() as the baseline object.
To hell with concurrency checking; load the DB version just before attaching and use that as the baseline object (NOT!!!)
Option (2) seems the most elegant method, particularly if I can find a way of storing a reference to the data context when the object is created. But - how?
Any other ideas?
EDIT
I tried to follow Jason Punyon's advice and create a concurrency field on on table as a test case. I set all the right properties (Time Stamp = true etc.) on the field in the dbml file, and I now have a concurrency field... and a different error:
System.NotSupportedException: An attempt has been made to Attach or Add an entity that is not new, perhaps having been loaded from another DataContext. This is not supported.
So what the heck am I supposed to attach, then, if not an existing entity? If I wanted a new record, I would do an InsertOnSubmit()! So how are you supposed to use Attach()?
Edit - FULL DISCLOSURE
OK, I can see it's time for full disclosure of why all the standard patterns aren't working for me.
I have been trying to be clever and make my interfaces much cleaner by hiding the DataContext from the "consumer" developers. This I have done by creating a base class
public class LinqedTable<T> where T : LinqedTable<T> {
...
}
... and every single one of my tables has the "other half" of its generated version declared like so:
public partial class MyClass : LinqedTable<MyClass> {
}
Now LinqedTable has a bunch of utility methods, most particularly things like:
public static T Get(long ID) {
// code to load the record with the given ID
// so you can write things like:
// MyClass obj = MyClass.Get(myID);
// instead of:
// MyClass obj = myDataContext.GetTable<MyClass>().Where(o => o.ID == myID).SingleOrDefault();
}
public static Table<T> GetTable() {
// so you can write queries like:
// var q = MyClass.GetTable();
// instead of:
// var q = myDataContext.GetTable<MyClass>();
}
Of course, as you can imagine, this means that LinqedTable must somehow be able to have access to a DataContext. Up until recently I was achieving this by caching the DataContext in a static context. Yes, "up until recently", because that "recently" is when I discovered that you're not really supposed to hang on to a DataContext for longer than a unit of work, otherwise all sorts of gremlins start coming out of the woodwork. Lesson learned.
So now I know that I can't hang on to that data context for too long... which is why I started experimenting with creating a DataContext on demand, cached only on the current LinqedTable instance. This then led to the problem where the newly created DataContext wants nothing to do with my object, because it "knows" that it's being unfaithful to the DataContext that created it.
Is there any way of pushing the DataContext info onto the LinqedTable at the time of creation or loading?
This really is a poser. I definitely do not want to compromise on all these convenience functions I've put into the LinqedTable base class, and I need to be able to let go of the DataContext when necessary and hang on to it while it's still needed.
Any other ideas?

Updating with LINQ to SQL is, um, interesting.
If the data context is gone (which in most situations, it should be), then you will need to get a new data context, and run a query to retrieve the object you want to update. It's an absolute rule in LINQ to SQL that you must retrieve an object to delete it, and it's just about as iron-clad that you should retrieve an object to update it as well. There are workarounds, but they are ugly and generally have lots more ways to get you in trouble. So just go get the record again and be done with it.
Once you have the re-fetched object, then update it with the content of your existing object that has the changes. Then do a SubmitChanges() on the new data context. That's it! LINQ to SQL will generate a fairly heavy-handed version of optimistic concurrency by comparing every value in the record to the original (in the re-fetched) record. If any value changed while you had the data, LINQ to SQL will throw a concurrency exception. (So you don't need to go altering all your tables for versioning or timestamps.)
If you have any questions about the generated update statements, you'll have to break out SQL Profiler and watch the updates go to the database. Which is actually a good idea, until you get confidence in the generated SQL.
One last note on transactions - the data context will generate a transaction for each SubmitChanges() call, if there is no ambient transaction. If you have several items to update and want to run them as one transaction, make sure you use the same data context for all of them, and wait to call SubmitChanges() until you've updated all the object contents.
If that approach to transactions isn't feasible, then look up the TransactionScope object. It will be your friend.

I think 2 is not the best option. It's sounding like you're going to create a single DataContext and keep it alive for the entire lifetime of your program which is a bad idea. DataContexts are lightweight objects meant to be spun up when you need them. Trying to keep the references around is also probably going to tightly couple areas of your program you'd rather keep separate.
Running a hundred ALTER TABLE statements one time, regenerating the context and keeping the architecture simple and decoupled is the elegant answer...

find the data context that originally created the object and use that to submit changes
Where did your datacontext go? Why is it so hard to find? You're only using one at any given time right?
So what the heck am I supposed to attach, then, if not an existing entity? If I wanted a new record, I would do an InsertOnSubmit()! So how are you supposed to use Attach()?
You're supposed to attach an instance that represents an existing record... but was not loaded by another datacontext - can't have two contexts tracking record state on the same instance. If you produce a new instance (ie. clone) you'll be good to go.
You might want to check out this article and its concurrency patterns for update and delete section.

The "An entity can only be attached as modified without original state if it declares a version member" error when attaching an entitity that has a timestamp member will (should) only occur if the entity has not travelled 'over the wire' (read: been serialized and deserialized again). If you're testing with a local test app that is not using WCF or something else that will result in the entities being serialized and deserialized then they will still keep references to the original datacontext through entitysets/entityrefs (associations/nav. properties).
If this is the case, you can work around it by serializing and deserializing it locally before calling the datacontext's .Attach method. E.g.:
internal static T CloneEntity<T>(T originalEntity)
{
Type entityType = typeof(T);
DataContractSerializer ser =
new DataContractSerializer(entityType);
using (MemoryStream ms = new MemoryStream())
{
ser.WriteObject(ms, originalEntity);
ms.Position = 0;
return (T)ser.ReadObject(ms);
}
}
Alternatively you can detach it by setting all entitysets/entityrefs to null, but that is more error prone so although a bit more expensive I just use the DataContractSerializer method above whenever I want to simulate n-tier behavior locally...
(related thread: http://social.msdn.microsoft.com/Forums/en-US/linqtosql/thread/eeeee9ae-fafb-4627-aa2e-e30570f637ba )

You can reattach to a new DataContext. The only thing that prevents you from doing so under normal circumstances is the property changed event registrations that occur within the EntitySet<T> and EntityRef<T> classes. To allow the entity to be transferred between contexts, you first have to detach the entity from the DataContext, by removing these event registrations, and then later on reattach to the new context by using the DataContext.Attach() method.
Here's a good example.

When you retrieve the data in the first place, turn off object tracking on the context that does the retrieval. This will prevent the object state from being tracked on the original context. Then, when it's time to save the values, attach to the new context, refresh to set the original values on the object from the database, and then submit changes. The following worked for me when I tested it.
MyClass obj = null;
using (DataContext context = new DataContext())
{
context.ObjectTrackingEnabled = false;
obj = (from p in context.MyClasses
where p.ID == someId
select p).FirstOrDefault();
}
obj.Name += "test";
using (DataContext context2 = new ())
{
context2.MyClasses.Attach(obj);
context2.Refresh(System.Data.Linq.RefreshMode.KeepCurrentValues, obj);
context2.SubmitChanges();
}