Does exist the possibility to clone an object coming from a linqToOject query?
Yes.
You can call any method in a LINQ-to-Objects query, including Clone().
To answer the question, no; that won't happen.
Related
Is this something that's been changed in MVC 3? There is no SubmitChanges() method anywhere and I can't figure out, I seem to have the proper namespace too (System.Data.Linq).
The only thing I find is SaveChanges() instead. Are they the same? Or have I run into something strange here?
They are conceptually the same, although their implementations are different.
SaveChanges is part of ObjectContext in Entity Framework.
SubmitChanges is part of DataContext in Linq to SQL.
In my project, i use several linq queries for getting prices list.
I need to calculate values based on these prices.
Is it possible to call a user method (who can, ideally, be in the entity class) directly from the linq query, for example, doing like this would be perfect
from foo in Foo
select new {
price = foo.Price,
priceclass = foo.GetClassOfPrice()
}
There would be no data access from GetClassOfPrice, just static code based on the price.
Thank's by advance !
Linq-To-Entities can call only special type of methods defined in conceptual model (EDMX). These methods are called Model defined functions. So if you define your method this way you will be able to call it. You can also check this blog post.
You can only call the method via LINQ to Objects as there is no translation to SQL for the method call. If you materialize the query -- bring it into memory -- first, then do the selection it should work.
var foos = context.Foo.ToList()
.Select( f => new
{
price = f.Price,
priceClass = f.GetClassOfPrice()
} );
Note that you should perform any conditional logic (Where) before doing the ToList so that you're only transferring the data that you actually need from the DB. I'm using extension methods because it's more natural for me and because you'd need to use the ToList or similar method anyway. I really dislike mixing LINQ syntax with the extension methods.
Unfortunately, this can't be done, because your LINQ query is translated to SQL. And your method isn't known to the so called provider that does this translation.
There are only a few functions that you can call when dealing with linq to entities and they are listed here:
http://msdn.microsoft.com/en-us/library/system.data.objects.sqlclient.sqlfunctions.aspx
http://msdn.microsoft.com/en-us/library/system.data.objects.entityfunctions.aspx
I have defined a GenericRepository class which does the db interaction.
protected GenericRepository rep = new GenericRepository();
And in my BLL classes, I can query the db like:
public List<Album> GetVisibleAlbums(int accessLevel)
{
return rep.Find<Album>(a => a.AccessLevel.BinaryAnd(accessLevel)).ToList();
}
BinaryAnd is an extension method which checks two int values bit by bit. e.g. AccessLevel=5 => AccessLevel.BinaryAnd(5) and AccessLevel.binaryAnd(1) both return true.
However I cannot use this extension method in my LINQ queries. I get a runtime error as follows:
LINQ to Entities does not recognize the method 'Boolean BinaryAnd(System.Object, System.Object)' method, and this method cannot be translated into a store expression.
Also tried changing it to a custom method but no luck. What are the workarounds?
Should I get all the albums and then iterate them through a foreach loop and pick those which match the AccessLevels?
I realize this already has an accepted answer, I just thought I'd post this in case someone wanted to try writing a LINQ expression interceptor.
So... here is what I did to make translatable custom extension methods: Code Sample
I don't believe this to be a finished solution, but it should hopefully provide a good starting point for anyone brave enough to see it through to completion.
You can only use the core extension methods and CLR methods defined for your EF provider when using Entity Framework and queries on IQueryable<T>. This is because the query is translated directly to SQL code and run on the server.
You can stream the entire collection (using .ToEnumerable()) then query this locally, or convert this to a method that is translatable directly to SQL by your provider.
That being said, basic bitwise operations are supported:
The bitwise AND, OR, NOT, and XOR operators are also mapped to canonical functions when the operand is a numeric type.
So, if you rewrite this to not use a method, and just do the bitwise operation on the value directly, it should work as needed. Try something like the following:
public List<Album> GetVisibleAlbums(int accessLevel)
{
return rep.Find<Album>(a => (a.AccessLevel & accessLevel > 0)).ToList();
}
(I'm not sure exactly how your current extension method works - the above would check to see if any of the flags come back true, which seems to match your statement...)
There are ways to change the linq query just before EF translates it to SQL, at that moment you'd have to translate your ''foreign'' method into a construct translatable by EF.
See an previous question of mine How to wrap Entity Framework to intercept the LINQ expression just before execution? and mine EFWrappableFields extension which does just this for wrapped fields.
Does LINQ to Objects queries cache by LINQ provider when it execute for the second time?
There is nothing to cache in LINQ-to-Objects, which simply uses a series of extension method calls to generate a chain (or graph) of iterators. It isn't like LINQ-to-SQL, which has to compile the graph into a SQL statement before executing it.
No it doesn't. Because linq to objects it is just extensions that roll your enumerable into another enumerable or execute it immediately. It is easier to understand how linq works by reading this article.
I understand that a IQueryable cannot be serialized. That means that queries can not be serialized, sent to a webservice, deserialized, queried and then sent back.
I was wondering if it is possible to convert a hibernate linq query to hql to be sent over the wire.
Is there another route I am missing?
I think I've seen ADO.NET Data Services as advertised to work with NHibernate:
http://wildermuth.com/2008/07/20/Silverlight_2_NHibernate_LINQ_==_Sweet
http://ayende.com/Blog/archive/2008/07/21/ADO.Net-Data-Services-with-NHibernate.aspx
This is an old post, and not sure how maintained this feature is, but its worth a shot.
I have a suggestion to you. Do not try to serialize query. Rather provide your user with an ability to compose arbitrary LINQ expression. Then send the expression over the wire (there is a gotcha here, since expressions are not serializable) to your server.
I have recently submitted to NHibernate.Linq project a change to their NHibernateExtensions class - a new ISession extension method List, which accepts an expression and fills the given list with the respective data, much like ICriteria.List method. The change was accepted (see here) so downloading the recent version of NHibernate.Linq should contain this method.
Anyway, you can find the details of this approach here.