I'm using LINQ to SQL in ASP.NET MVC. I wrote some new code to update Orders in our system, and as far as I can tell it's exactly like a hundred other similar pieces of code I've written (grab the object, update some fields, submit changes).
This time though, when I try to run the update, I get a "Row not found or changed" LINQ exception on this call stack:
System.Data.Linq.dll!System.Data.Linq.DataContext.SubmitChanges(System.Data.Linq.ConflictMode failureMode) + 0x14c bytes
System.Data.Linq.dll!System.Data.Linq.DataContext.SubmitChanges() + 0x14 bytes
If I just refresh the page after the exception, it just works with no issues.
How can I get it to work correctly the first time?
I've seen answers on the net relating to DateTime precision and Update checks, but nothing about the submission simply working the second time, not the first.
My code is basically like this:
Order order = myDataContext.Orders.SingleOrDefault(order.OrderID == orderID);
order.Field1 = true;
order.Boolean2 = true;
order.Double1 = 300.0;
myDataContext.SubmitChanges();
The issue turned out to be that I was changing a related table in between fetching the Order, editing, and saving it.
When the related table was changed, the Order table was indirectly changed because of the parent-child relationship. That caused the change checking validation to fail.
All I needed to do is pull all the related code (fetch, change, save) into one tight block, with no other code in-between. I'm not having any problems now.
Your code looks fine.
There must be something else in myDataContext that is causing the problem.
Where is myDataContext defined? I am guessing that there is some code in a if is postback block, or something similar, to cause the code that runs to take a different path.
Are you creating a new datacontext when rebinding (after calling SubmitChanges)?
You should never reuse a datacontext for selecting, after performing insert/update/delete actions with it.
Related
We have a breeze client solution in which we show parent entities with lists of their children. We do hard deletes on some child entities. Now when the user is the one doing the deletes, there is no problem, but when someone else does, there seems to be no way to invalidate the children already loaded in cache. We do a new query with the parent and expanding to children, but breeze attaches all the other children it has already heard of, even if the database did not return them.
My question: shouldn't breeze realize we are loading through expand and thus completely remove all children from cache before loading back the results from the db? How else can we accomplish this if that is not the case?
Thank you
Yup, that's a really good point.
Deletion is simply a horrible complication to every data management effort. This is true no matter whether you use Breeze or not. It just causes heartache up and down the line. Which is why I recommend soft deletes instead of hard deletes.
But you don't care what I think ... so I will continue.
Let me be straight about this. There is no easy way for you to implement a cache cleanup scheme properly. I'm going to describe how we might do it (with some details neglected I'm sure) and you'll see why it is difficult and, in perverse cases, fruitless.
Of course the most efficient, brute force approach is to blow away the cache before querying. You might as well not have caching if you do that but I thought I'd mention it.
The "Detached" entity problem
Before I continue, remember the technique I just mentioned and indeed all possible solutions are useless if your UI (or anything else) is holding references to the entities that you want to remove.
Oh, you'll remove them from cache alright. But whatever is holding references to them now will continue to have a reference to an entity object which is in a "Detached" state - a ghost. Making sure that doesn't happen is your responsibility; Breeze can't know and couldn't do anything about it if it did know.
Second attempt
A second, less blunt approach (suggested by Jay) is to
apply the query to the cache first
iterate over the results and for each one
detach every child entity along the "expand" paths.
detach that top level entity
Now when the query succeeds, you have a clear road for it to fill the cache.
Here is a simple example of the code as it relates to a query of TodoLists and their TodoItems:
var query = breeze.EntityQuery.from('TodoLists').expand('TodoItems');
var inCache = manager.executeQueryLocally(query);
inCache.slice().forEach(function(e) {
inCache = inCache.concat(e.TodoItems);
});
inCache.slice().forEach(function(e) {
manager.detachEntity(e);
});
There are at least four problems with this approach:
Every queried entity is a ghost. If your UI is displaying any of the queried entities, it will be displaying ghosts. This is true even when the entity was not touched on the server at all (99% of the time). Too bad. You have to repaint the entire page.
