If I make changes to an existing linq object by assigning a "new" object of the same type (with different values), SubmitChanges does not make the changes in the database. why not?
existing= new Data.Item{a=1, b=2...};
vs
existing.a= 1;
existing.b= 2;
Because you are not changing the object, you are assigning a new object to the variable.
You need to assign to fields one by one, (or InsertOnSubmit... but that will create a new object in the database and it does not sound like that is what you want to do).
This approach will sort of work if you we assigning the newly created object to a field of an object that LINQ to SQL knows about, but once again, that would be creating a new object rather than changing the one that field previously pointed to (which could result in a bunch of garbage rows in your database if you never get rid of them).
Related
I've done some searches (over the web and SO) but so far have been unable to find something that directly answer this:
Is there anyway to force L2S to use a Stored Procedure when acessing a Database?
This is different from simply using SPROC's with L2S: The thing is, I'm relying on LINQ to lazy load elements by accessing then through the generated "Child Property". If I use a SPROC to retrieve the elements of one table, map then to an entity in LINQ, and then access a child property, I believe that LINQ will retrieve the register from the DB using dynamic sql, which goes against my purpose.
UPDATE:
Sorry if the text above isn't clear. What I really want is something that is like the "Default Methods" for Update, Insert and Delete, however, to Select. I want every access to be done through a SPROC, but I want to use Child Property.
Just so you don't think I'm crazy, the thing is that my DAL is build using child properties and I was accessing the database through L2S using dynamic SQL, but last week the client has told me that all database access must be done through SPROCS.
i don't believe that there is a switch or setting that out of the box and automagically would map to using t sprocs the way you are describing. But there is now reason why you couldn't alter the generated DBML file to do what you want. If I had two related tables, a Catalog table and CatalogItem tables, the Linq2SQL generator will naturally give me a property of CatalogItems on Catalog, code like:
private EntitySet<shelf_myndr_Previews_CatalogItem> _shelf_myndr_Previews_CatalogItems;
[global::System.Data.Linq.Mapping.AssociationAttribute(Name="CatalogItem", Storage="_CatalogItems", ThisKey="Id", OtherKey="CatalogId")]
public EntitySet<CatalogItem> CatalogItems
{
get
{
return this._CatalogItems;
//replace this line with a sproc call that ultimately
//returns the expected type
}
set
{
this._CatalogItems.Assign(value);
//replace this line with a sproc call that ultimately
//does a save operation
}
}
There is nothing stopping you from changing that code to be sproc calls there. It'd be some effort for larger applications and I'd be sure that you be getting the benefit from it that you think you would.
How about loading the child entities using the partial OnLoaded() method in the parent entity? That would allow you to avoid messing with generated code. Of course it would no longer be a lazy load, but it's a simple way to do it.
For example:
public partial class Supplier
{
public List<Product> Products { get; set; }
partial void OnLoaded()
{
// GetProductsBySupplierId is the SP dragged into your dbml designer
Products = dataContext.GetProductsBySupplierId(this.Id).ToList();
}
}
Call your stored procedure this way:
Where GetProductsByCategoryName is the name of your stored procedure.
http://weblogs.asp.net/scottgu/archive/2007/08/16/linq-to-sql-part-6-retrieving-data-using-stored-procedures.aspx
In LINQ-to-SQL if I update an object in the context but haven't called SubmitChanges, is there a way to "undo" or abandon that update so that the changes won't get submitted when I eventually call SubmitChanges?
For example, if I've updated several objects and then decide I want to abandon the changes to one of them before submitting.
Part 2: same question for Entity Framework, v3.5
Both LINQ to SQL and Entity Framework will use the same call (assuming you still have the active Context):
_dbContext.Refresh(RefreshMode.OverwriteCurrentValues, yourObj);
A more appropriate way would be to treat the Context as a Unit of Work, in which case you would no longer have an active context when refreshing the object. You would simply dispose of the object you're using currently and get a fresh copy from a new context.
I think you can use the .GetOriginalEntityState(yourEntity) to retrieve the original values. Then set your updated entity back to the original
dim db as new yourDataContext
//get entity
dim e1 as yourEntity = (from x in db.table1).take(1)
//update entity
e1.someProperty = 'New Value'
//get original entity
dim originalEntity = db.table1.getOrignalEntityState(e1)
e1 = originalEntity
db.submitChanges()
Very pseudo-code but I think it conveys the right idea. Using this method, you could also just undo one or more property changes without refreshing the entire entity.
I have a user object called UserSystem, which is created by a static factory class that returns User Systems. Because the factory class only exists to create this object once, then disposes, is it possible to associate my persisted UserSystem object with another instance of my database context that I create at a later point?
I would like to avoid having to query my new DatabaseContext to find the matching UserSystem object and simply associate the persisted user object from the first DatabaseContext class with my new DatabaseContext.
Thanks!
George
You probably want to Attach your object to the DataContext. There are many articles about this, for example this one. Be careful though - this method is not intended to allow you to attach objects that are already attached to another DataContext, it is only for deserialized objects that are completely unattached, which I assume is what you have.
You may use the Attach method on the Table<T> object to insert a detached data object into it. You may insert it in a modified state, or in an unmodified state. If you insert it in a modified state, the next SubmitChanges() call will include it.
The Table(Of TEntity) Attach method overloads
If I have a LINQ to SQL table that has a field called say Alias.
There is then a method stub called OnAliasChanging(string value);
What I want to do is to grab the value, check the database whether the value already exists and then set the value to the already entered value.
