I am using Linq.Dynamic. I have already added another SelectMany extension to all for creating a new anonymous object with the data. But, I have ran into another issue that I can not seem to solve.
I want to have extension method chaining as follows, but using the dynamic methods:
var customerandorderflat = db.Customers
.SelectMany(c => c.Orders.SelectMany(o => o.Order_Details,
(ord, orddetail) => new
{
OrderID = ord.OrderID,
UnitPrice = orddetail.UnitPrice
}).DefaultIfEmpty(),
(cus, ord) => new
{
CustomerId = cus.CustomerID,
CompanyName = cus.CompanyName,
OrderId = ord.OrderID == null ? -1 : ord.OrderID,
UnitPrice = ord.UnitPrice
});
Ideally I would like to chain the dynamic SelectMany as follows:
db.Customers.SelectMany(c => c.Orders.SelectMany("Order_Details", "new(outer.OrderID, inner.UnitPrice)"), "new(outer.CustomerID, inner.OrderID)");
Or something to that affect. The problem is that I can not get a signature to match.
I have tried many different options to get it to allow chaining. But it just doesn't work. I am thinking ideally it would look like this:
public static IQueryable SelectMany(this IQueryable source, IQueryable innerExpression, string resultsSelector, params object[] values)
But, it doesn't recognize c => c.Orders as IQueriable. I also need to figure out how to do DefaultIfEmpty on the results to allow for LEFT JOINs.
Please help.
c.Orders is an EntitySet. EntitySet doesn't implement IQueryable. Try c.Orders.AsQueryable()
It was getting the wrong definition. Actual error when on right definition: Cannot convert lambda expression to type 'System.Linq.IQueryable' because it is not a delegate type
Related
How to use string extension method in linq query:
public NewsType GetNewsType(string name)
{
var newsType = db.NewsTypes.FirstOrDefault(x => x.Name.ToFriendlyUrl() ==
name.ToFriendlyUrl());
return newsType;
}
Above query x.Name.ToFriendlyUrl() is not allowed at the minute. Is anyone know how to achieve with it.
Extension methods are indeed allowed in LINQ queries, moreover the LINQ methods themselves are implemented as extension methods.
It's quite another issue however, to use extension methods (or most other methods) in LINQ-to-SQL or LINQ-to-Entities queries. Those queries are not actually run in the C# code, but they are treated like expressions that are translated to SQL. I.e.
db.News.Where(x => x.Published).Select(x => x.Name)
is translated to the SQL Statement
Select Name
From News
Where Published = 1
and it's results are returned to the C# code.
Since there is not way to transfer the ToFriendlyUrl() method to SQL, your code throws an error.
You have basically, two solutions/workarounds. One is to transform the call to a form could be translated into SQL, e.g. if the ToFriendlyUrl() method was just:
public static string ToFriendlyURL(this string value)
{
return value.ToLower();
}
you can inline that code in the LINQ call, and that would work. If however, the methods is more complex, than your only solution is to just fetch the data from the base and then process it on the C# side:
var newsTypeQuery = db.NewsTypes.Where(x => // other conditions, if any);
var newsTypes = newsTypes.ToList(); //forces execution of the query
// the result is now a C# list
var newsType = newsTypes.FirstOrDefault(x =>
x.Name.ToFriendlyUrl() == name.ToFriendlyUrl());
Assuming the NewsTypes is an IQueryable this is a result of Entity Framework not being able to convert you extension method into SQL (how should it?). Unless you can rewrite your predicate into something that Entity Framework can translate into SQL you will have to perform the query client side:
public NewsType GetNewsType(string name)
{
var newsType = db.NewsTypes.AsEnumerable().FirstOrDefault(x => x.Name.ToFriendlyUrl() == name.ToFriendlyUrl());
return newsType;
}
Notice how AsEnumerable() has been added before FirstOrDefault. Unfortunately this may pull all the rows returned by NewsTypes from the server to client and thus may be quite costly.
This
var newsType = db.NewsTypes.FirstOrDefault(
x => x.Name.ToFriendlyUrl() == name.ToFriendlyUrl());
can't be done in Entity Framework. ToFriendlyUrl is an extension method. It's something that is in the "client" computer. The query will be executed on the SQL server. The SQL server doesn't have a ToFriendlyUrl function.