You may be able to do that. But in many respects this technique is almost as impractical as the first. It means that ever view is in a potentially invalid state after any query takes place anywhere.
Detaching an entity has side-effects. All other entities that depend on the one you detached are instantly (a) changed and (b) orphaned. There is no easy recovery from this, as explained in the "orphans" section below.
This technique wipes out all pending changes among the entities that you are querying. We'll see how to deal with that shortly.
If the query fails for some reason (lost connection?), you've got nothing to show. Unless you remember what you removed ... in which case you could put those entities back in cache if the query fails.
Why mention a technique that may have limited practical value? Because it is a step along the way to approach #3 that could work
Attempt #3 - this might actually work
The approach I'm about to describe is often referred to as "Mark and Sweep".
Run the query locally and calculate theinCache list of entities as just described. This time, do not remove those entities from cache. We WILL remove the entities that remain in this list after the query succeeds ... but not just yet.
If the query's MergeOption is "PreserveChanges" (which it is by default), remove every entity from the inCache list (not from the manager's cache!) that has pending changes. We do this because such entities must stay in cache no matter what the state of the entity on the server. That's what "PreserveChanges" means.
We could have done this in our second approach to avoid removing entities with unsaved changes.
Subscribe to the EntityManager.entityChanged event. In your handler, remove the "entity that changed" from the inCache list because the fact that this entity was returned by the query and merged into the cache tells you it still exists on the server. Here is some code for that:
var handlerId = manager.entityChanged.subscribe(trackQueryResults);
function trackQueryResults(changeArgs) {
var action = changeArgs.entityAction;
if (action === breeze.EntityAction.AttachOnQuery ||
action === breeze.EntityAction.MergeOnQuery) {
var ix = inCache.indexOf(changeArgs.entity);
if (ix > -1) {
inCache.splice(ix, 1);
}
}
}
If the query fails, forget all of this
If the query succeeds
unsubscribe: manager.entityChanged.unsubscribe(handlerId);
subscribe with orphan detection handler
var handlerId = manager.entityChanged.subscribe(orphanDetector);
function orphanDetector(changeArgs) {
var action = changeArgs.entityAction;
if (action === breeze.EntityAction.PropertyChange) {
var orphan = changeArgs.entity;
// do something about this orphan
}
}
detach every entity that remains in the inCache list.
inCache.slice().forEach(function(e) {
manager.detachEntity(e);
});
unsubscribe the orphan detection handler
Orphan Detector?
Detaching an entity can have side-effects. Suppose we have Products and every product has a Color. Some other user hates "red". She deletes some of the red products and changes the rest to "blue". Then she deletes the "red" Color.
You know nothing about this and innocently re-query the Colors. The "red" color is gone and your cleanup process detaches it from cache. Instantly every Product in cache is modified. Breeze doesn't know what the new Color should be so it sets the FK, Product.colorId, to zero for every formerly "red" product.
There is no Color with id=0 so all of these products are in an invalid state (violating referential integrity constraint). They have no Color parent. They are orphans.
Two questions: how do you know this happened to you and what do your do?
Detection
Breeze updates the affected products when you detach the "red" color.
You could listen for a PropertyChanged event raised during the detach process. That's what I did in my code sample. In theory (and I think "in fact"), the only thing that could trigger the PropertyChanged event during the detach process is the "orphan" side-effect.
What do you do?
leave the orphan in an invalid, modified state?
revert to the equally invalid former colorId for the deleted "red" color?
refresh the orphan to get its new color state (or discover that it was deleted)?
There is no good answer. You have your pick of evils with the first two options. I'd probably go with the second as it seems least disruptive. This would leave the products in "Unchanged" state, pointing to a non-existent Color.
It's not much worse then when you query for the latest products and one of them refers to a new Color ("banana") that you don't have in cache.
The "refresh" option seems technically the best. It is unwieldy. It could easily cascade into a long chain of asynchronous queries that could take a long time to finish.
The perfect solution escapes our grasp.
What about the ghosts?
Oh right ... your UI could still be displaying the (fewer) entities that you detached because you believe they were deleted on the server. You've got to remove these "ghosts" from the UI.
