I'm writing a simple ApiController for getting product stocks, but I'm having a strange issue. I get the data from a method that returns a System.Linq.IQueryable (In a library), but I can't apply any of the Linq methods, like Count or ToList(). The import directive is present and doesn't report any problem. Also intellisense shows Data as System.Linq.IQueryable.
The code:
Imports System.Web.Http
Imports System.Linq
Public Class ProductSearchController
Inherits ApiController
Public Function Get(...) As IQueryable
Dim nvc As NameValueCollection = HttpUtility.ParseQueryString(Request.RequestUri.Query)
Dim sEcho As String = nvc("sEcho").ToString()
Dim iDisplayStart As Integer = CType(nvc("iDisplayStart"), Integer)
'Signature of Stock: public IQueryable Stock(...)
Dim Data = New GenericSearches().Stock(...)
Dim Count As Integer = Data.Count() 'Error Here!
Dim result = New DataTableResult With {
.sEcho = sEcho,
.iTotalRecords = Count,
.iTotalDisplayRecords = Count,
.aaData = Data.ToList() 'Error Here!
}
Return result
End Function
End Class
Also, I noticed that error correction and intellisense asks me to choose those methods from a weird Devexpress library something like DevXpress.Data.Helpers.Linq.
The non-generic IQueryable interface doesn't have extension methods of Count and ToList(). Only the generic IQueryable<T> does.
If you can modify the library to return a suitable IQueryable<T>, I suggest you do so. If you can't, you may need to call Cast<T>() or OfType<T>() to create an appropriate IQueryable<T> with the right extension methods.
Related
Inspired by this post dynamic-repositories-in-lightspeed I am trying to build my own like this.
I have a abstract GenericRepository like this. I have omitted most of the code for simplicity (Its just normal Add/Update/Filtering methods).
public abstract class GenericRepository<TEntity, TContext> :
DynamicObject,
IDataRepository<TEntity>
where TEntity : class, new()
where TContext : DbContext, new()
{
protected TContext context;
protected DbSet<TEntity> DbSet;
}
As you can see, my abstract GenericRepository extends from DynamicObject to support dynamic repositories.
I also have a abstract UnitOfWork implementation which generated a repository for a given entity at runtime like this. Again, base classes and other details are irrelevant for the question, but I'm happy to provide them if you require.
public abstract class UnitOfWorkBase<TContext> : IUnitOfWork
where TContext : DbContext, new()
{
public abstract IDataRepository<T> Repository<T>()
where T : class, IIdentifiableEntity, new();
// Code
}
Following class implements abstract method of the above class.
public class MyUnitOfWorkBase : UnitOfWorkBase<MyDataContext>
{
public override IDataRepository<T> Repository<T>()
{
if (Repositories == null)
Repositories = new Hashtable();
var type = typeof(T).Name;
if (!Repositories.ContainsKey(type))
{
var repositoryType = typeof(GenericRepositoryImpl<,>);
var genericType = repositoryType.MakeGenericType(typeof(T), typeof(InTeleBillContext));
var repositoryInstance = Activator.CreateInstance(genericType);
Repositories.Add(type, repositoryInstance);
}
return (IDataRepository<T>)Repositories[type];
}
}
Now, whenever I want to create a dynamic repository for basic CRUD functions, I can do it like this.
var uow = new MyUnitOfWorkBase();
var settingsRepo = uow.Repository<Settings>();
var settingsList = settingsRepo.Get().ToList();
Now, What I want to do is something like this.
dynamic settingsRepo = uow.Repository<Settings>();
var result = settingsRepo.FindSettingsByCustomerNumber(774278L);
Here, FindSettingsByCustomerNumber() is a dynamic method. I resolve this method using this code.
public class GenericRepositoryImpl<TEntity, TContext> :
GenericRepository<TEntity, TContext>
where TEntity : class, IIdentifiableEntity, new()
where TContext : DbContext, new()
{
public override bool TryInvokeMember(InvokeMemberBinder binder,
object[] args, out object result)
{
// Crude parsing for simplicity
if (binder.Name.StartsWith("Find"))
{
int byIndex = binder.Name.IndexOf("By");
if (byIndex >= 0)
{
string collectionName = binder.Name.Substring(4, byIndex - 4);
string[] attributes = binder.Name.Substring(byIndex + 2)
.Split(new[] { "And" }, StringSplitOptions.None);
var items = DbSet.ToList();
Func<TEntity, bool> predicate = entity => entity.GetType().GetProperty(attributes[0]).GetValue(entity).Equals(args[0]);
result = items.Where(predicate).ToList();
return true;
}
}
return base.TryInvokeMember(binder, args, out result);
}
}
This is the problem I am having.
using this line var items = DbSet.ToList(); works well, but if I were to query a large table with 1000's of data, then performance issues occur.
