I am a bit of a newbie when it comes to Linq to SQL but I hope you can help out. I've written the following Linq to SQL statement with Extension Methods:
Cedb.ClassEvents.Where(c => c.ClassID == 1).Select(c => c).Single()
Where Cedb is the Datacontext, ClassEvents is a table (for classes and events being held at a facility) and ClassID is a unique integer key.
This query runs fine in LinqPad (without Cedb). When it returns, it says that the return type is "ClassEvent". In Intellisense in Visual studio, it tells me that the return type of this query is ClassEvent (created in my data model). However, when I try to place the results in a variable:
var classEvent = Cedc.ClassEvents.Where(c.ClassID == 1).Select(c => c).Single();
then I get an error: InvalidCastException: Specified cast is not valid. The same thing happens if I use the "ClassEvent" class in place of the var. I'm new to this but this one seems like a true slam dunk rather than a bug. Is there something about the Single method that I don't know that is leading to the error? Any help would be appreciated!
Slace - and any other interested parties. The cause of the "Invalid Cast Exception" error was a change in the underlying data model. A smallint field had been changed to bit. Thus, when the system tried to map the query results onto the "ClassEvent" data structure, the conflict between the model (which had not been updated) and the data table emerged.
Nonetheless, I do appreciate the answer!
You don't need to do both a Select and a Single, in fact, you don't even need the Where, you can get away with (see http://msdn.microsoft.com/en-us/library/bb535118.aspx):
var classEvent = Cedc.ClassEvents.Single(c => c.ClassID == 1);
I'd also recommend against using Single unless you're 100% sure that the Func<T, bool> will always return a value, as if it doesn't return a value you will have an exception thrown. Better is using SingleOrDefault and doing a null check before interaction with the object (http://msdn.microsoft.com/en-us/library/bb549274.aspx)
Related
When you query an EntitySet property on a model object in Linq-to-SQL, it returns all rows from the entityset and does any further querying client-side.
This is confirmed in a few places online and I've observed the behavior myself. The EntitySet does not implement IQueryable.
What I've had to do is convert code like:
var myChild = ... ;
// Where clause performed client-side.
var query = myChild.Parents().Where(...) ;
to:
var myChild = ... ;
// Where clause performed in DB and only minimal set of rows returned.
var query = MyDataContext.Parents().Where(p => p.Child() == myChild) ;
Does anyone know a better solution?
A secondary question: is this fixed in the Entity Framework?
An EntitySet is just a collection of entities. It implements IEnumerable, not IQueryable. The Active Record pattern specifies that entities be directly responsible for their own persistence. OR mapper entities don't have any direct knowledge of the persistence layer. OR Mappers place this responsibility, along with Unit Of Work, and Identity Map responsibilities into the Data Context. So if you need to query the data source, you gotta use the context (or a Table object). To change this would bend the patterns in use.
I had a similar problem: How can I make this SelectMany use a join. After messing with LINQPad for a good amount of time I found a decent workaround. The key is to push the EntitySet you are looking at inside a SelectMany, Select, Where, etc. Once it's inside that it becomes an Expression and then the provider can turn it into a proper query.
Using your example try this:
var query = from c in Children
where c == myChild
from p in c.Parents
where p.Age > 35
select p;
I'm not able to 100% verify this query as I don't know the rest of your model. But the first two lines of the query cause the rest of it to become an Expression that the provider turns into a join. This does work with my own example that is on the question linked to above.
I have a Linq2Sql query that looks like this:
var data = from d in dc.GAMEs
where (d.GAMEDATE + d.GAMETIME.Value.TimeOfDay) >= DateTime.Now
&& d.GAMESTAT == 'O' && d.GAMETYPE == 0 select d;
Resharper is underlining the "d.GAMETIME.Value.TimeOfDay" in blue and telling me it's a possible System.InvalidOperationException. While I get that if it were C# code, referencing Value without checking if it has a value would be such, i'm not sure if that is true of a Linq query.
The actual SQL generated looks horrendous, and makes me want to burn my eyes out, but I see nothing that looks like it could be a null reference. Can I safely ignore this?
(ignore for the moment the other issues, such as if it returns the expected results)
EDIT:
Upon further thought, I can see how the above might cause an exception in a LinqToObjects query, and possibly other kinds (XML?). So yeah, I suppose Resharper is just being safe.
When dealing with expression trees (as this LINQ to SQL query) it totally depends on the LINQ provider used (in your case LINQ to SQL). Therefore, it is (almost) impossible for Resharper to say anything useful about your query. I think it just interprets this code as normal C# delegates. I would say it is safe to ignore, but perhaps add a comment for the next developer.
Without seeing the data schema my guess is that GAMETIME is Nullable<DateTime> - i.e. maps to datetime/time field in the DB that can be null. Resharper is simply giving you a static analysis warning that you are referencing Nullable<T>.Value without checking that it has a value.
You can rewrite the query in this way:
var data = from d in dc.GAMEs
where (d.GAMEDATE + (d.GAMETIME.HasValue ? d.GAMETIME.TimeOfDay : new TimeSpan())) >= DateTime.Now
&& d.GAMESTAT == 'O' && d.GAMETYPE == 0 select d;
The above query will just use a TimeSpan of 0 when GAMETIME is NULL.
Considering that GAMEDATE is a non-nullable database field and GAMETIME is a nullable one, I recommend that you make GAMETIME non-nullable too. This way the two fields are consistent and do not need extra logic to handle NULL values.
EDIT I have just confirmed that trying to call Nullable<T>.Value does indeed throw InvalidOperationException, not NullReferenceException.