I'm sure you can figure out how to remove them. But you have to learn what they are first.
You could iterate over every entity that you are displaying and see if it is in a "Detached" state. YUCK!
Better I think if the cleanup mechanism published a (custom?) event with the list of entities you detached during cleanup ... and that list is inCache. Your subscriber(s) then know which entities have to be removed from the display ... and can respond appropriately.
Whew! I'm sure I've forgotten something. But now you understand the dimensions of the problem.
What about server notification?
That has real possibilities. If you can arrange for the server to notify the client when any entity has been deleted, that information can be shared across your UI and you can take steps to remove the deadwood.
It's a valid point but for now we don't ever remove entities from the local cache as a result of a query. But.. this is a reasonable request, so please add this to the breeze User Voice. https://breezejs.uservoice.com/forums/173093-breeze-feature-suggestions
In the meantime, you can always create a method that removes the related entities from the cache before the query executes and have the query (with expand) add them back.
I am using EF 4.4 and I would like to update many entities, but some other user can modified many of the entities that the first user is modified. So I get a concurrency exception. Other case is that the first user tries to add many new registers and other user added some of them meanwhile. So I have an exception that exists some of the registers (unique constraint).
I would like to ensure that the first user finish his operation add only the registers that does no exists yet (add all his entities except the entities that are added by the second user).
To do that, I need to update the entities in my dbContext so I see that there at least two options.
First, in the catch when I capture the update exception, I can do:
ex.Entries.Single().Reload();
The second option is:
myContext.Entry<MyTable>(instance).Reload();
I guess that the second option only refreshes the entity that I use as parameter, but if the problem is that I need to refresh many entities, how can I do that?
What really does the first option, Single().Reload?
When you do
ex.Entries.Single().Reload();
you are sure that the offending entity is refreshed. What is does is taking the one and only (Single) entity from the DbUpdateConcurrencyException.Entries that could not be saved to the database (in case of a concurrency exception this is always exactly one).
When you do
myContext.Entry(instance).Reload();
You are not sure that you refresh the right entity unless you know that only one entity had changes before SaveChanges was called. If you save an entity with child entities any one of them can cause a concurrency problem.
In EF 6.x (6.1.3), below code will let you find all the changes; the way you asked in your question!
try
{
var listOfRefreshedObj = db.ChangeTracker.Entries().Select(x => x.Entity).ToList();
var objContext = ((IObjectContextAdapter)your_db_context).ObjectContext;
objContext.Refresh(System.Data.Entity.Core.Objects.RefreshMode.ClientWins, listOfRefreshedObj);
await db.Entry(<yourentity>).ReloadAsync();
return Content(HttpStatusCode.<code>, "<outputmessage>"); ;
}
catch (Exception e)
{
return Content(HttpStatusCode.<code>, "<exception>");
}
Explaination:
Query Entries in the ChangeTracker and store them in a list
var listOfRefreshedObj = db.ChangeTracker.Entries().Select(x => x.Entity).ToList();
Next is to refresh the context. In some cases (row is removed etc.), this will throw an exception which you can catch. RefreshMode.ClientWins tells EF to accept all client units as modified when next update occurs. In some cases, you might want to prompt the users with the changes and let them decide. RefreshMode Enumeration. An example is here ObjectContext.Refresh Method Example
objContext.Refresh(System.Data.Entity.Core.Objects.RefreshMode.ClientWins, listOfRefreshedObj);
You're probably doing this whole thing after you receive DbUpdateConcurrencyException anyways!
Question in short:
Is there a way to make EF throw an exception if the code accesses a navigation property and the data being requested is not loaded in memory and lazy loading is disabled?
A bit longer:
I'm using Entity Framework 4 (model first) and have disabled lazy loading. Before, if I run something like the following, the code would run fine but be slow since every call to customer.Orders resulted in a new SQL query being executed:
int totalValue = 0;
foreach(var customer in efContext.Customers)
foreach (var order in customer.Orders)
totalValue += order.Value;
I wanted to disable lazy loading completely to prevent such code from be introduced in the code base I'm working on. What I wanted to happen is that when the code executes customer.Orders, an exception should be thrown stating something along the lines of "Entity not loaded. Lazy loading is off.". But instead, the list of orders is simply empty because the orders has not been loaded.
