ExecuteQuery() method returns an IEnumerable but is there a way to make it return IQueryable?
Well, you can call AsQueryable, but it won't do any good. The problem is that when you use ExecuteQuery, the query isn't composable because LINQ to SQL doesn't "understand" it as such.
One of the core purposes of IQueryable<T> is to allow the various aspects of a query to be composed together and then LINQ to SQL can convert them into a single SQL query. That just doesn't work when one of the bits of the query is effectively opaque.
If you want, you can export the result of your query to a list, and next convert it to IQueryable. See the next example code:
public IQueryable<Data> GetData()
string query = #"select ...";
object[] parameters = new object[...]{...};
var resultQuery = this.DataContext.ExecuteQuery<SICDB.Data>(query, parameters);
var tempList = resultQuery .ToList();
return tempList.AsQueryable();
}
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 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.
I am trying to send a Linq query as a string to a method to be used in a where clause. Since IEnumerable wouldn't work for this, I have converted my IEnumerable to IQueryable and still it throws error. The following is the code:
public static void FilterData(string Query)
{
if((List<MemberMaintenanceData>)HttpContext.Current.Session["Allmembers"] != null)
{
//Get the IEnumerable object colection from session
var data = (List<MemberMaintenanceData>) HttpContext.Current.Session["Allmembers"];
//Convert it to IQueryable
IQueryable<MemberMaintenanceData> queryData = data.AsQueryable();
//This line doesn't compile!!
queryData = queryData.Where(Query);
HttpContext.Current.Session["Allmembers"] = queryData.AsEnumerable().ToList();
}
}
I intended passing "a => a.AccountId == 1000" as Query
There is a free (and open source) library, provided by Microsoft for parsing strings into Lambda expressions that can then be used in Linq queries. It also contains versions of the standard query operators such as Where() that take a string parameter. You can find it described in Scott Guthries blog post on Dynamic Linq.
For example, you can do queries like this (adapted from a snippet from the Scott guthrie link)
// imagine these have come from a drop down box or some other user input...
string thingToSelectBy = "City";
string citySelectedByUser = "London";
int minNumberOfOrders = 10;
string whereClause = String.Format("{0} = #0 and Orders.Count >= #1", thingToSelectBy);
var query = db.Customers
.Where(whereClause, citySelectedByUser, minNumberOfOrders)
.OrderBy("CompanyName")
.Select("new(CompanyName as Name, Phone");
The Where clause in thisw code snippet shows how you create a where clause using a parameterised string and then dynamically inject values for the parameters at run time, for example, based on user input. This works for parameters of any type.
In your example, the where clause would be
whereClause = "AccountId = 1000";
So in effect you would be doing something like
var newFilteredQueryData = queryData.Where("AccountId = 1000");
That link also contains the location where you can download the source code and a comprehensive document describing the dynamic query API and expression language.
Given a class such as:
public class foo
{
public int AccountID {get;set;}
}
You should be able to do something like this:
Expression<Func<foo, bool>> filter = f => f.AccountID == 1000;
And then pass that as your query. If it is really needed as a string you can do this:
filter.ToString();
//By Using this library
using System.Linq.Dynamic.Core;
InventoryList = Repository.GetAll(); // IQueryable
string filterString = "UnitPrice > 10 And Qty>100 OR Description.Contains("Dairy")";
var filteredGenericList = InventoryList.Where(filterString);
I am trying to convert a Linq query that I have working in Linq to be able to work in dynamic linq (using System.Linq.Dynamic) because I want a user to be able to form their own queries and this query string would be added onto other query strings at runtime.
I have a query:
db.incidents.Where(a => a.incidentLocations.Single().location.street.Contains(location);
and I have tried to convert it to the following dynamic linq string:
query =
string.Concat("incidentLocations.Single().location.street.Contains(\"", location, "\")");
db.incidents.Where(query);
Where location is a string that includes search text.
I have managed to convert all my other queries to dynamic linq but this one i am struggling with the exception error:
"No applicable aggregate method 'Single' exists"
I understand that dynamic linq does not support all extension methods, could someone possibly tell me how I could get round this problem.
Get the source of Linq.Dynamic, copy paste the Where method, change the signature and the string with the function name inside the method and you're good to go. I did it to add Single First etc, I can't copy it here because I'm not on my dev machine but I'll do it later if necessary ;)
EDIT: here's the Single method if you decide to use it:
public static object Single(this IQueryable source)
{
if (source == null) throw new ArgumentNullException("source");
return source.Provider.Execute(
Expression.Call(
typeof(Queryable), "Single",
new Type[] { source.ElementType },
source.Expression));
}
Sorry to digg up a very old thread, but I thought I could add somevaluable information!
I had to do this for First()/FirstOrDefault() with Linq to Entities instead of your Linq to SQL and I can confirm #Guillaume86 's solution sure works!
Here's how I modified MicrosoftDynamic.sql:
I added this inside the static DynamicQueryable class:
public static object FirstOrDefault(this IQueryable source)
{
if (source == null) throw new ArgumentNullException("source");
return source.Provider.Execute(
Expression.Call(
typeof(Queryable), "FirstOrDefault ",
new Type[] { source.ElementType },
source.Expression));
}
I also modified interface IEnumerableSignatures as
void FirstOrDefault();
(I used FirstOrDefault because First() isn't supported when it's not the last call in linq to entities)
You can repeat this for any supported function :)
Since there is no Linq to DB2 yet (c'mon IBM!), and I want to deal with IQueryables or IEnumerables in my code, how would I convert a DataTable to an IQueryable? Or an IEnumerable?
I have an interface and a class that matches the columns in the datatable...
IQueryable<IMyData> GetAS400Data(..parameters..)
{
DataSet d = GetData();
...
//Some code to convert d to IQueryable<IMyData>
}
DataTable.Rows does not support .AsQueryable, since MSFT yanked it, so I'm not sure what to do here.
table.AsEnumerable()...
table.AsEnumerable().AsQueryable()...
However, you'd need to write your own translation (Select) to your type; and the IQueryable<T> would still be using LINQ-to-Objects; the only purpose (in this scenario) of using IQueryable<T> over IEnumerable<T> would be to use expressions for some other reason - perhaps for the dynamic LINQ library.
you can use something like this.
DataSet ds = GetData();
DataTable dt= ds.Tables[0];
var query =
from row in dt.AsEnumerable()
select new IMyData()
{
property1= row[0],
property2= row[1]
};
Take a look here it seems that a provider with entity framework for DB2 exists.