I know this has been asked before in several ways but none of the answers seem applicable to me - or correct - or current, so I'll try again.
I have a large model with several intstances of inherited entities. One example is a Timetable that contains a collection of TimetableEvents. There are several sub-types of TimetableEvent, such as an InterviewTimetableEvent, BreakTimetableEvent and an ExercisetimeTableEvent. ExerciseTimetableEvent has a relationship to an Exercise entity.
I need to use self-tracking entities as I'm using a WCF back end to serve up data to several WPF clients in a stateless fashion.
So, I need to eager load everything and I thought that self-tracking entities would automatically do this but it appears they dont.
So, to get a timetable I need to do something like this:
var tt = (from s in ESSDataContainer.Timetables
.Include("TimetableEvents")
where s.TimetableId == timetableid
select s).FirstOrDefault();
This will give me the TimetableEvents but not the Exercises that are related to the ExerciseTimetableEvents. Ive tried the following (and several other suggestions) without luck:
var tt = (from s in ESSDataContainer.Timetables
.Include("TimetableEvents")
.Include("ExerciseTimetableEvents.Exercise")
where s.TimetableId == timetableid
select s).FirstOrDefault();
Is there a solution to this?
If not I'll go back to normal context tracking and connect to the database from a local container rather than using WCF.
Cheers
It's a bit tricky, but possible:
var tt = (from s in ESSDataContainer.Timetables
where s.TimetableId == timetableid
select new
{
TimeTable = s,
Events = s.TimeTableEvents,
Exercise = s.TimeTableEvents.OfType<ExerciseTimetableEvents>()
.Select(ett => ett.Exercise)
}).Select(s => s.TimeTable)
.AsEnumerable()
.FirstOrDefault();
Clear as mud, but, hey: No magic strings! Also, it has the advantage that it actually works....
There is a Proposal for this issua at Microsoft Connect:. If you think this worthy you can vote for it.
Related
After upgrading from .Net Core 2.2 to 3.0, the apps is throwing this err message.
Cannot use multiple DbContext instances within a single query execution
What is the workaround?
IQueryable<ApplicationUser> query;
var queryJoin = from ci in _courseInstructorRepository.Table
join uc in _userCourseRepository.Table on ci.CourseId equals uc.CourseId
select new { ci, uc };
if (userId > 0)
queryJoin = queryJoin.Where(x => x.ci.UserId == userId);
if (courseId > 0)
queryJoin = queryJoin.Where(x => x.uc.CourseId == courseId);
if (classId > 0)
queryJoin = queryJoin.Where(x => x.uc.CourseClassId == classId);
query = queryJoin.Select(x => x.uc.User).Distinct();
if (!string.IsNullOrEmpty(studentFirstChar))
query = query.Where(x => x.FirstName.StartsWith(studentFirstChar));
if (schoolId > 0)
query = query.Where(x => x.SchoolId == schoolId);
query = query.OrderBy(x => x.UserName);
return new PagedList<ApplicationUser>(query, pageIndex, pageSize);
You have a couple of design flaws in your code that EF core 2 swept under the carpet.
Your repositories don't share one context instance. EF core 2 couldn't create one SQL query from your code either, but it silently switched to client-side evaluation. That is, it just executed two SQL queries and joined them in memory. This must have been highly inefficient. One of the best design decisions in EF core 3 was to abandon automatic client-side evaluation, so now you're getting this error.
You don't use navigation properties. Using an ORM like EF, using manual joins should hardly ever be necessary. The Instructor class should have a navigation property like Courses, and Course a navigation property like Instructor.
Don't use this redundant repository layer anyway. As you're already experiencing in this small piece of code, it usually makes things harder than necessary without any added value.
One of your variables was created using another instance of DBContext, so when you try to use it as part of another DBContext's query it throws. The work around is to close the first DBContext and invoke DBContext.Attach(model) on the second.
I haven't been able to find any answers online for this.
What are the advantages/disadvantages of using Multiple DB Contexts against using a single?
Is the below solution ok for setting related objects, (when saving to DB) (to make it more efficient, because I already have the ID, no need to fetch object)
I've heard its reccommened to use contexts like Using(MyContext c = MyContext) {}, at the moment I'm using the default MVC way, of having the context instantiated in the controller, is this ok?