If I directly try to use the IQueryble interface and call it like this
Func predicate = entity => entity.GetType().GetProperty(attributes[0]).GetValue(entity).Equals(args[0]);
result = DbSet.Where(predicate).ToList();
It gives me an error saying there is no method GetProperty() in LINQ to Entities.
Is it possible to make it work using LINQ to Entities?
You need to know that LINQ-to-Entities needs to convert your expression (given by the predicate) into a SQL query. entity is replaced by the database column. Additionally LINQ2Entities supports various expressions (e.g. EqualExpression, etc.). However it cannot support the whole .NET Framework. Especially: what should GetType() on a database column return?
Therefore you need to use the Expresson API to generate the predicate and use only expressions supported by LINQ2Entities. For example: Use a MemberAccess expression for accessing a property (LINQ2Entities is able to map that to an SQL query).
Hint: we are doing predicate generation for Entity Framework and had to overcome some additional problems which we could solve using the library LinqKit.
If you do not know about the .NET Expression API yet, you need to gather skills in that area before you can resume your dynamic repository idea.
BTW: I don't think that it is a very good idea to have this kind of automatic calls. They are not refactoring safe (i.e. what if you rename the DB column? All your method calls run into problems, and it is not detectable by the compiler).
I would use it only to generate predicates for Where() clauses from Filter-like DTO types.
Unusual pattern - dynamic methods on a repository patterns.But that is another topic.
Dynamic invocation of the repository you have.
So now you need to understand Linq to Entities a little more.
Linq to Entities language reference what you can do with linq to Entities.
Given the expression tree has to be converted in to DB instructions,
it isnt surprising there are restrictions.
In case you are interested The EF provider specs and links to samples
So given you want to Dynamic EF, you have a few options.
I concentrate on dynamic wheres, but you can apply to other EF methods.
Check out
Dynamic Linq on codeplex
which allows things like
public virtual IQueryable<TPoco> DynamicWhere(string predicate, params object[] values) {
return Context.Set<TPoco>().Where(predicate, values);
}
This Where is an IQueryable extension that accepts strings...
Samples of using this string based predicate parser
LinqKit or even PM> Install-Package LinqKit
Linqkit takes dynamic EF to the next level,
Offers amazing features like
public IQueryable<TPoco> AsExpandable() {
return Context.Set<TPoco>().AsExpandable();
}
which allows you build AND and ORs progressively.
Expression trees
Expression Building API is the most powerful tool to support you here .
Learning the API is hard. using the tool harder.
eg Dealing with concatenation very hard. BUT if you can understand the API and how expressions work.
It is possible.
Here is a SIMPLE example. (imagine something complex)
public static Expression<Func<TPoco, bool>> GetContainsPredicate<TPoco>(string propertyName,
string containsValue)
{
// (tpoco t) => t.propertyName.Contains(value ) is built
var parameterExp = Expression.Parameter(typeof(TPoco), #"t");
var propertyExp = Expression.Property(parameterExp, propertyName);
MethodInfo method = typeof(string).GetMethod(#"Contains", new[] { typeof(string) });
var someValue = Expression.Constant(containsValue, typeof(string));
var containsMethodExp = Expression.Call(propertyExp, method, someValue);
return Expression.Lambda<Func<TPoco, bool>>(containsMethodExp, parameterExp);
}
I'm new to NHibernate and not great at Linq, but this is seriously kicking my butt, and I can't find any really clear examples on SO.
I need to get Thread information from the database, but I need to include a subquery that does a count on the number of Posts on a particular thread.
Here is a SQL Statement
Select ID, ThreadName,
(Select Count(Posts.ID) From Posts Where ThreadID = Threads.ID) as Replies
From Threads
And Class Structure:
Class Thread
Property ID as Integer
Property ThreadName as String
Property Replies as Integer
End Class
Class Post
Property ID as Integer
Property ThreadID as Integer
Property PostText as String
End Class
Any help would be much appreciated. And bonus points to supply both a LINQ example and a one using the NHibernate syntax.
So the query would be like this:
C#
var query =
from thrs in session.Query<YourNamespace.Thread>() // in C# Thread would need
select new YourNamespace.Thread // precise NS to distinguish System.Threading
{
ID = thrs.ID,
ThreadName = thrs.ThreadName,
Replies = thrs.Posts.Count()
};
var list = query.ToList(); // the above statement was executed
VB:
Dim query = From t As Thread In session.Query(Of Thread)()
Select New Thread With {
.ID = t.ID,
.ThreadName= t.ThreadName,
.Replies = t.Posts.Count
}
Dim list as List(of Thread) = query.ToList()
The very important think here is, that the Thread must have a mapping to the collection of Posts
C#
public class Thread
{
...
