Over the weekend I realized that an application I'm working on which uses NHibernate as an ORM to a sqlite database has a concurrency issue.
I'm essentially looping through a collection in javascript and executing the following:
var item = new Item();
item.id = 1;
item.name = 2;
$.post("Item/Save", $.toJSON(item), function(data, testStatus) {
/*User can be notified that the item was saved successfully*/
}, "text");
And my server code looks like this:
public ActionResult Save()
{
string json = Request.Form[0];
var serializer = new DataContractJsonSerializer(typeof(JsonItem));
var memoryStream = new MemoryStream(Encoding.Unicode.GetBytes(json));
JsonItem item = (JsonItem)serializer.ReadObject(memoryStream);
memoryStream.Close();
SaveItem(item);
return Content("success");
}
The concurrency issue obviously occurs in the loop calling Save() for each element iterated, but I'm not sure how to accommodate for and prevent this. Any advice is appreciated.
What is the concurrency issue?
I didn't understand your problem with concurrency.
Comment: if you iterate the collection, AND in the postback you reload the window... hmmm... there is a potential problem here. The first postback will throw away any pending work, refreshing completely the page.
Suggestion: don't iterate, send the complete collection in one Ajax call.
Related
I am having trouble saving my entities after updating them. I can add new entities like this: add(student); but if I tried this:
if (ModelState.IsValid)
{
db.Entry(student).State = EntityState.Modified;
db.SaveChanges();
return RedirectToAction("someView");
}
I get this error message:
System.Data.Entity.Infrastructure.DbUpdateConcurrencyException was unhandled by user code
Message=Store update, insert, or delete statement affected an unexpected number of rows (0). Entities may have been modified or
deleted since entities were loaded. Refresh ObjectStateManager
entries.
Here’s my controller method:
[HttpPost]
public ActionResult ClassAttendance(InstructorIndexData viewModel, FormCollection frmcol)
{
var instructorData = new InstructorIndexData();
string[] AllFstMNames = frmcol["item.Student.FirstMidName"].Split(',');
string[] AllLstNames = frmcol["item.Student.LastName"].Split(',');
string[] AllAddresses = frmcol["item.Student.Address"].Split(',');
string[] AllEnrollmentDates = frmcol["item.Student.EnrollmentDate"].Split(',');
//more of the same code…
var student = new Student();
var enrollment = new Enrollment();
for ( int i = 0; i < AllFstMNames.Count(); i++)
{
student.FirstMidName = AllFstMNames[i];
student.LastName = AllLstNames[i];
student.Address = AllAddresses[i];
student.EnrollmentDate = Convert.ToDateTime(AllEnrollmentDates[i]);
if (!string.IsNullOrEmpty(frmcol["item.Grade"]))
{
enrollment.Grade = Convert.ToInt32(AllGrades[i]);
}
enrollment.StudentID = Convert.ToInt32(AllStudentIds[i]);
enrollment.attendanceCode = Convert.ToInt32(AllAttendanceCodes[i]);
enrollment.classDays = AllclassDays[i];
enrollment.CourseID = Convert.ToInt32 (AllCourseIds[i]);
//update rows
}
if (ModelState.IsValid)
{
db.Entry(student).State = EntityState.Modified;
db.SaveChanges();
return RedirectToAction("someView");
}
Can you help me with just being able to update values in the database?
While I was looking at the code here, my initial thought is that it doesn't seem quite right to have a for loop that updates the student and enrollment objects multiple times and then to have only one call to db.SaveChanges outside the loop. This is concerning because only the last iteration of the for loop will be applied when the data is saved to the database. (You have a comment to "update rows" at the end of the for loop - perhaps some code is missing or misplaced?)
Then, I started thinking about why it would be necessary to manually set the Entry(...).State property. Wouldn't the db automatically know that an object is modified and needs to be saved? That lead me to this question: Where is db defined? What technology stack is being used there?
Finally, after making an assumption that the db object might work something like the MS LINQ-to-SQL feature, I noticed that the the student object is newly instantiated before the for loop. This is fine for inserting new data, but if you are wanting to update existing data, I believe you need to first get a copy of the object from the database and then update the properties. This allows the db object to monitor the changes (again, assuming that it has this capability). (If this is not the case, then it leads me to wonder how the db will know which record in the database to update since you are not setting anything that appears to be a primary key, such as StudentId, on the student object in the loop.)