Person P = new Person();
P.QuickSetCar(CarID);
db.People.Add(P);
db.saveChanges();
and
private void QuickSetCar(int CarID)
{
if(this.Car == null) {
Car C = new Car();
C.ID = ID;
this.Car = C;
}
}
Using multiple contexts is always a disadvantage unless you have no choice (e.g. your data is dispersed across multiple databases).
Almost, but you can simplify your new car initialization to:
private void QuickSetCar(int CarID) {
if(this.Car == null)
this.Car = new Car(){ ID = CarID };
}
That's fine. Just don't use more than one context for the duration of the web transaction (request/response) and don't keep it around for longer than that either.
Multiple contexts are normally only useful for very large models spread over multiple databases. If you are starting a project it is normally best to use a single context.
Your method for using IDs is fine.
Creating contexts within controllers is certainly workable but I would strongly suggest against it. This approach is often shown in demos and for basic scaffolding but in real world applications it is not a great idea. Large amounts of code will often be duplicated by instatiating and querying contexts in each controller. Also have a central repository will allow for much easier caching to improve performance.
I have two tables (there are more in the database but only two are involved here).
Account and AccountStatus, an account can have an AccountStatus (active,inactive etc).
I create a new Account and set a couple of properties but when I reach this code:
1. var status = db.AccountStatuses.SingleOrDefault(s => s.ID == (long)AccountStatusEnum.Active);
2. account.AccountStatus = status;
3. db.Accounts.AddObject(account);
The first line executes fine, but when I reach the second line it takes a REALLY long time, and when I step in to the code it seems that every single account is loaded from the database.
I don't see why it should even want to load all the accounts?
We use Entity Framework 4 and Poco and we have lazy loading enabled.
Any suggestions?
Cheers
/Jimmy
You have to be careful which constructs you use to fetch data, as some will pull in the whole set and filter afterword. (aside: the long time delay may be the database being created and seeded, if there isn't one already, it will occur the first time you touch it, likely with a query of some sort. Also remember that when you retrieve a whole dataset, you may in actuality only have what amounts to a compiled query that won't be evaluated until you interact with it).
Try this form instead and see if you have the same issue:
var status = db.AccountStatuses.Where(s => s.ID == (long)AccountStatusEnum.Active);
I want to include a table (Events) from another database in my LINQ to SQL class.
How should I format the Data Source of that table?
I have tried with IP.dbo.Events, IP.DatabaseName.dbo.Events and so on, but can not get it to work.
The reason for all this is to select Events that a Client have attended.
I have also tried to have the table in another LINQ to SQL class, but then it complains on the Data Context.
public static IQueryable<Event> ByClientID(this IQueryable<Event> events, int clientID)
{
events = from e in events
from c in new MyDataContext().ClientCourses
where e.EventID == c.CourseID &&
c.ClientID == clientID
select e;
return events;
}
You can only use tables that reside on the same physical SQL Server in two different instances. I did this once as someone had "cleverly" put an application's DB across two database instances.
There is a blog post on it here that may help.
Could you create a view that returns the data from the 2nd database and use this instead? (Not tried this so absolutely no idea if it'll work :)
Obviously this is no good if you need to be saving to the other database too..
When is the "unit of work" no longer a "unit"? Which scenario is better on resources? THe first creates one connection wheras the second creates 4.
using (DataContext dc=new DataContext)
{
var orders= from o in dc.orders
select ( new Product { property a= from a in ..join... select x,
property b= from b in ..join... select y,
property c= from c in ..join... select z.}
)
}
OR
using (DataContext dc=new DataContext)
{
var orders= from o in dc.orders
select ( new Product { property a = GetPropertyA(),
property b = GetPropertyB(),
property c = GetPropertyC()}
)
}
When using LINQ if you add one entity to another and they use a different data context you are going to get an error. I would suggest keeping the same data context for each unit of work you are using. You can easily identify the unit of work you are processing by using the repository pattern to write your data access classes. If you're not clear on the repository pattern, check out this entry
Generally speaking the less round trips to your database the better. However, in high scale apps we often make our databases more and more 'dumb' so we end up not doing any joins so we don't tie up CPU and memory on the SQL database server. It really depends on which resources you find more scarce. If your SQL server has plenty of resources do the join there, otherwise pulling it back separately would be better. Have you looked at the results of this in the SQL profiler? Are you sure it's 4 connections, or just 4 separate calls?