// really sorry for C# ... I will learn VB syntax ...
public virtual IList<Post> Posts { get; set; }
VB
Public Class Thread
Public Overridable Property Posts As IList(Of Post)
If this collection of Posts, would be mapped in NHibernate, then the above LINQ syntax will work out of the box
I've wired up the MvcMiniProfiler to my app, and it's reporting Duplicate Queries.
I've set a BreakPoint in my Repository
Public Function Read() As System.Linq.IQueryable(Of [Event]) Implements IEventRepository.Read
Dim events = (From e In dc.Events
Select e)
Return events.AsQueryable ''# BREAKPOINT HERE
End Function
And I've hit the page in question.
My code hits the Read() function twice through my service layer (this is by design since I can't figure out how to reduce the calls)
Dim eventcount = EventService.GetHotEventCount() ''# First Hit
Dim eventlist = EventService.GetHotEvents((page - 1) * 5) ''# Second Hit
Dim model As EventsIndexViewModel = New EventsIndexViewModel(eventlist, page, eventcount)
Return View("Index", model)
The EventService does a simple query against the IQueryable Read
Public Function GetHotEvents(ByVal skip As Integer) As List(Of Domain.Event) Implements IEventService.GetHotEvents
Return _EventRepository.Read() _
.Where(Function(e) e.EventDate >= Date.Today AndAlso
e.Region.Name = RegionName) _
.OrderByDescending(Function(e) (((e.TotalVotes) * 2) + e.Comments.Count)) _
.ThenBy(Function(e) e.EventDate) _
.Skip(skip) _
.Take(5) _
.ToList()
End Function
Unfortunately I can't figure out why MiniProfiler is saying there are 8 Duplicate queries (13 in total).
Revised
So it appears as though Sam has stated that I'm not pre-loading my relationships within my queries.
How do I appropriately pre-load relationships in Linq to SQL? Can anyone lend any advice?
Edit
Here's the ViewModel that's being created.
Public Class EventsIndexViewModel
Public Property Events As List(Of Domain.ViewModels.EventPreviewViewModel)
Public Property PageNumber As Integer
Public Property TotalEvents As Integer
Public Property MapEventsList As List(Of Domain.Pocos.MapPin)
Public Property JsonMapEventsList As String
Sub New()
End Sub
Sub New(ByVal eventlist As List(Of Domain.Event), ByVal page As Integer, ByVal eventcount As Integer)
_PageNumber = page
__TotalEvents = eventcount
Dim mel As New List(Of MapPin)
_Events = New List(Of Domain.ViewModels.EventPreviewViewModel)
For Each e In eventlist
_Events.Add(New Domain.ViewModels.EventPreviewViewModel(e))
mel.Add(New MapPin(e.Location.Latitude, e.Location.Longitude, e.Title, e.Location.Name, e.Location.Address))
Next
_MapEventsList = mel
_JsonMapEventsList = (New JavaScriptSerializer()).Serialize(mel)
End Sub
End Class
Edit - added screenshot
You basically have two options to avoid SELECT n+1 with LINQ to SQL:
1) Use DataLoadOptions - http://msdn.microsoft.com/en-us/library/system.data.linq.dataloadoptions.loadwith.aspx
DataLoadOptions enables you to specify per entity exactly that related tables should be eager-loaded. In your case, for the entity Event, you could specify LoadWith for both Comments and Locations. Whenever you load Events, Comments and Locations will then be preloaded.
The DataLoadOptions is a property you can set on the DataContext itself.
2) Use projection to fetch all the data you need in one specific query, instead of relying on lazy loading the related entities.
You have imposed a repository on top of your DataContext, so this might not be the approach you want to take, but:
Instead of selecting a list of Events and then using this entity's properties Comments and Locations, you could have your query return exactly what you need in a specific ViewModel class. LINQ to SQL would then fetch everything in a single SQL query.
I consider this the best approach if you don't absolutely NEED to abstract away the DataContext behind a repository interface. Even if you do, you could consider having the repository return View specific results, i.e.
dc.Events
.Where(something)
.Skip(something)
.Select(event => new EventViewModel
{
Event = event
Locations = event.Locations,
Comments = event.Comments
}
);
with EventViewModel being
public class EventViewModel
{
Event Event;
List<Location> Locations;
List<Comment> Comments;
}
you're going to want to .Include("Locations") and .Include("Comments") in the respective queries. I believe it goes before the .Where(), but I'm not positive about that.
I know there is a way to use Expressions and Lambdas to accomplish this but I having a hard time piecing it all together. All I need is a method that will dynamically query an Entity Framework DBSet object to find the row where the propery with the given name matches the value.
