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why Linq to sql can't track update information if we directly assign new object value to old .?

I have the following code
Customer customer = (from x in db.Customers where x.CustomerID.Equals(cust.CustomerID.Value) select x).SingleOrDefault();
customer = newcustomer;
db.SubmitChanges();
by executing the above code I am able to get new data into the customer object,
But by executing SubmitChanges() its can't update into the database.
I need to write following kind of mechanism to update value into database.
customer.CustomerName = "James";
customer.DisplayName = "Jimmy";
db.SubmitChanges();
Is there any suggestion for that ?

You can't do so, because you change the reference that customer points to. So the ORM lose the changes you make to the object you retrieve from the database. Actually, when you fetch a record from the dabase for a specific customer like below:
Customer customer = (from x in db.Customers
where x.CustomerID.Equals(cust.CustomerID.Value)
select x).SingleOrDefault();
You create an object, whose properties have the values that are in the record you fetched. When later you change for instance the value of the CustomerName
customer.CustomerName = "James";
and then you call the SubmitChanges method:
db.SubmitChanges();
a process runs, which detects any changes made to the object. This process finds that you have changed the value of CustomerName and the appopriate UPDATE statement would be created by the ORM. Last this statement is sent to the database, in order to be executed.
On the other hand if you change the value of customer, the changes detection process would decide that nothing changed regarding the retrieved record. Why this happens is due to the fact that the only way the ORM is aware of the record is through the reference it has to the created object after the retrieval. Changing this you lose it from your context.

If you see msdn, you get the answer.
Regardless of how many changes you make to your objects, changes are
made only to in-memory replicas. You have made no changes to the
actual data in the database
When you call SubmitChanges, LINQ to SQL examines the set of known objects >to determine whether new instances have been attached to them. If they have, >these new instances are added to the set of tracked objects.
There are 2 statements
customer = newcustomer;
this statement just update the value or reference in memory.
db.SubmitChanges();
This statement commit the all changes i.e insert, update, delete statement in memory to db end with proper validate the record.
So we need use second statement whenever we need to force and update the data.

DbContext with read-only set properties

I was reading this article http://blogs.msdn.com/b/adonet/archive/2011/01/27/using-dbcontext-in-ef-feature-ctp5-part-2-connections-and-models.aspx and was trying to figure out how to create private setters (the section in the article DbContext with read-only set properties is right before the summary). How would you create private setters? I was playing around with different methods but nothing seemed to work. I am doing this because I need to group the original table based on a query I have because the original table is a heap and I need a primary key for the entity. So anytime a client asks for this table it is already grouped. Not even sure if this is the correct way to do that. Thanks.
EDIT: sorry for being vague. I am doing code first. For example there exists a SQL Table with JobNbr, Qty and Date and I need to group by JobNumber, sum on Qty and take the oldest expiration date, and that will be my entity since this table has no primary key. The way I am doing it now gives me the error below from a method I created in the DbContext class. I do have a EntityTypeConfiguration class. Do I do this in that class?
EDIT: : you might be wondering why I am doing this. Basically I need to get data from the heap and save it in another database. My original approach was database.SqlQuery() to get grouped rows from the heap, but sometimes I have too many parameters for execute_sql. So I decided to create an entity for the grouped query without tracking changes (since all I am doing is reading from the table and saving to another DB). See my post here with the issue I am having https://stackoverflow.com/questions/22106030/entity-framework-6-this-database-sqlquery-character-limitation-with-sp-executes. The only way I know to get around it is to create an entity (even though in this case the entity is a query and not a table).
The entity or complex type
' cannot be
constructed in a LINQ to Entities query.

Entity Framework doesn't update value which is modified by a trigger

My table Sections (SQL Server) has ID as a primary key (int, identity) and SortIndex column (int) for sorting purposes.
The database has a trigger which sets SortIndex := ID at each INSERT. Obviously I want to change the sorting index later, by swapping the values for two rows.
I access the data using Entity Framework, all with MVC3 web application.
The problem is, Entity Framework doesn't update the value of SortIndex after I insert a new object into the table. It also caches all the data, so the following call to get all objects from this table will also give wrong SortIndex value for this object.
I tried changing StoreGeneratedPattern for this column in EDMX. This seems to be great and elegant but doesn't solve the problem.
If I set to Identity, it causes EF to properly update the value, but it becomes read only (exception thrown when trying to change). Setting it to Computed is similar, but instead of exception being thrown the values are just not written to the DB.
I can recreate the EF object every time if I need to use it after inserting an object, just by doing:
DatabaseEntities db = new DatabaseEntities()
But it seems like ugly workaround for me.
What's a solution to this problem?
Obviously something, what doesn't require me to do some action after every insert (and take a risk that it's forgotten and unnoticed) is preferred.

