Why I am getting this error:
The method 'Single' is not supported by LINQ to Entities. Consider using the method 'First' instead.
public ActionResult Details(int id)
Line 27: {
var result = (from d in _db.MovieSet
Line 29: where d.Id == id
Line 30: select d).Single();
//
//
}
Code compiles safe, but only breaks if call is made to the respective section. I am new to LINQ, therefore do not know which methods are for LINQtoSQL or LINQtoEntities. This means more errors! We cannot remember all methods this way.
My question is, if there are limitations to the methods applicable to certain types / scenarios, why do they appear in Intellisense?
EDIT: Any work-around / technique helpful to have an idea if one is supported ?
Microsoft has a complete list of supported and unsupported methods in Linq to Entities. That's where to go to find out this information.
You'll notice that the Single and SingleOrDefault methods are in fact listed as "not supported" in the section on Paging Methods.
As Jared pointed out, the compiler does not know at compile time which provider you are using, so it has no way to enforce compile-time safety of extension methods that the provider may or may not implement. You'll have to rely on the documentation instead.
In the case of LINQtoSQL / Entities, the queries are all broken down into expression trees which are then passed to the provider APIs. The providers cannot provide compile time information about the trees they do or do not support because there is no syntactic difference. The only choice is for them to provide runtime data.
For example once in expression tree form, both the Single and First appear as a MethodCallExpression instance.
Unfortunately, it's yet another indication of both the relative immaturity of EF and the Object Relational Impedance Mismatch.
Documentation is your friend if you choose to go this route.
Related
Usually the distinction between LINQ to SQL and LINQ to Objects isn't much of an issue, but how can I determine which is happening?
It would be useful to know when writing the code, but I fear one can only be sure at run time sometimes.
It's not micro optimization to make the distinction between Linq-To-Sql and Linq-To-Objects. The latter requires all data to be loaded into memory before you start filtering it. Of course, that can be a major issue.
Most LINQ methods are using deferred execution, which means that it's just building the query but it's not yet executed (like Select or Where). Few others are executing the query and materialize the result into an in-memory collection (like ToLIst or ToArray). If you use AsEnumerable you are also using Linq-To-Objects and no SQL is generated for the parts after it, which means that the data must be loaded into memory (yet still using deferred execution).
So consider the following two queries. The first selects and filters in the database:
var queryLondonCustomers = from cust in db.customers
where cust.City == "London"
select cust;
whereas the second selects all and filters via Linq-To-Objects:
var queryLondonCustomers = from cust in db.customers.AsEnumerable()
where cust.City == "London"
select cust;
The latter has one advantage: you can use any .NET method since it doesn't need to be translated to SQL (e.g. !String.IsNullOrWhiteSpace(cust.City)).
If you just get something that is an IEnumerable<T>, you can't be sure if it's actually a query or already an in-memory object. Even the try-cast to IQueryable<T> will not tell you for sure what it actually is because of the AsQueryable-method. Maybe you could try-cast it to a collection type. If the cast succeeds you can be sure that it's already materialized but otherwise it doesn't tell you if it's using Linq-To-Sql or Linq-To-Objects:
bool isMaterialized = queryLondonCustomers as ICollection<Customer> != null;
Related: EF ICollection Vs List Vs IEnumerable Vs IQueryable
The first solution comes into my mind is checking the query provider.
If the query is materialized, which means the data is loaded into memory, EnumerableQuery(T) is used. Otherwise, a special query provider is used, for example, System.Data.Entity.Internal.Linq.DbQueryProvider for entityframework.
var materialized = query
.AsQueryable()
.Provider
.GetType()
.GetGenericTypeDefinition() == typeof(EnumerableQuery<>);
However the above are ideal cases because someone can implement a custom query provider behaves like EnumerableQuery.
I had the same question, for different reasons.
Judging purely on your title & initial description (which is why google search brought me here).
Pre compilation, given an instance that implements IQueryable, there's no way to know the implementation behind the interface.
At runtime, you need to check the instance's Provider property like #Danny Chen mentioned.
public enum LinqProvider
{
Linq2SQL, Linq2Objects
}
public static class LinqProviderExtensions
{
public static LinqProvider LinqProvider(this IQueryable query)
{
if (query.Provider.GetType().IsGenericType && query.Provider.GetType().GetGenericTypeDefinition() == typeof(EnumerableQuery<>))
return LinqProvider.Linq2Objects;
if (typeof(ICollection<>).MakeGenericType(query.ElementType).IsAssignableFrom(query.GetType()))
return LinqProvider.Linq2Objects;
return LinqProvider.Linq2SQL;
}
}
In our case, we are adding additional filters dynamically, but ran into issues with different handling of case-sensitivity/nullreference handling on different providers.