My context:
public class MyContext : DbContext
{
public IDbSet<Account> Accoounts{ get { return Set<Account>(); } }
}
The method that I'm looking to write:
public T Get<T>(string property, object value) : where T is Account
{...}
I would rather not have to use Dynamic SQL to accomplish this so no need to suggest it because I already know it's possible. What I'm really looking for is some help to accomplish this using Expressions and Lambdas
Thanks in advance, I know it's brief but it should be pretty self-explanatory. Comment if more info is needed
I'm trying to avoid dynamic linq as much as possible because the main point of linq is strongly typed access. Using dynamic linq is a solution but it is exactly the oppose of the linq purpose and it is quite close to using ESQL and building the query from sting concatenation. Anyway dynamic linq is sometimes real time saver (especially when it comes to complex dynamic ordering) and I successfully use it in a large project with Linq-to-Sql.
What I usually do is defining some SearchCriteria class like:
public class SearchCriteria
{
public string Property1 { get; set; }
public int? Property2 { get; set; }
}
And helper query extension method like:
public static IQueryable<SomeClass> Filter(this IQueryable<SomeClass> query, SearchCriteria filter)
{
if (filter.Property1 != null) query = query.Where(s => s.Property1 == filter.Property1);
if (filter.Property2 != null) query = query.Where(s => s.Property2 == filter.Property2);
return query;
}
It is not generic solution. Again generic solution is for some strongly typed processing of classes sharing some behavior.
The more complex solution would be using predicate builder and build expression tree yourselves but again building expression tree is only more complex way to build ESQL query by concatenating strings.
Here's my implementation:
public T Get<T>(string property, object value) : where T is Account
{
//p
var p = Expression.Parameter(typeof(T));
//p.Property
var propertyExpression = Expression.Property(p, property);
//p.Property == value
var equalsExpression = Expression.Equal(propertyExpression, Expression.Constant(value));
//p => p.Property == value
var lambda = Expression.Lambda<Func<T,bool>>(equalsExpression, p);
return context.Set<T>().SingleOrDefault(lambda);
}
It uses EF 5's Set<T>() method. If you are using a lower version, you'll need to implement a way of getting the DbSet based on the <T> type.
Hope it helps.
Dynamic Linq may be an option. Specify your criteria as a string and it will get built as an expression and ran against your data;
An example from something I have done;
var context = new DataContext(ConfigurationManager.ConnectionStrings["c"].ConnectionString);
var statusConditions = "Status = 1";
var results = (IQueryable)context.Contacts.Where(statusConditions);
http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx
I am using Entity Framework 4.0 and trying to use the "Contains" function of one the object sets in my context object. to do so i coded a Comparer class:
public class RatingInfoComparer : IEqualityComparer<RatingInfo>
{
public bool Equals(RatingInfo x, RatingInfo y)
{
var a = new {x.PlugInID,x.RatingInfoUserIP};
var b = new {y.PlugInID,y.RatingInfoUserIP};
if(a.PlugInID == b.PlugInID && a.RatingInfoUserIP.Equals(b.RatingInfoUserIP))
return true;
else
return false;
}
public int GetHashCode(RatingInfo obj)
{
var a = new { obj.PlugInID, obj.RatingInfoUserIP };
if (Object.ReferenceEquals(obj, null))
return 0;
return a.GetHashCode();
}
}
when i try to use the comparer with this code:
public void SaveRatingInfo2(int plugInId, string userInfo)
{
RatingInfo ri = new RatingInfo()
{
PlugInID = plugInId,
RatingInfoUser = userInfo,
RatingInfoUserIP = "192.168.1.100"
};
//This is where i get the execption
if (!context.RatingInfoes.Contains<RatingInfo>(ri, new RatingInfoComparer()))
{
//my Entity Framework context object
context.RatingInfoes.AddObject(ri);
context.SaveChanges();
}
}
i get an execption:
"LINQ to Entities does not recognize the method 'Boolean Contains[RatingInfo](System.Linq.IQueryable1[OlafCMSLibrary.Models.RatingInfo], OlafCMSLibrary.Models.RatingInfo,
System.Collections.Generic.IEqualityComparer1[OlafCMSLibrary.Models.RatingInfo])' method, and his method cannot be translated into a store expression."
Since i am not proficient with linQ and Entity Framework i might be making a mistake with my use of the "var" either in the "GetHashCode" function or in general.
If my mistake is clear to you do tell me :) it does not stop my project! but it is essential for me to understand why a simple comparer doesnt work.
Thanks
Aaron
LINQ to Entities works by converting an expression tree into queries against an object model through the IQueryable interface. This means than you can only put things into the expression tree which LINQ to Entities understands.
It doesn't understand the Contains method you are using, so it throws the exception you see. Here is a list of methods which it understands.
Under the Set Methods section header, it lists Contains using an item as supported, but it lists Contains with an IEqualityComparer as not supported. This is presumably because it would have to be able to work out how to convert your IEqualityComparer into a query against the object model, which would be difficult. You might be able to do what you want using multiple Where clauses, see which ones are supported further up the document.