In short StoreGeneratedPattern means: the value is handled by the store and your application will never modify it. In such case you will get store generated value automatically after you call SaveChanges.
If you don't use StoreGeneratedPattern you will not get value and you will have to force another query execution to refresh your entity. You can for example do:
objectContext.Refresh(RefreshMode.StoreWins, yourSection);
Generally situations where you need to update values in both database through triggers and application don't play very nicely with EF (and probably also other ORM tools).

I found the answer from 'Ladislav Mrnka' being exact and marked it as accepted. Here are other workarounds, which I found while trying to find some solution. However, the solution I was looking for is in general not possible.
One of possibilities is to set StoreGeneratedPattern = Computed to let EF know, this value is calculated. And then, make a Stored Procedure to actually change the value of SortIndex. Typically it would change values in two rows (swap them), to change the sorting order. This procedure along with a trigger at INSERT gives guarantee the data stays consistent in the DB. It's not possible to create new row without proper value set in SortIndex, it's not possible to make two objects have the same value (unless stored procedure has a bug) and it's not possible to manually break the value somehow, because it's not possible to edit through EF. Looks like a great solution.
It's easily possible to have stored procedures mapped to functions in EF.
The problem is, it's now fine to enter a new row and EF properly updates data in its cache, but the cache is not updated after calling the stored procedure. Still some manual updated or refresh function is needed. Otherwise the following call to get objects sorted by SortIndex will give wrong results.
Other than that, it's possible to set MergeOption = MergeOption.OverwriteChanges for several entities, which causes EF to update data from the DB somewhat better. With this being done, it's possible to reread the object after inserting it or calling stored procedure and it will get refreshed. However, reading a collection of objects with db.Section.OrderBy(o => o.SortIndex) will still return cached results with wrong sorting order.
If anyone is interested, it's possible to make MergeOption default to something else by adding EF partial class and then partial method OnContextCreated, like here:
public partial class DatabaseEntities
{
partial void OnContextCreated()
{
Subsection.MergeOption = MergeOption.OverwriteChanges;
Section.MergeOption = MergeOption.OverwriteChanges;
Function.MergeOption = MergeOption.OverwriteChanges;
}
}

Do you know if you'll work with that column again in the same request?
I would use the context per request scenario, which usually gets you out of many problem, because a new EF context is created with every request, so you have a fresh data once per request.
With long lived context, there can grow incosistencies as you described.
Anyways the StoreGeneratedPattern setted to computed should be right. But it updates itself only when you're storing the actual entity. It's not getting updated by inserting or updating any other entity.
from http://msdn.microsoft.com/en-us/library/dd296755(v=vs.90).aspx
If you create a new entity or change an existing entity, the values of properties with StoreGeneratedPattern set to Computed are retrieved from the server when you call the SaveChanges method in your application.
If you assign a value to a property with StoreGeneratedPattern set to Computed in your application, the value will be overwritten with the server-generated value when you call the SaveChanges method.
We're using the computed value option for SQL sequenced GUID, and it's working OK.

I had a similar situation with a Sql Server Quote table with a varchar QuoteNumber column that is a non-primary unique key whose value is generated by an after-insert trigger. The trigger is used because the generated value is derived by fetching data from a foreign key table. Sql Server schema identity declarations do not allow you to pull information from other tables.
I'd like EF to treat this varchar column like an identity and do nothing to it on update and reread it after insert. EF will do so if there is a .HasDatabaseGeneratedOption(System.ComponentModel.DataAnnotations.Schema.DatabaseGeneratedOption.Identity) property to a non-identity column in the code it generates to configure the Entity like so (scroll right):
public QuoteConfiguration(string schema)
{
ToTable("Quote", schema);
HasKey(x => x.ID);
Property(x => x.ID).HasColumnName(#"ID").HasColumnType("int").IsRequired().HasDatabaseGeneratedOption(System.ComponentModel.DataAnnotations.Schema.DatabaseGeneratedOption.Identity);
Property(x => x.QuoteNumber).HasColumnName(#"Quote_Number").HasColumnType("varchar").IsOptional().IsUnicode(false).HasMaxLength(64).HasDatabaseGeneratedOption(System.ComponentModel.DataAnnotations.Schema.DatabaseGeneratedOption.Identity);
}
My EF model is code first and generated by Simon Hughes' EntityFramework Reverse POCO Generator. At first, I could not figure out how to make the generator add this property to a column that is not declared as an identity in Sql Server.
Rereading the entire Quote entity after insert did not retrieve the auto-generated QuoteNumber. Then I discovered that re-reading just the QuoteNumber column after insert defeated the entity cache. But, I felt dirty doing it.
Finally, I worked with Simon Hughes to discover how to get his EF Reverse POCO to do it for me. You just extend the UpdateColumn function in your *.tt file like so:
Settings.UpdateColumn = (Column column, Table table) =>
{
if (table.Name.Equals("Quote", StringComparison.InvariantCultureIgnoreCase)
&& column.Name.Equals("Quote_Number", StringComparison.InvariantCultureIgnoreCase))
{
column.IsStoreGenerated = true;
}
}