The "standard" solution is to save in a second column named FriendlyName a precalculated version of the ToFriendlyUrl(), so your query becomes:
var friendlyName = name.ToFriendlyUrl();
var newsType = db.NewsTypes.FirstOrDefault(
x => x.FriendlyName == friendlyName);
Instead try like this
public NewsType GetNewsType(string name)
{
var newsType = db.NewsTypes.FirstOrDefault(x => x.Name == name).ToFriendlyUrl();
return newsType;
}
I need to return a number of Linq query results into a List object based on a foreign key value. What is the syntax for doing this? I am new to using Linq, so below is my best guess so far. I receive an error in the .Where() "clause" stating "The name 'pt' does not exist in the current context. Any help would be greatly appreciated!
List<AgentProductTraining> productTraining = new List<AgentProductTraining>();
var prodCodes = productTraining.Select(pt => new[]
{
pt.ProductCode,
pt.NoteId,
pt.ControlId
})
.Where(pt.CourseCode == course.CourseCode);
You would need to switch the locations of where and select if you're using extension methods:
var prodCodes = productTraining.Where(pt => pt.CourseCode == course.CourseCode)
.Select(pt => new SomeRandomType
{
ProductCode = pt.ProductCode,
NoteId = pt.NoteId,
ControlId = pt.ControlId
});
I also recommend, as you can see above, that you create a type for that select statement so that you're not relying on anonymous types. You should put in into an object type that you know everything about.
Also, if CourseCode is a string, that should be pt.CourseCode.Equals(course.CourseCode).
I am working with code first approach in EDM and facing an error for which I can't the solution.Pls help me
LINQ to Entities does not recognize the method 'Boolean
CheckMeetingSettings(Int64, Int64)' method, and this method cannot be
translated into a store expression.
My code is following(this is the query which I have written
from per in obj.tempPersonConferenceDbSet
where per.Conference.Id == 2
select new PersonDetials
{
Id = per.Person.Id,
JobTitle = per.Person.JobTitle,
CanSendMeetingRequest = CheckMeetingSettings(6327,per.Person.Id)
}
public bool CheckMeetingSettings(int,int)
{
///code I have written.
}
Please help me out of this.
EF can not convert custom code to SQL. Try iterating the result set and assigning the property outside the LINQ query.
var people = (from per in obj.tempPersonConferenceDbSet
where per.Conference.Id == 2
order by /**/
select new PersonDetials
{
Id = per.Person.Id,
JobTitle = per.Person.JobTitle,
}).Skip(/*records count to skip*/)
.Take(/*records count to retrieve*/)
.ToList();
people.ForEach(p => p.CanSendMeetingRequest = CheckMeetingSettings(6327, p.Id));
With Entity Framework, you cannot mix code that runs on the database server with code that runs inside the application. The only way you could write a query like this, is if you defined a function inside SQL Server to implement the code that you've written.
More information on how to expose that function to LINQ to Entities can be found here.
Alternatively, you would have to call CheckMeetingSettings outside the initial query, as Eranga demonstrated.
Try:
var personDetails = obj.tempPersonConferenceDbSet.Where(p=>p.ConferenceId == 2).AsEnumerable().Select(p=> new PersonDetials
{
Id = per.Person.Id,
JobTitle = per.Person.JobTitle,
CanSendMeetingRequest = CheckMeetingSettings(6327,per.Person.Id)
});
public bool CheckMeetingSettings(int,int)
{
///code I have written.
}
You must use AsEnumerable() so you can preform CheckMeetingSettings.
Linq to Entities can't translate your custom code into a SQL query.
You might consider first selecting only the database columns, then add a .ToList() to force the query to resolve. After you have those results you van do another select where you add the information from your CheckMeetingSettings method.
I'm more comfortable with the fluid syntax so I've used that in the following example.
var query = obj.tempPersonConferenceDbSet
.Where(per => per.Conference.Id == 2).Select(per => new { Id = per.Person.Id, JobTitle = per.Person.JobTitle })
.ToList()
.Select(per => new PersonDetails { Id = per.Id,
JobTitle = per.JobTitle,
CanSendMeetingRequest = CheckMeetingSettings(6327, per.Person.Id) })
If your CheckMeetingSettings method also accesses the database you might want to consider not using a seperate method to prevent a SELECT N+1 scenario and try to express the logic as part of the query in terms that the database can understand.
Am testing out MvcContrib's grid for sorting.
Am using LightSpeed as my ORM
Problem: getting compile error on: listOfRfidTags = ...