From the horse's mouth (bolding is mine):
The two fundamental members of the
Nullable structure are the HasValue
and Value properties. If the HasValue
property for a Nullable object is
true, the value of the object can be
accessed with the Value property. If
the HasValue property is false, the
value of the object is undefined and
an attempt to access the Value
property throws an
InvalidOperationException.
This code is a no-go
var errors = (from error in db.ELMAH_Error
select new
{
error.Application,
error.Host,
error.Type,
error.Source,
error.Message,
error.User,
error.StatusCode,
error.TimeUtc
}).ToList();
return View(errors);
as it results in a 'requires a model of type IEnumerable' error. The following code of course works fine, but selects all the columns, some of which I'm simply not interested in:
var errors = (from error in db.ELMAH_Error
select error).ToList();
return View(errors);
I'm brand spanking new to MVC2 + L2E, so maybe I'm just not thinking in the right mindset yet, but this seems counter-intuitive. Is there an easy way to select a limited number of columns, or is this just part of using an ORM?
This isn't an issue with L2E / L2S, but rather with MVC strongly-typed views.
I'm betting it's not just "requires a model of type IEnumerable", but "a model of type IEnumerable<Something>". If you have a strongly-typed view bound to the type of your errors table, then trying to pass it the anonymous type created by your query (via select new {...}) will cause a type mismatch.
You could either create a new type which contains only the rows that you want, and use that as your view page Model, or you could just do the full select and then limit the columns in your ASP code.
I am working on a database access layer project and decided to use Linq to SQL for it. One of the things I wanted to do was provide an API that can take Linq Expressions as arguments to retrieve data. For instance, I wrote this API now that looks like this:
public Result GetResults(System.Linq.Expressions.Expression<Func<Result, bool>> predicate)
{
Result result = db.Results.SingleOrDefault(predicate);
return result;
}
You can then use this API to query the database for a Result row that satisfies certain conditions, for example:
Result result = Provider.GetResults(r => r.ID == 11);
This works very well. I am able to get the one row I want based on my conditions.
Next step was taking this to be able to get multiple objects back from the database.
The way I got it to work was like this:
public List<Result> GetResults(System.Linq.Expressions.Expression<Func<Result, bool>> predicate)
{
List<Result> results = db.Results.Select(r => r).Where(r => r.ID == 11).ToList<Result>();
return results;
}
As you can see, I call a Select with r => r, this gives me back everything and then I use a Where to filter to what I need.
It works... but something tells me that I am doing it really ugly. I could be wrong, but doesn't this pull EVERYTHING out of the Results table then filters it? or does it put together the correct SQL statement that does filter at the database level?
Anyway... I would highly appreciate some guidance on how I can accomplish this task. How do I write an API that takes a Linq Expression as an argument and returns a set of objects from the database based on that expression.
Thanks!
The Select(r=>r) does nothing (except change from Table<T> to IQueryable<T> - but nothing useful). And I assume you intended to pass predicate to the Where?
Indeed, this doesn't pull everything out and filter it - the appropriate WHERE (TSQL) clause is generated. This is possible because of "deferred execution" and "composability" - meaning: it doesn't actually execute anything until you start iterating the data (in the ToList()) - until then you are simply shaping the query.
You can see this by doing somthing like:
db.Log = Console.Out;
and look at the TSQL. Or run a TSQL trace. To make it prettier, simplify it to:
return db.Results.Where(predicate).ToList();
I know you said you wanted to pass a predicate and return a List but have you considered returning an IQueryable
Then you could call:
GetResults().Where(r => r.SomeProperty == "SomeValue")
.OrderBy(r => r.SomeOtherProperty)
.Skip(10)
.Take(10); //etc.
With your current API design you would return all records and then have to get 10, where as the above would return only the 10 you needed...
Just some thoughts...
I'm working my way through the MVC Storefront code and trying to follow the path of a repository, a service and a model that is a poco, outside of the dbml/data context. It's pretty easy to follow actually, until I started writing tests and things failed in a way I just don't understand.
In my case, the primary key is a uniqueidentifier instead of an int field. The repository returns an IQueryable:
public IQueryable<Restaurant> All()
{
return from r in _context.Restaurants select new Restaurant(r.Id)
{
Name = r.Name
};
}
In this case, Restaurant is a Models.Restaurant of course, and not _context.Restaurants.Restaurant.
Filtering in the service class (or in repository unit tests) against All(), this works just as expected:
var results = Repository.All().Where(r => r.Name == "BW<3").ToList();
This works just fine, has one Model.Restaurant. Now, if I try the same things with the pkid:
var results = Repository.All().Where(r => r.Id == new Guid("088ec7f4-63e8-4e3a-902f-fc6240df0a4b")).ToList();
If fails with:
The member 'BurningPlate.Models.Restaurant.Id' has no supported translation to SQL.
If seen some similiar posts where people say it's because r => r.Id is [Model.Restaurants] is a class level the linq2sql layer isn't aware of. To me, that means the the first version shouldn't work either. Of course, if my pk is an int, it works just fine.
What's really going on here? Lord knows, it's not very intuitive to have one work and one not work. What am I misunderstanding?
I think the problem here is due to using a constructor overload, and expecting the query to fill it in. When you do a projection like this, you have to put all the things you want to be in the projection query in the actual projection itself. Otherwise Linq won't include that in the SQL query.
So, rewrite your bits like so:
return from r in _context.Restaurants select new Restaurant()
{
Id=r.Id,
Name = r.Name
};
This should fix it up.
Not having actually typed this code out, have you tried
var results = Repository.All().Where(r => r.Id.Equals(new Guid("088ec7f4-63e8-4e3a-902f-fc6240df0a4b")).ToList()
Ninja
this probably has to do with fact that you trying to instantiate the guid in the query, and I think that LINQ to SQL is trying to convert that to actual SQL code before the object is created.
Try instantiating before the query and not on the query.