I have the following extension method to calculate Product's average rating.
public static class Extensions
{
public static decimal CalculateAverageRating(this ICollection<ProductReview> productReviews)
{
// Calculate and return average rating
return averageRatings;
}
}
I want to use this method in an EF queryable where like this:
var products = _context.Products
.Include(pr => pr.ProductReviews)
.AsQueryable();
if(searchParams.Rating != 0)
products = products.Where(p => p.ProductReviews.CalculateAverageRating() == searchParams.Rating);
However im keep hitting the error "ArgumentException: Expression of type 'System.Collections.Generic.IAsyncEnumerable1[Products.Reviews.ProductReview]' cannot be used for parameter of type 'System.Collections.Generic.IEnumerable1[System.Object]' of method ".
Can we use extension method in a EF Where? Please advice me on this.
no problem with extension method in EF. The error is about wrong data type. So please try this way
public static class Extensions
{
public static decimal CalculateAverageRating(this IAsyncEnumerable<ProductReview> productReviews)
{
// Calculate and return average rating
return averageRatings;
}
}
var products = _context.Products
.Include(pr => pr.ProductReviews)
.AsQueryable().ToAsyncEnumerable();
if(searchParams.Rating != 0)
products = products.Where(p => p.ProductReviews.CalculateAverageRating() == searchParams.Rating);
PS : I have not checked above code
First of all sorry for coming late to the party...
But like Gert Arnold said, this will all be done in memory.
So EF will fetch the entire products table in memory with the product reviews, this can be catastrophical for performance.
What you might want to do is (2 is the better way):
If you really want to keep using your extension method, project the needed proprties from the product and product reviews into a vm and then do the filtering.
Just use the average method that sql offers you
I am fairly new to C# and .Net, so apologies if something doesn't make sense, I will try my best to explain my problem.
I have two methods which are basically going to use the similar query but with slight differences. So instead of repeating the query in both methods I created a third private method which will return the common part of the query and then the functions can add more clauses in query as they require.
Here is a generic function which returns the IQueryable object with common part of the query
private IQueryable<OfferViewModel> GetOffersQueryForSeller(int sellerId)
{
return Db.Offers
.Where(o => o.Sku.SellerId == sellerId && o.IsActive && !o.IsDiscontinued)
.Select(o => new OfferViewModel
{
Id = o.Id,
Name = o.Sku.Name,
ImageUrl = o.Sku.ImageUrl ?? o.Sku.Upcq.Upc.ImageUrl,
QuantityName = o.Sku.QuantityName
});
}
Following are the two method which are reusing the IQueryable object
public async Task<List<OfferViewModel>> GetSellerOffers(int sellerId)
{
var query = GetOffersQueryForSeller(sellerId);
return await query.ToListAsync();
}
public async Task<List<OfferViewModel>> GetDowngradableSellerOffers(int sellerId)
{
var query = GetOffersQueryForSeller(sellerId);
return await query
.Where(o => o.Sku.Id == monthlySkuId)
.ToListAsync();
}
Now GetSellerOffers works just fine but GetDowngradableSellerOffers throws a run time error with message The specified type member 'Sku' is not supported in LINQ to Entities.. I asked around and one of the guys told me that I cannot add additional where after adding a select which uses a ViewModel because then my records will be mapped to ViewModel and LINQ will attempt to look up props of ViewModel instead of database columns.
Now I have two questions,
In the docs I read Entity Framework will only run query when I try to fetch the results with methods like ToList and if I haven't done that why it wouldn't allow me to apply conditions on database fields/
How can I reuse the common query in my scenario?
How about the following code:
(The type Offer should be replaced by the type of the Elements that Db.Offers holds)
private IQueryable<OfferViewModel> GetOffersQueryForSeller(int sellerId, Func<Offer,bool> whereExtension)
{
return Db.Offers
.Where(o => ... && whereExtension.Invoke(o))
.Select(o => new OfferViewModel { ... });
}
private IQueryable<OfferViewModel> GetOffersQueryForSeller(int sellerId)
{
return GetOffersQueryForSeller(sellerId, (o) => true);
}
And then call it in GetDowngradableSellerOffers like this:
public async Task<List<OfferViewModel>> GetDowngradableSellerOffers(int sellerId)
{
var query = GetOffersQueryForSeller(sellerId, (o) => o.Sku.Id == monthlySkuId);
return await query.ToListAsync();
}
This Linq statement works perfectly except that I realized that I am required to use the repository and unit of work ...
So I have this Linq query
var query = (from rg in ReportGroups
join rd in ReportDefinitions on rg.ReportGroupID equals rd.ReportGroupID
where rd.ReportGroupID == 5
select rg).Count();
What I am seeing is that
ReportGroups uses GetReportGroups ( for GetAll() )
ReportDefinitions uses GetReportDefinitions (for GetAll() )
So an example is
object responseObject = null;
responseObject = _reportService.GetReportGroups("en-us").ToList();
responseObject = _reportService.GetReportDefinitions("en-us").Where(e => e.ReportGroupID == Convert.ToInt32(id)).ToList();
So you can see that is how I'm currently having to retrieve data.