Hence, at runtime we had to tweak the filters that we add based on the type of provider, and ended up adding this extension method:
Using EF core in net core 6
To see if the provider is an EF provider, use the following code:
if (queryable.Provider is Microsoft.EntityFrameworkCore.Query.Internal.EntityQueryProvider)
{
// Queryable is backed by EF and is not an in-memory/client-side queryable.
}
One could get the opposite by testing the provider against System.Linq.EnumerableQuery (base type of EnumerableQuery<T> - so you don't have to test generics).
This is useful if you have methods like EF.Functions.Like(...) which can only be executed in the database - and you want to branch to something else in case of client-side execution.
This question already has an answer here:
Method cannot be translated into a store expression
(1 answer)
Closed 7 years ago.
The closest to my questions that I've found is this one and as far I can see, it's not regarding the same issue. I might be mistaken, in which case, I hope someone can explain how the similarity goes (except for the same words referring to the error, that is). With that being said, I have the following issue.
This works as supposed to.
List<Typo> things = context.Things.Where(thing => thing.Nice).ToList();
This, however, doesn't.
List<Typo> things = context.Things.Where(thing => IsNice(thing)).ToList();
...
private bool IsNice(Typo thing) { return thing.Nice; }
context is of type ModelContainer deriving DbContext. I've been told that it's a standard setup for EF and I have no reason to suspect otherwise. The error message claims the following.
{"LINQ to Entities does not recognize the method XXX method, and this method cannot be translated into a store expression."}
I have no experience with this error and, frankly, the research I've made gave me very little clarity. What can I look into to (a) make it work my way and (b) investigate it further.
You can't use your own method in a lambda expression of Linq to Entities, even if it only encapsulates your entities properties because Linq to Entities doesn't know how to translate them into a valid SQL statement.
For getting your methods working you must get the entities from the database with a valid LinQ to Entities query and fill with them a C# collection (like a List). You can use the extension methods .AsEnumerable(), .ToList(), Select(), ...
Here you have a list of .NET methods that LinQ to Entities can translate and you can use in your lambda expression, also with a brief explanation of your exception:
CLR Method to Canonical Function Mapping
And here you have a list with the LinQ methods supported by LinQ to Entities:
Supported LinQ methods
IMO, it's still the same issue as the question you mention: LINQ to Entities cannot interpret IsNice, and thus, you need 'normal' LINQ to resolve the Where method. You can force this with the AsEnumberable method:
List<Typo> things = context.Things.AsEnumerable().Where(thing => IsNice(thing)).ToList();
Or even shorter.
List<Typo> things = context.Things.Where(IsNice).ToList();
The following linq2entities code would appear to call the IEnumerable version of FirstOrDefault extension, because PersonHistories is an ICollection. At runtime however, it actually calls the IQueryable version.
var test = db.Persons.Where(d => d.PersonKey == 4)
.Select(f => f.PersonHistories.FirstOrDefault());
The problem I am having is the custom query provider I am using does not perform this automatic conversion, and I get the error "...ICollection cannot be used for parameter of type IQueryable". Thus requires explicitly calling AsQueryable to work around this, but for complex queries it gets very redundant and not feeling very DRY:
db.Persons.Where(d => d.PersonKey == 4)
.Select(f => f.PersonHistories.AsQueryable().FirstOrDefault());
I have dug around in the framework reference source trying to find the Linq 2 Entities visitors/provider stuff, but have had no luck(perhaps is not part of any of the open reference sources). How do the base providers accomplish this implicit use of AsQueryable?
I do understand that these are translated to expression trees.
I do understand that the Enumerable.FirstOrDefault is replaced with Queryable.FirstOrDefault by the provider. That is the premise of the question.
How do the base providers accomplish this implicit use of AsQueryable?
They don't. Your code doesn't really execute FirstOrDefault() at all. It builds an expression tree which represents the call, but that isn't executed directly. The query provider sees that, works out that the f.PersonHistories is actually based on an entity which is in the database, and converts the query appropriately.
As far as I understand the only thing LINQ supports, which Scala currently doesn't with its collection library, is the integration with a SQL Database.
As far as I understand LINQ can "accumulate" various operations and can give "the whole" statement to the database when queried to process it there, preventing that a simple SELECT first copies the whole table into data structures of the VM.
If I'm wrong, I would be happy to be corrected.
If not, what is necessary to support the same in Scala?
Wouldn't it possible to write a library which implements the collection interface, but doesn't have any data structures backing it but a String which gets assembled with following collection into the required Database statement?
Or am I completely wrong with my observations?
As the author of ScalaQuery, I don't have much to add to Stilgar's explanation. The part of LINQ which is missing in Scala is indeed the expression trees. That is the reason why ScalaQuery performs all its computations on Column and Table types instead of the basic types of those entities.
