I'm attempting to use Odata and web-api with mongoDb. The web-api library applies linq queries across an IQueryable in order to query it. One of the queries I'm required to support involves a cast. web-api returns an IQueryable with the expression
value.where(it => ((it.property as Child).Value == "example"));
This results in
Unable to determine the serialization information for the expression:
<TypeAs>it.Property.Value
is there a way to modify the expression once I get the IQueryable so that it performs an
is check and then a cast, as this is supported by the C# driver. Alternatively is there a library which provides this support already?
Related
I use in my project a lot of LINQ queries and business methods.
To allow these business method to be used from an Iqueryable :
I defined UDF functions in SQL Server (with the needed parameters)
Add this UDF to the EDMX model of the application
And make a gateway between UDF and LinQ with a method like this in a
partial class who inherits from the dbcontext :
[EdmFunction("MyProject.Store", "GetTaxesOfProduct")]
public static Decimal GetTaxesOfProduct(Decimal amount, Int32 TaxMethod)
{
throw new NotSupportedException("Not direct access possible, use with E-SQL or LINQ");
}
This works perfectly for IQueryable.
But the problem is that, to use this method from a simple object (not linked to a database record), i need to make something creepy like this :
var query = from foo in context.JustATable select context.GetTaxesOfProduct(15.55, 3);
And recently i came across this http://blogs.msdn.com/b/charlie/archive/2008/01/31/expression-tree-basics.aspx who explain how, with expression, you can make a method who is usable from C# objects and IQueryable
So, with expression, is it possible to make business methods like my method but without the use of UDF and just expressions ?
Thank you by advance !
It depends on the content of your UDF. Expression can work only with entities defined in your model and use only operations provided by Entity Framework provider for your database. So if you use any complex SQL statement with not supported equivalent for LINQ or non mapped features inside your UDF it will not work.
Within the context of C# on .Net 4.0, are there any built-in objects that implement IQueryable<T>?
IQueryable objects are produced by Queryable Providers (ex. LINQ to SQL, LINQ to Entities/Entity Framework, etc). Virtually nothing you can instantiate with new in the basic .NET Framework implements IQueryable.
IQueryable is an interface designed to be used to create Queryable providers, which allow the LINQ library to be leveraged against an external data store by building a parse-able expression tree. By nature, Queryables require a context - information regarding what exactly you're querying. Using new to create any IQueryable type, regardless of whether it's possible, doesn't get you very far.
That being said, any IEnumerable can be converted into an IQueryable by using the AsQueryable() extension method. This creates a superficially-similar, but functionally very different construct behind the scenes as when using LINQ methods against a plain IEnumerable object. This is probably the most plentiful source of queryables you have access to without setting up an actual IQueryable provider. This changeover is very useful for unit-testing LINQ-based algorithms as you don't need the actual data store, just a list of in-memory data that can imitate it.
Well, your question is kinda weird... but I believe that if you look at an interface in Reflector, it will give you a list of implementers in the loaded assemblies.
As a disclaimer I have not used Reflector since it went pay-for-play so I might be wrong.
EntityCollection does, as does EnumerableQuery.
Not that I think either of these is going to get you anywhere. To help, we need to know what you are really trying to solve. If you are writing a LINQ provider, you should read this: http://msdn.microsoft.com/en-us/library/bb546158.aspx.
They recommend writing your own implementation.
If you are looking for a way to instantiate an empty list of IQueryable, then you can use this:
IQueryable<MyEntity> = Enumerable.Empty<MyEntity>().AsQueryable()
I've got a REST web service that uses LinqtoNh to query entities and return them as DTO, plain classical stuff. I want the service to apply some filter/order clause on the returned entities, and I'm asking if there is some way of serialize the linqtonh expression in order to send it an the wire without creating some custom strategy. A plus would be not having a NH reference on the client.
Any suggestion?
Take a look at WCF Ria Services: It expose a WCF Rest service that supports the LINQ filtering, sorting, paging e grouping.
Server side the DomainService will create a LINQ query with "Where", "Order*", "Take", "Skip"...