I have method with code:
using (var cc = new MyDBContext())
{
var myList = (from user in cc.Users
where user.UserGroup.Name == "smth"
orderby user.ID ascending
select user);
if (startIndex != null)
return View(myList.Skip((int)startIndex).Take(50));
else
return View(myList);
}
In view I catch exception The ObjectContext instance has been disposed and can no longer be used for operations that require a connection.
Some people says that .ToList() must solve problem, but it throws an exception with myList.ToList() too. What is my problem?
P.S. in debug mode I have exception at #item.FullName in view, but if I move mouse on FullName property I can see correct value.
Sorry for my bad english.
Take the "return View()" statements outside of the "using" block completely. That will ensure you have retrieved the complete data sets before your DbContext object is disposed. Like this:
using (var cc = new MyDBContext())
{
var myList = (linq).ToList();
}
return View(myList);
I'm pretty sure the problem is that you are returning an IEnumerable to the View, which means the items haven't actually been retrieved yet. But when you return the object to your View, the DbContext is getting disposed before the view has a chance to retrieve the rows.
The problem was in lazy loaded sub property of User entity. I add to link statement Include("PropName") and it works good.
I'm currently stuck on an issue that I cannot understand:
The following code produce a Datatable ("outDT") in order to populate a gridview later on. The datatable is build using rows of others datatables issued from various SQL quieres (those "get" function that return also Datatable).
The issue come when I call the "getAverage" method. Right after the call, the "pcDT" Datatable is nullified and cause a further "Collection was modified; enumeration operation might not execute." error on next loop.
I never modified a single row of the "pcDT" Datatable in the foreach loop, nor in the problematic "getAverage" method.
public DataTable getReportTable(int idClient)
{
Object thisLock = new Object();
DataTable outDT = new DataTable();
outDT.Columns.Add("PC Name");
DataTable pcDT = getPCNames(idClient);
*foreach (DataRow pcRow in pcDT.Rows)*
{
DataRow outRow = outDT.NewRow();
outRow["PC Name"] = pcRow["Name"];
**DataTable collectedDT = getAverage((int)pcRow["idPC"]);**
foreach (DataRow dataRow in collectedDT.Rows)
{
outDT.Columns.Add(dataRow["name"].ToString());
outRow[dataRow["name"].ToString()] = dataRow["AVG(MeasurePoint.dataValue)"];
}
outDT.Rows.Add(outRow);
}
return outDT;
}
(*)This foreach cause the famous "Collection was modified; enumeration operation might not execute." error
(**) And here his the method call that reset the "pcDT" Datatable. This function simply call a mySQL queries and retrieve a Datatable.
It's ok, I've found my mistake. It was in the class that manage basic SQLQueries; I used a Datatable instance that is linked to the class and not to the method, so when I recall a method that used this instance, I lost previous filled Datatable...
My bad :/
I have the following question:
It is easy to insert an oBject in database with a form.
Just create an object
link it to the fields in your from.
Post back to controller,
create a new datacontext and do datacontext.InsertOnSubmit(object)
.
public static void AddPage(string lang, Page page)
{
using (var db = new CardReaderDataContext())
{
page.Lang = lang;
page.URL = UrlHelper.CreateValidSeoUrl(page.Name, "-");
db.Pages.InsertOnSubmit(page);
db.SubmitChanges();
}
}
But if you want to update an object, it is a tedious job.