I am wanting to do a Join, but i'm not sure how I can do this.
I was thinking about multiple calls, many a for loop...
Also I noticed how an existing service is called in a different method with lambda statement
private IEnumerable<SelectListItem> GetActivityList()
{
return _activityService
.GetAllActivities()
.OrderBy(n => n.ActivityName)
.Select(a => new SelectListItem
{
Text = a.ActivityName,
Value = a.Id.ToString(CultureInfo.InvariantCulture)
});
}
I realize that people cannot see all the underlying data abstractions, and I can certainly provide anything asked, I'm just a bit stumped as I used Linqpad to connect to the database and wrote my query, but now I realize that the join etc.. is maybe not so easy with the service layers.
return DbSet.Include(x => x.ReportGroups )
.Include(x => x.ReportDefinitions ).Where(//id condition);
I'm adding a "search" functionality to a web app I'm working on and I have the following action method:
public PartialViewResult SearchEmployees(string search_employees)
{
var employeeList = _db.Employees.ToList();
var resultList = employeeList.Where(t => t.FirstName.Contains(search_employees)).ToList();
return PartialView(resultList)
}
here I'm trying to filter out all employees that have a first name that contains the search string, however I keep getting a null list. Am I using the lambda expression wrong?
another question, is .Contains case sensitive? (I know in java theres .equals and .equalsIgnoreCase, is there something similar to this for .Contains?)
The problem here was the .ToList() in the first line.
.NET's string.Contains() method is, by default, case sensitive. However, if you use .Contains() in a LINQ-to-Entities query, Contains() will follow the case sensitivity of the database (most databases are case insensitive).
When you called .ToList() in the first line, it pulled down all of your data from the database, so the second line was doing an ordinary .NET .Contains(). Not only did it give you unexpected results, it's also terrible for performance, so please make a point to use a query before you use .ToList() (if you even use .ToList() at all).
public PartialViewResult SearchEmployees(string search_employees)
{
var employeeList = _db.Employees;
var resultList = employeeList.Where(t => t.FirstName.Contains(search_employees))
.ToList();
return PartialView(resultList)
}
Can you try the following code?
public PartialViewResult SearchEmployees(string search_employees)
{
var employeeList = _db.Employees.ToList();
var resultList = employeeList;
if(!String.IsNullOrEmpty(search_employees))
resultList = employeeList.Where(t => t.FirstName.Contains(search_employees)).ToList();
return PartialView(resultList)
}
Thanks,
Amit
I'm migrating some stuff from one mysql server to a sql server but i can't figure out how to make this code work:
using (var context = new Context())
{
...
foreach (var item in collection)
{
IQueryable<entity> pages = from p in context.pages
where p.Serial == item.Key.ToString()
select p;
foreach (var page in pages)
{
DataManager.AddPageToDocument(page, item.Value);
}
}
Console.WriteLine("Done!");
Console.Read();
}
When it enters into the second foreach (var page in pages) it throws an exception saying:
LINQ to Entities does not recognize the method 'System.String
ToString()' method, and this method cannot be translated into a store
expression.
Anyone know why this happens?
Just save the string to a temp variable and then use that in your expression:
var strItem = item.Key.ToString();
IQueryable<entity> pages = from p in context.pages
where p.Serial == strItem
select p;
The problem arises because ToString() isn't really executed, it is turned into a MethodGroup and then parsed and translated to SQL. Since there is no ToString() equivalent, the expression fails.
Note:
Make sure you also check out Alex's answer regarding the SqlFunctions helper class that was added later. In many cases it can eliminate the need for the temporary variable.
As others have answered, this breaks because .ToString fails to translate to relevant SQL on the way into the database.
However, Microsoft provides the SqlFunctions class that is a collection of methods that can be used in situations like this.
For this case, what you are looking for here is SqlFunctions.StringConvert:
from p in context.pages
where p.Serial == SqlFunctions.StringConvert((double)item.Key.Id)
select p;
Good when the solution with temporary variables is not desirable for whatever reasons.
Similar to SqlFunctions you also have the EntityFunctions (with EF6 obsoleted by DbFunctions) that provides a different set of functions that also are data source agnostic (not limited to e.g. SQL).
The problem is that you are calling ToString in a LINQ to Entities query. That means the parser is trying to convert the ToString call into its equivalent SQL (which isn't possible...hence the exception).
All you have to do is move the ToString call to a separate line:
var keyString = item.Key.ToString();
var pages = from p in context.entities
where p.Serial == keyString
select p;
Cast table to Enumerable, then you call LINQ methods with using ToString() method inside:
var example = contex.table_name.AsEnumerable()
.Select(x => new {Date = x.date.ToString("M/d/yyyy")...)