Avoiding the NH reference it's a great idea, implement your own IQueryable, link it to NHibernate (maybe you want to use an IoC engine to do so) and return it in the exposed queries! You're almost done, most of the LINQ2NH call are standard LINQ extensions method, you would have to write somethign else if you need to use the eager fetching extension method (Fetch*) or other pretty NHibernate-stuff.
As far the client, WCF Ria is initially designed for SL but supports everything, they have even JS client called RIA/JS
I've been learning about IQueryable and lazy loading/deferred execution of queries.
Is it possible to expose this functionality over WCF? I'd like to expose a LINQ-to-SQL service that returns an IQueryable which I can then perform additional queries on at the client, and finally execute using a .ToList(). Is OData format applicable at all in this context?
If possible, what is the term for this technique and what are some good tutorials I can follow? Thank you.
You should check WCF Data Services which will allow you to define Linq query on the client. WCF Data Services are probably the only solution for your requirement.
IQueryable is still only interface and the functionality depends on the type implementing the interface. You can't directly expose Linq-To-Sql or Linq-To-Entities queries. There are multiple reasons like short living contexts or serialization which will execute the query so the client will get list of all objects instead of the query.
as far as i know, the datacontext is not serializable meaning that you cannot pass it around with WCF
You can use http://interlinq.codeplex.com/ which allows you to send Linq query over WCF.
WCF Data Services only can be using with webHttpBinding and not all Linq queries can be expressed. Writing queries when WCF Data Service is used is not so attractive - requires string expressions such as:
.AddQueryOption("$filter", "Id eq 100");
https://remotelinq.codeplex.com/ is another choice. But it works in AppDomain to scan the Current Assemblies and serialize them. This tech is not suitable to WinRT as no Domains for WinRT App
I've been struggling with the same question, and realized that a well formed question is a problem solved.
IQueryable basically serves to filter the query before sending it to your DB call so instead of getting 1000 records and filter only 10, you get those 10 to begin with. That filtering belongs to your Service Layer, but if you are building an API I would assume you would map it with AND/OR parameters in your URL.
http://{host}/{entity}/q?name=john&age=21.
So you end up with something like this:
Filter:Column1=Value1 > http://{host}/{entity}q?column1=value1 > SELECT *
FROM Entity
WHERE Column1=Value1
MVC > WCF > DB
You can find a very good sample [here]
Lastly, since your payload from the WCF will be most likely a JSON, you can (and should) then deserialize them in your Domain Models inside a collection. It is until this point where the paging should occur, so I would recommend some WCF caching (and since its HTTP, it's really simple). You still will be using LINQ on the WebApp side, just w/o "WHERE" LINQ clause (unless you want to dynamically create the URL expressed above?)
For a complex OR query, you mind end up with multiple WCF queries (1 per "AND") and then concatenate them all together
If it's possible to send IQuerable<> over WCF it's not a good thing security wise, since IQuerable<> could expose stuff like the connection string to the database.
Some of the previous comments seem promising though.
I would like to be able to use custom string querying within my NHibernate Linq expressions. Let's say for example (and this is just an example) I would like to be able to select entities containing a property which is an anagram of a particular string:
var myEntities = EntityRepository.AllEntities.Where(x => x.Description.IsAnagramOf('hits');
I imagine the steps involved in this process would be:
Define a SQL Server UDF to determine
whether two strings are anagrams.
Define an extension method called
IsAnagramOf() for the String
class.
(And this is the tricky one). Modify
Linq to NHibernate's
component for parsing expression
trees so that it converts calls to
the extension method into the
appropriate SQL UDF call.
My question is this. Does Linq to NHibernate contain some kind of extensibility model enabling me to 'slot in' my own custom string operations, or would I literally have to modify the existing source code to add in my shiznit to the expression tree parsing component?
The extensibility is built in NH 3.0 (final release next month).
You can see a full working example at http://fabiomaulo.blogspot.com/2010/07/nhibernate-linq-provider-extension.html