So I may be changing my alias from "griegs" to "slappy" and if slappy exists then I want to revert to the already existing value of "griegs".
So I have;
partial void OnaliasChanging(string value)
{
string prevValue = this.alias;
this.Changed = true;
}
When I check the value of prevValue it's always null.
How can I get the current value of a field?
Update
If I implement something like;
partial void OnaliasChanging(string value)
{
if (this.alias != null)
this.alias = "TEST VALUE";
}
it goes into an infinte loop which is unhealthy.
If I include a check to see whether alias already == "TEST VALUE" the infinate loop still remains as the value is always the original value.
Is there a way to do this?
The code snippets you've posted don't lend themselves to any plausible explanation of why you'd end up with an infinite loop. I'm thinking that this.alias might be a property, as opposed to a field as the character casing would imply, but would need to see more. If it is a property, then you are invoking the OnAliasChanging method before the property is ever set; therefore, trying to set it again in the same method will always cause an infinite loop. Normally the way to design this scenario is to either implement a Cancel property in your OnXyzChanging EventArgs derivative, or save the old value in the OnXyzChanging method and subsequently perform the check/rollback in the OnXyzChanged method if you can't use the first (better) option.
Fundamentally, though, what you're trying to do is not very good design in general and goes against the principles of Linq to SQL specifically. A Linq to SQL entity is supposed to be a POCO with no awareness of sibling entities or the underlying database at all. To perform a dupe-check on every property change not only requires access to the DataContext or SqlConnection, but also causes what could technically be called a side-effect (opening up a new database connection and/or silently discarding the property change). This kind of design just screams for mysterious crashes down the road.
In fact, your particular scenario is one of the main reasons why the DataContext class was made extensible in the first place. This type of operation belongs in there. Let's say that the entity here is called User with table Users.
partial class MyDataContext
{
public bool ChangeAlias(Guid userID, string newAlias)
{
User userToChange = Users.FirstOrDefault(u => u.ID == userID);
if ((userToChange == null) || Users.Any(u => u.Alias == newAlias))
{
return false;
}
userToChange.Alias = newAlias;
// Optional - remove if consumer will make additional changes
SubmitChanges();
return true;
}
}
This encapsulates the operation you want to perform, but doesn't prevent consumers from changing the Alias property directly. If you can live with this, I would stop right there - you should still have a UNIQUE constraint in your database itself, so this method can simply be documented and used as a safe way to attempt a name-change without risking a constraint violation later on (although there is always some risk - you can still have a race condition unless you put this all into a transaction or stored procedure).
If you absolutely must limit access to the underlying property, one way to do this is to hide the original property and make a read-only wrapper. In the Linq designer, click on the Alias property, and on the property sheet, change the Access to Internal and the Name to AliasInternal (but don't touch the Source!). Finally, create a partial class for the entity (I would do this in the same file as the MyDataContext partial class) and write a read-only wrapper for the property:
partial class User
{
public string Alias
{
get { return AliasInternal; }
}
}
You'll also have to update the Alias references in our ChangeAlias method to AliasInternal.
Be aware that this may break queries that try to filter/group on the new Alias wrapper (I believe Linq will complain that it can't find a SQL mapping). The property itself will work fine as an accessor, but if you need to perform lookups on the Alias then you will likely need another GetUserByAlias helper method in MyDataContext, one which can perform the "real" query on AliasInternal.
Things start to get a little dicey when you decide you want to mess with the data-access logic of Linq in addition to the domain logic, which is why I recommend above that you just leave the Alias property alone and document its usage appropriately. Linq is designed around optimistic concurrency; typically when you need to enforce a UNIQUE constraint in your application, you wait until the changes are actually saved and then handle the constraint violation if it happens. If you want to do it immediately your task becomes harder, which is the reason for this verbosity and general kludginess.
One more time - I'm recommending against the additional step of creating the read-only wrapper; I've put up some code anyway in case your spec requires it for some reason.
Is it getting hung up because OnaliasChanging is firing during initialization, so your backing field (alias) never gets initialized so it is always null?
Without more context, that's what it sounds like to me.
I ran into an interesting problem while using DLINQ. When I instantiate an entity, calling .SubmitChanges() on the DataContext will insert a new row into the database - without having ever called .Insert[All]OnSubmit(...).
//Code sample:
Data.NetServices _netServices = new Data.NetServices(_connString);
Data.ProductOption[] test = new Data.ProductOption[]
{
new Data.ProductOption
{
Name="TEST1",
//Notice the assignment here
ProductOptionCategory=_netServices.ProductOptionCategory.First(poc => poc.Name == "laminate")
}
};
_netServices.SubmitChanges();
Running the code above will insert a new row in the database. I noticed this effect while writing an app to parse an XML file and populate some tables. I noticed there were 1000+ inserts when I was only expecting around 50 or so - then I finally isolated this behavior.
How can I prevent these objects from being persisted implicitly?
Thanks,
-Charles
Think of the relationship as having two sides. When you set one side of the relationship the other side needs to be updated so in the case above as well as setting the ProductOptionCategory it is effectively adding the new object to the ProductOptions relationship on the laminate ProductOptionCategory side.
The work-around is as you have already discovered and to set the underlying foreign key instead so LINQ to SQL will not track the objects in the usual way and require implicit indication it should persist the object.
Of course the best solution for performance would be to determine from the source data which objects you don't want to add and never create the instance in the first place.