But be careful, when you calling AsEnumerable or ToList methods because you will request all data from all entity before this method. In my case above I read all table_name rows by one request.
Had a similar problem.
Solved it by calling ToList() on the entity collection and querying the list.
If the collection is small this is an option.
IQueryable<entity> pages = context.pages.ToList().Where(p=>p.serial == item.Key.ToString())
Hope this helps.
Upgrading to Entity Framework Version 6.2.0 worked for me.
I was previously on Version 6.0.0.
Hope this helps,
Change it like this and it should work:
var key = item.Key.ToString();
IQueryable<entity> pages = from p in context.pages
where p.Serial == key
select p;
The reason why the exception is not thrown in the line the LINQ query is declared but in the line of the foreach is the deferred execution feature, i.e. the LINQ query is not executed until you try to access the result. And this happens in the foreach and not earlier.
If you really want to type ToString inside your query, you could write an expression tree visitor that rewrites the call to ToString with a call to the appropriate StringConvert function:
using System.Linq;
using System.Data.Entity.SqlServer;
using System.Linq.Expressions;
using static System.Linq.Expressions.Expression;
using System;
namespace ToStringRewriting {
class ToStringRewriter : ExpressionVisitor {
static MethodInfo stringConvertMethodInfo = typeof(SqlFunctions).GetMethods()
.Single(x => x.Name == "StringConvert" && x.GetParameters()[0].ParameterType == typeof(decimal?));
protected override Expression VisitMethodCall(MethodCallExpression node) {
var method = node.Method;
if (method.Name=="ToString") {
if (node.Object.GetType() == typeof(string)) { return node.Object; }
node = Call(stringConvertMethodInfo, Convert(node.Object, typeof(decimal?));
}
return base.VisitMethodCall(node);
}
}
class Person {
string Name { get; set; }
long SocialSecurityNumber { get; set; }
}
class Program {
void Main() {
Expression<Func<Person, Boolean>> expr = x => x.ToString().Length > 1;
var rewriter = new ToStringRewriter();
var finalExpression = rewriter.Visit(expr);
var dcx = new MyDataContext();
var query = dcx.Persons.Where(finalExpression);
}
}
}
In MVC, assume you are searching record(s) based on your requirement or information.
It is working properly.
[HttpPost]
[ActionName("Index")]
public ActionResult SearchRecord(FormCollection formcollection)
{
EmployeeContext employeeContext = new EmployeeContext();
string searchby=formcollection["SearchBy"];
string value=formcollection["Value"];
if (formcollection["SearchBy"] == "Gender")
{
List<MvcApplication1.Models.Employee> emplist = employeeContext.Employees.Where(x => x.Gender == value).ToList();
return View("Index", emplist);
}
else
{
List<MvcApplication1.Models.Employee> emplist = employeeContext.Employees.Where(x => x.Name == value).ToList();
return View("Index", emplist);
}
}
I got the same error in this case:
var result = Db.SystemLog
.Where(log =>
eventTypeValues.Contains(log.EventType)
&& (
search.Contains(log.Id.ToString())
|| log.Message.Contains(search)
|| log.PayLoad.Contains(search)
|| log.Timestamp.ToString(CultureInfo.CurrentUICulture).Contains(search)
)
)
.OrderByDescending(log => log.Id)
.Select(r => r);
After spending way too much time debugging, I figured out that error appeared in the logic expression.
The first line search.Contains(log.Id.ToString()) does work fine, but the last line that deals with a DateTime object made it fail miserably:
|| log.Timestamp.ToString(CultureInfo.CurrentUICulture).Contains(search)
Remove the problematic line and problem solved.
I do not fully understand why, but it seems as ToString() is a LINQ expression for strings, but not for Entities. LINQ for Entities deals with database queries like SQL, and SQL has no notion of ToString(). As such, we can not throw ToString() into a .Where() clause.
But how then does the first line work? Instead of ToString(), SQL have CAST and CONVERT, so my best guess so far is that linq for entities uses that in some simple cases. DateTime objects are not always found to be so simple...
My problem was that I had a 'text' data type for this column (due to a migration from sqlite).
Solution: just change the data type to 'nvarchar()' and regenerate the table.
Then Linq accepts the string comparison.
I am working on retiring Telerik Open Access and replacing it with Entity Framework 4.0. I came across same issue that telerik:GridBoundColumn filtering stopped working.
I find out that its not working only on System.String DataTypes. So I found this thread and solved it by just using .List() at the end of my Linq query as follows:
var x = (from y in db.Tables
orderby y.ColumnId descending
select new
{
y.FileName,
y.FileSource,
y.FileType,
FileDepartment = "Claims"
}).ToList();
Just turn the LINQ to Entity query into a LINQ to Objects query (e.g. call ToArray) anytime you need to use a method call in your LINQ query.