The type arguments for method 'System.Linq.Enumerable.OrderBy(System.Collections.Generic.IEnumerable, System.Func, System.Collections.Generic.IComparer)' cannot be inferred from the usage. Try specifying the type arguments explicitly.
public ActionResult Index(GridSortOptions sort)
{
IEnumerable<RfidTag> listOfRfidTags = uow.RfidTags;
if(sort.Column != null) {
listOfRfidTags = listOfRfidTags.OrderBy(sort.Column, sort.Direction);
}
ViewData["sort"] = sort;
return View(listOfRfidTags);
}
view:
#Html.Grid(Model).Columns(column =>{
column.For(a => Html.ActionLink("Edit", "Edit", new { id = a.Id })).Named("Edit");
column.For(a => a.TagCode).Named("TagCode").Sortable(true);
column.For(a => a.Number);
})
You are getting this compiling error because you are trying to use an OrderBy extension method that is only defined in MvcContrib and not in System.Linq.
In order to fix it, you just need to use the following line:
using MvcContrib.Sorting;
And then you can use the OrderBy method as in your original code:
listOfRfidTags = listOfRfidTags.OrderBy(sort.Column, sort.Direction);
Although itowlson answer works, he just reimplements what the OrderBy extension method in MvcContrib already does (see SortExtensions.cs).
The OrderBy extension method takes a delegate for getting the sort key, not a column and direction. So this line:
listOfRfidTags = listOfRfidTags.OrderBy(sort.Column, sort.Direction);
needs to look something like this:
listOfRfidTags = listOfRfidTags.OrderBy(r => r.SomeProperty);
(or OrderByDescending depending on the sort.Direction). The trouble is that SomeProperty can't be determined at compile time because you want it to come from sort.Column. This means that if you want to use LINQ then you'll probably need to use Dynamic LINQ or Reflection to extract the property you want to sort on e.g.
PropertyInfo property = typeof(RfidTag).GetProperty(sort.Column);
listOfRfidTags = listOfRfidTags.OrderBy(r => property.GetValue(r));
However, since you are using LightSpeed as your ORM, you can bypass LINQ and use the core API, which does allow dynamic column names:
Order order = Order.By(sort.Column);
if (sort.Direction == SortDirection.Descending))
order = order.Descending();
IList<RfidTag> listOfRfidTags = uow.Find<RfidTag>(new Query { Order = order });
This has the side benefit that the sorting will happen on the database instead of in the Web application.
how come this work
public IQueryable<Category> getCategories(int postId)
{
subnusMVCRepository<Categories> categories = new subnusMVCRepository<Categories>();
subnusMVCRepository<Post_Category_Map> postCategoryMap = new subnusMVCRepository<Post_Category_Map>();
var query = from c in categories.GetAll()
join pcm in postCategoryMap.GetAll() on c.CategoryId equals pcm.CategoryId
where pcm.PostId == 1
select new Category
{
Name = c.Name,
CategoryId = c.CategoryId
};
return query;
}
but this does not
public IQueryable<Category> getCategories(int postId)
{
subnusMVCRepository<Categories> categories = new subnusMVCRepository<Categories>();
subnusMVCRepository<Post_Category_Map> postCategoryMap = new subnusMVCRepository<Post_Category_Map>();
var query = from c in categories.GetAll()
join pcm in postCategoryMap.GetAll() on c.CategoryId equals pcm.CategoryId
where pcm.PostId == postId
select new Category
{
Name = c.Name,
CategoryId = c.CategoryId
};
return query;
}
The issue is most likely in the implementation of the query provider.
pcm.PostId == 1
and
pcm.PostId == postId
actually have a big difference. In the expression tree the first is generated as a ConstantExpression which doesnt need to be evaulated.
With the second, the compiler actually generates an inner class here (this is the _DisplayClassX that you see). This class will have a property (will most likely be the same name as your parameter) and the expression tree will create a MemberAccessExpression which points to the auto-generated DisplayClassX. When you query provider comes accross this you need to Compile() the Lambda expression and evaluate the delegate to get the value to use in your query.
Hope this helps.
cosullivan
The problem is not the linq itself,
you need to be sure that the context or provider object is able to fetch the data.
try testing the
subnusMVCRepository<Categories> categories = new subnusMVCRepository<Categories>();
subnusMVCRepository<Post_Category_Map> postCategoryMap = new subnusMVCRepository<Post_Category_Map>();
objects and see if they are populated or if they behaving as required.
you may want to search the generated code for c__DisplayClass1 and see what you can see there. some times the generated code dose some weird things.
when you step into you code check the locals and the variable values. this may also give you some clues.
Edit : Have you tried to return a List<> collection ? or an Enumerable type?
Edit : What is the real type of the item and query may not be iterable