I feel that this behavior may also be problematic, just in a different way than lazy loading. A developer who was tasked with checking that every customer has a contact could write something like the following:
if (customer.Contact != null)
throw new BlablablaException("Customer lacks contact");
But with lazy loading disabled, customer.Contact would always be null, and the developer might think that his code works properly. Sure, he should do his job properly and test both scenarios (contact existing, contact not existing) but it would be much more clear if an exception was thrown.
In my opinion, it seems strange that Entity Framework does not throw an exception if I try to access data which is not available and instead just returns an empty list.
Is there an explanation to this behavior? And is there a way to make EF throw exception when a navigation property is called and the data is not avaialble?
You could test each entry to be sure it's loaded.
If it's a collection:
model.Entry(customer).Collection(c => c.Customers).IsLoaded
If it's a not a collection:
model.Entry(customer).Reference(c => c.Customer).IsLoaded
However, I think it would be better if you just eagerly loaded your collection prior to iterating through it, guaranteeing that the collections are loaded, rather than worrying if the collection is loaded or not:
foreach(var customer in efContext.Customers.Include(c => c.Orders))
In my app, I have a code like this:
// 1
Foo.get(123).example = "my example" // as expected, don't change value in db
// 2
Foo.get(123).bars.each { bar ->
bar.value *= -1 // it's changing "value" field in database!! WHY?
}
note: Foo and Bar are tables in my DB
Why is gorm saving in database is second case?
I don't have any save() method in code.
Tks
SOLVED:
I need to use read() to get a readonly session.
(Foo.discard() also works)
Doc: http://grails.org/doc/latest/guide/5.%20Object%20Relational%20Mapping%20%28GORM%29.html#5.1.1%20Basic%20CRUD
(In the first case, I guess I made mistest)
Both should save, so the first example appears to be a bug. Grails requests run in the context of an OpenSessionInView interceptor. This opens a Hibernate session at the beginning of each request and binds it to the thread, and flushes and closes it at the end of the request. This helps a lot with lazy loading, but can have unexpected consequences like you're seeing.
Although you're not explicitly saving, the logic in the Hibernate flush involves finding all attached instances that have been modified and pushing the updates to the database. This is a performance optimization since if each change had been pushed it would slow things down. So everything that can wait until a flush is queued up.
So the only time you need to explicitly save is for new instances, and when you want to check validation errors.
I'm learning about concurrency in conjunction with EF4.0 and have a question about the locking pattern used.
Say I configure a fixed concurrency mode on a version number property.
Now say I fetch a record (entity) from the database (context) and edit some property. Version gets incremented and when SaveChanges is called on its context. If the current database (context) version matches the version of the original record (entity) the save continues, otherwise an OptimisticConcurrencyException gets thrown by EF.
Now, my point of interest is the following: between the check of the versions there's always a small period of time, however small, it is there. So in theory someone else could've just updated the record between the comparison and the actual save, thus possibly corrupting the data.
How does this get solved? It feels as if the problem just gets pushed forward.
There is no period of time between checking versions and updating record because the database command looks like:
UPDATE SomeTable
SET SomeColumn = 'SomeValue'
WHERE Id = #Id AND Version = #OldVersion
SELECT ##ROWCOUNT
The check and update is one atomic operation. Rowcount will return 0 if no record with Id = #Id and Version = #OldVersion exists and that zero is translated to the exception.
This can (and probably is) solved using locking hints.
For SQL Server, EF can query (SELECT) from the database WITH UPDLOCK.
This tells the Database Engine that, you want to read a/several records, and nobody else can change those records until you perform an update thereafter.
If you want to see this for yourself, check out the Sql Server Profiler which will show you the queries in real-time.
Hope that helps.
CAVEAT: I can't say for sure that this is the way EF handles this scenario because I haven't checked myself but, certainly if you were going to do it yourself, this is one way to do it.