You declare a table as a Table object with a projection (tuple) of its columns, e.g.:
class User extends Table[(Int, String)] {
def id = column[Int]("id", O.PrimaryKey, O.AutoInc)
def name = column[String]("name")
def * = id ~ name
}
User.id and User.name are now of type Column[Int] and Column[String] respectively. All computations are performed in the Query monad (which is a more natural representation of database queries than the SQL statements that have to be created from it). Take the following query:
val q = for(u <- User if u.id < 5) yield u.name
After some implicit conversions and desugaring this translates to:
val q:Query[String] =
Query[User.type](User).filter(u => u.id < ConstColumn[Int](5)).map(u => u.name)
The filter and map methods do not have to inspect their arguments as expression trees in order to build the query, they just run them. As you can see from the types, what looks superficially like "u.id:Int < 5:Int" is actually "u.id:Column[Int] < u.id:Column[Int]". Running this expression results in a query AST like Operator.Relational("<", NamedColumn("user", "id"), ConstColumn(5)). Similarly, the "filter" and "map" methods of the Query monad do not actually perform filtering and mapping but instead build up an AST that describes these operations.
The QueryBuilder then uses this AST to construct the actual SQL statement for the database (with a DBMS-specific syntax).
An alternative approach has been taken by ScalaQL which uses a compiler plugin to work directly with expression trees, ensure that they only contain the language subset which is allowed in database queries, and construct the queries statically.
I should mention that Scala does have experimental support for expression trees. If you pass an anonymous function as an argument to a method expecting a parameter of type scala.reflect.Code[A], you get an AST.
scala> import scala.reflect.Code
import scala.reflect.Code
scala> def codeOf[A](code: Code[A]) = code
codeOf: [A](code:scala.reflect.Code[A])scala.reflect.Code[A]
scala> codeOf((x: Int) => x * x).tree
res8: scala.reflect.Tree=Function(List(LocalValue(NoSymbol,x,PrefixedType(ThisType(Class(scala)),Class(scala.Int)))),Apply(Select(Ident(LocalValue(NoSymbol,x,PrefixedType(ThisType(Class(scala)),Class(scala.Int)))),Method(scala.Int.$times,MethodType(List(LocalValue(NoSymbol,x$1,PrefixedType(ThisType(Class(scala)),Class(scala.Int)))),PrefixedType(ThisType(Class(scala)),Class(scala.Int))))),List(Ident(LocalValue(NoSymbol,x,PrefixedType(ThisType(Class(scala)),Class(scala.Int)))))))
This has been used in the bytecode generation library 'Mnemonics', which was presented by its author Johannes Rudolph at Scala Days 2010.
With LINQ the compiler checks to see if the lambda expression is compiled to IEnumerable or to IQueryable. The first works like Scala collections. The second compiles the expression to an expression tree (i.e. data structure). The power of LINQ is that the compiler itself can translate the lambdas to expression trees. You can write a library that builds expression trees with interface similar to what you have for collection but how are you goning to make the compiler build data structures (instead of JVM code) from lambdas?
That being said I am not sure what Scala provides in this respect. Maybe it is possible to build data structures out of lambdas in Scala but in any case I believe you need a similar feature in the compiler to build support for databases. Mind you that databases are not the only underlying data source that you can build providers for. There are numerous LINQ providers to stuff like Active Directory or the Ebay API for example.
Edit:
Why there cannot be just an API?
In order to make queries you do not only use the API methods (filter, Where, etc...) but you also use lambda expressions as arguments of these methods .Where(x => x > 3) (C# LINQ). The compilers translate the lambdas to bytecode. The API needs to build data structures (expression trees) so that you can translate the data structure to the underlying data source. Basically you need the compiler to do this for you.
Disclaimer 1:
Maybe (just maybe) there is some way to create proxy objects that execute the lambdas but overload the operators to produce data structures. This would result in slightly worse performance than the actual LINQ (runtime vs compile time). I am not sure if such a library is possible. Maybe the ScalaQuery library uses similar approach.
Disclaimer 2:
Maybe the Scala language actually can provide the lambdas as an inspectable objects so that you can retrieve the expression tree. This would make the lambda feature in Scala equivalent to the one in C#. Maybe the ScalaQuery library uses this hypothetical feature.