You do the same flow,
you get the object,
link it to your form,
post it, but THEN !!!
because it went outside your datacontext, you have to reload the object from the datacontext,
transfer all the variables and save it,
this is a little complex explained so I give an example:
To update an object that you modified in a form:
public static void Update(Page page)
{
using (var db = new CardReaderDataContext())
{
var _page = db.Pages.Where(p => p.Guid == page.Guid).Single();
_page.ModificationDate = DateTime.Now;
_page.Title = page.Title;
_page.Description = page.Description;
_page.Content = page.Content;
_page.Keywords = page.Keywords;
_page.Name = page.Name;
_page.WTLang = page.WTLang;
_page.WTSKU = page.WTSKU;
_page.WTTi = page.WTTi;
_page.WTUri = page.WTUri;
_page.URL = UrlHelper.CreateValidSeoUrl(page.Name, "-");
// _page.Order = GetMaxOrderByMenuGuid(page.MenuGuid);
db.SubmitChanges();
}
}
I don't know if it is clear, if it isn't comment me, I will edit
I think you're looking for DataContext.Attach, but you can only use that with linqtosql objects that have been serialised/deserialised.
Have a read of the answer to this question -
http://social.msdn.microsoft.com/forums/en-US/linqprojectgeneral/thread/384a1c03-3acf-43ef-9a25-b84f93025e63/
"It's also not a good idea to even
attempt to fetch the old version. By
doing that you are in effect turning
off optimistic concurrency, so unless
you intended that this is a bad
approach. What you need to do is
round trip both the original state and
the current state of the object."
I am using asp.net mvc for an application. I've taken some guidance from Rob Conery's series on the MVC storefront. I am using a very similar data access pattern to the one that he used in the storefront.
However, I have added a small difference to the pattern. Each class I have created in my model has a property called IsNew. The intention on this is to allow me to specify whether I should be inserting or updating in the database.
Here's some code:
In my controller:
OrderService orderService = new OrderService();
Order dbOrder = orderService.GetOrder(ID);
if (ModelState.IsValid)
{
dbOrder.SomeField1 = "Whatever1";
dbOrder.SomeField2 = "Whatever2";
dbOrder.DateModified = DateTime.Now;
dbOrder.IsNew = false;
orderService.SaveOrder(dbOrder);
}
And then in the SQLOrderRepository:
public void SaveOrder(Order order)
{
ORDER dbOrder = new ORDER();
dbOrder.O_ID = order.ID;
dbOrder.O_SomeField1 = order.SomeField1;
dbOrder.O_SomeField2 = order.SomeField2;
dbOrder.O_DateCreated = order.DateCreated;
dbOrder.O_DateModified = order.DateModified;
if (order.IsNew)
db.ORDERs.InsertOnSubmit(dbOrder);
db.SubmitChanges();
}
If I change the controller code so that the dbOrder.IsNew = true; then the code works, and the values are inserted correctly.
However, if I set the dbOrder.IsNew = false; then nothing happens...there are no errors - it just doesn't update the order.
I am using DebuggerWriter here: http://www.u2u.info/Blogs/Kris/Lists/Posts/Post.aspx?ID=11 to trace the SQL that is being generated, and as expected, when the IsNew value is true, the Insert SQL is generated and executed properly. However, when IsNew is set to false, there appears to be no SQL generated, so nothing is executed.
I've verified that the issue here (LINQ not updating on .SubmitChanges()) is not the problem.
Any help is appreciated.
In your SaveOrder method you are always creating a new ORDER object. You need to change this so that if order.IsNew is false, it retrieves the existing one from the DB and updates it instead.
public void SaveOrder(Order order)
{
ORDER dbOrder;
if (order.IsNew)
{
dbOrder = new ORDER();
dbOrder.O_ID = order.ID;
}
else
{
dbOrder = (from o in db.ORDERS where o.O_ID == order.ID select o).Single();
}
dbOrder.O_SomeField1 = order.SomeField1;
dbOrder.O_SomeField2 = order.SomeField2;
dbOrder.O_DateCreated = order.DateCreated;
dbOrder.O_DateModified = order.DateModified;
if (order.IsNew)
db.ORDERs.InsertOnSubmit(dbOrder);
db.SubmitChanges();
}
I think you have the problem that your entity is detached from your context.
You should try to attach your entity back to your context if you want to update. The downside of LINQtoSQL is that for the re-attachment you'll need the original state of the object when it was detached...
Another solution is to re-get your entity from the context and copy all the data from your entity in the parameter. This will do until you'll have more complex entities.
What tvanfosson said.
I would just like to add that I use logic where if Id equals default(0 or Empty if using guids), then I assume it is new. Otherwise if I have the id passed in, then I go get the existing object and update it.