How to force Linq to update last edit time of a row?

In database, I have a LastEditTime column of type datetime. I used this to track last row update/insert time. Linq throws exception claiming that it can not insert null at that column.
Naturally, since LastEditTime column is NOT NULL, an exception is expected. My hand generated query inserts getutcdate(). How can I ask Linq to do similar?

Create a partial class of whatever table it is. In the partial class have the following:
public partial class MyTable{
partial void OnValidate(System.Data.Linq.ChangeAction action)
{
LastEditTime = DateTime.Now;
}
}
OnValidate is always called before doing a database.submitchanges()

The simplest answer would be to assign a default value for the field in the database, i.e. getdate(). Which will ensure the field is populated with the current time/date but only if it's blank.
Linq to SQL is only an access layer on top of SQL so it's fairly dumb in that respect and you'd either need to craft the Linq to SQL classes yourself or ensure that you populate the value when you create your object instance that you are adding to the DB.
If you are looking for something that updates automatically to ensure concurrency every time you update the record then take a look at the timestamp field type.

You could create a trigger in the database and assign it to the update action. I know it works, we do this all the time on our Postgres database.
I don´t know if SQL Server has triggers, but you can use a stored procedure.

LINQ to SQL disconnected updating object from different data context

Link
I'm using ASP.NET with C# and trying to use linq to sql to update a data context as exhibited on the blog linked above. I created the timestamp field in the table just as stated and am using the following method:
private void updateRecord(TableName updatedRecord)
{
context db = new context();
db.TableName.Attach(updatedRecord,true);
db.SubmitChanges();
}
My question is, are you supposed to assign the timeStamp field to anything in your updatedRecord before trying to call the Attach method on your data context?
When I run this code I get the following exception: System.Data.Linq.ChangeConflictException: Row not found or changed. I update all of the fields, including the primary key of the record that I'm updating before passing the object to this update method. During debugging the TimeStamp attribute of the object shows as null. I'm not sure if it's supposed to be that way or not.
Every book and resource I have says that this is the way to do it, but none of them go into great detail about this TimeStamp attribute.
I know this is quick and easy, so if anybody knows, please let me know.

Since you say that you created the time stamp field in the table, I wonder if, in the case where this column was added later, the column properties may not be set correctly.
You may want to check the properties on your TimeStamp column in the DBML designer. Make sure that:
AutoGenerated = true
Auto-Sync = Always
Time Stamp = True
Update Check = Never
The server data type should be rowversion NOT NULL
If it is not set to be auto generated and synced always, the row version won't be returned from the insert since you haven't changed it when the insert was done. Even though this value is generated by the database the DataContext needs to know this so that it can handle it properly.
In addition, now that you have a timestamp column, UpdateCheck should be set to Never for all of the other columns.

If you have a timestamp column, then to update a record (from a vanilla object): yes, I would expect to have to assign it. Otherwise, you lose the ability to use the timestamp for optimistic concurrency checking.
The idea is you take a copy of the timestamp when you get hold of your (disconnected) object, then when you update you can use this column to verify that nobody else has edited the row.
There are two common scenarios:
1: if you are only performing a short lived operation, get the record out of the database first - make your changes to the object, and simply SumbitChanges() [all with the same data-context]. The data-context will handle concurrency for you.
2: if you are disconnecting the object (for example passing it to a client application for a while), then use something like serialization (LINQ-to-SQL objects support DataContractSerializer (optionally; you need to enable it)). So serialize the object at the server, pass it to the client - the client makes changes to their copy and passes it back. The server deserializes it and uses Attach() and SubmitChanges(). The record in memory should still have the timestamp that it had when extracted from the database, so we can perform optimistic concurrency spanning all the time the record has been disconnected.