Edit 2:
I did a bit of digging. It seems like ScalaQuery uses the library approach and overloads a bunch of operators to produce the trees at runtime. I am not entirely sure about the details because I am not familiar with Scala terminology and have hard time reading the complex Scala code in the article:
http://szeiger.de/blog/2008/12/21/a-type-safe-database-query-dsl-for-scala/
Like every object which can be used in or returned from a query, a table is parameterized with the type of the values it represents. This is always a tuple of individual column types, in our case Integers and Strings (note the use of java.lang.Integer instead of Int; more about that later). In this respect SQuery (as I’ve named it for now) is closer to HaskellDB than to LINQ because Scala (like most languages) does not give you access to an expression’s AST at runtime. In LINQ you can write queries using the real types of the values and columns in your database and have the query expression’s AST translated to SQL at runtime. Without this option we have to use meta-objects like Table and Column to build our own AST from these.
Very cool library indeed. I hope in the future it gets the love it deserves and becomes real production ready tool.
You probably want something like http://scalaquery.org/. It does exactly what #Stilgar answer suggests, except it's only SQL.
check out http://squeryl.org/
Forgive me if this has been asked before; I did a search but couldn't find anything that specifically answered this question, but I'd be happy to be referred.
I'm taking a peek at both the Entity Framework and LINQ to SQL, and while I like the systems (and of course the LINQ integration) I'm a little skeptical on the data-binding aspect. I've taken query results and inspected them, and they don't appear to implement the standard .NET list-binding interfaces (IBindingList, and more importantly ITypedList), leading me to believe that binding them to a grid (or anything else) is going to use reflection to get/set my entity properties. This seems like a comparatively expensive operation, especially when all of the code is generated anyway and could implement the interfaces.
Does anyone have any insight into this? Is reflection used to get/set the value of the properties? If yes, does this have a performance impact that's noticeable, and does anyone have any idea why they went this route?
Edit
I'm actually concentrating on whether or not ITypedList is implemented somewhere along the way, as that's what has the capability to provide an explicit mechanism for defining and interacting with properties without resorting to reflection. While I didn't have a LINQ to SQL project up, I did inspect a simple Entity Framework entity query result, and it did not appear to implement ITypedList. Does anyone know if I'm just looking at something incorrectly, or if not why this is the case?
Edit 2
After accepting Marc's answer, I thought it might be helpful for others if I posted some simple code I used to seamlessly implement his solution. I put the following in the static constructor for my class:
public static ContextName()
{
foreach(Type type in System.Reflection.Assembly.GetExecutingAssembly()
.GetTypes())
{
if (type.GetCustomAttributes(typeof(System.Data.Linq.Mapping
.TableAttribute), true) != null)
{
System.ComponentModel.TypeDescriptor.AddProvider(
new Hyper.ComponentModel.HyperTypeDescriptionProvider(
System.ComponentModel.TypeDescriptor.GetProvider(
type)),
type);
}
}
}
While this is for LINQ to SQL, I'm sure an equivalent version could be written for the Entity Framework. This will scan the assembly for any types with the Table attribute and add a custom provider for each of them.
The Expression API that underpins LINQ etc is founded on reflection (MemberInfo), not the component-model (PropertyDescriptor etc) - so there is not a huge requirement for ITypedList. Instead, the contents are inferred from the T in IQueryable<T>, IEnumerable<T> and IList<T> etc.
The closest you might get is IListSource, but that will still just be a shallow wrapper around a proper typed list.
If performance of runtime binding (to PropertyDescriptor) is key, you might want to look at HyperDescriptor - which uses Reflection.Emit and TypeDescriptionProvider to wrap the component-model.
Re the "why" etc; note that in almost all cases with LINQ-to-SQL and EF the Expression (or the "create and set members" part) is going to be compiled to a delegate before it is invoked - so at runtime there is no huge reflection cost. And likewise, with LINQ-to-Objects everything is already compiled (by the C# compiler).
In my experience with LINQ to SQL, I have concluded for a few reasons that LINQ is using reflection to set and get field values in entity class instances. I don't know if I can remember all the reasons, but here's what I can tell you.
If I recall correctly, LINQ does not call any property procedures, but directly sets and reads all the private field values directly, which can only be done via reflection, as far as I know.
The names provided by the MetaData (in the attributes on the entity class properties) provide field name information in string form (if the database column and property name are different, for example). You can conclude from this that LINQ must be using reflection to look up the member to access it.
I think it does this to enforce simplicity -- you can rely on the values in the fields in the entity class directly mirroring the database column values, and there's not much overhead in retrieving the object from the database (populating a newly created entity to correspond to values retrieved from the database should not be routed through property procedures). One thing I have done to represent enumerated values retrieved from the database, for example, is to make some of the LINQ-generated properties private and wrap them in a manually coded property in a partial class that reads and writes the LINQ property.
From looking at what MSDN has to say on LINQ to SQL data binding, it does seem to use IListSource or IBindingList.
Btw, LINQ uses Expression Trees, and that is not reflection as much as meta-programming. Performance should be much better than reflection. Charlie Calvert covers this a bit in this article.