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.
Related
I have been struggling with an issue for a couple of days. I am sharing a Content Provider with two different apps (app A and app B). All the stuff regarding DB creation and Content Provider management is done by app A. App B just accesses it using the corresponding Authorities and a Content Provider Client.
ContentProviderClient myCPClient = this.miContext.getContentResolver().acquireContentProviderClient(this.miUri);
The problem comes up when trying to query the database in a more complex way, i.e. using some key words like GROUP BY, HAVING, etc. I need to get unique references according to one specific column (I want to use GROUP BY), and I have found out that there is no rawQuery() method for a ContentProviderClient, but a simplified query() method (compared to the one available in the class SQLiteDatabase, which allows to formulate proper MySQL commands).
I have checked this answer, but since my ContentProvider is accessed from a different app, I do not have any class like MyContentProvider.
To sum up, is there any way to make a proper query (like rawQuery()) to a ContentProvider which was generated by a different app?
I have finally got to a solution which is rather simple and sensible. I got a very good explanation about Content Providers and Content Resolvers. The latter is used to access the former, which means that they can not control what is in the provider, but get data from them. This means that you can not make a Content Provider Client to use a rawQuery() if it is not implemented (override) in the query() method of the corresponding ContentProvider.
To work around my problem, I have used a flag in my provider client and modify my content provider to read it so I can make use of GROUP BY. I just wanted to get unique references from the database according to a particular column.
Here it is the solution, which is not a very clean one, but it works quite well.
For the ContentProviderClient,
ContentProviderClient myCPClient = this.miContext.getContentResolver().acquireContentProviderClient(this.miUri);
//I declare some variables for the query
//'selection' will get all the rows whose "_id" is greater than 0, i.e. all the rows
String selection = BaseDatosParam.Tabla._ID + ">?";
String[] selectionArgs = {"0"};
//'groupBy' is not formatted in any particular way. I just need it to contain the pattern "GROUP BY"
String groupBy = "GROUP BY" + BaseDatosParam.Tabla.REF;
//the last field of the query corresponds to 'sortOrder', but I
Cursor c = myCPClient.query(Uri.parse(miUri.toString()),
projection, selection, selectionArgs, groupBy);
In the ContentProvider,
public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder)
{
String where = selection;
String groupBy = null;
SQLiteDatabase db = this.miBDManager.getWritableDatabase();
//We just check out whether 'sortOrder' includes the pattern "GROUP BY", otherwise that field will remain null
Pattern myPat = Pattern.compile("GROUP BY");
Matcher myMat = myPat.matcher(sortOrder);
if (myMat.find())
groupBy = myMat.replaceFirst("");
Cursor c = db.query(BaseDatosParam.Tabla.NOMBRE_TABLA, projection, where, selectionArgs, groupBy, null, null);
c.setNotificationUri(this.getContext().getContentResolver(), uri);
return c;
}
Regards,
I have some tables in a MySQL database to represent records from a sensor. One of the features of the system I'm developing is to display this records from the database to the web user, so I used ADO.NET Entity Data Model to create an ORM, used Linq to SQL to get the data from the database, and stored them in a ViewModel I designed, so I can display it using MVCContrib Grid Helper:
public IQueryable<TrendSignalRecord> GetTrends()
{
var dataContext = new SmgerEntities();
var trendSignalRecords = from e in dataContext.TrendSignalRecords
select e;
return trendSignalRecords;
}
public IQueryable<TrendRecordViewModel> GetTrendsProjected()
{
var projectedTrendRecords = from t in GetTrends()
select new TrendRecordViewModel
{
TrendID = t.ID,
TrendName = t.TrendSignalSetting.Name,
GeneratingUnitID = t.TrendSignalSetting.TrendSetting.GeneratingUnit_ID,
//{...}
Unit = t.TrendSignalSetting.Unit
};
return projectedTrendRecords;
}
I call the GetTrendsProjectedMethod and then I use Linq to SQL to select only the records I want. It is working fine in my developing scenario, but when I test it in a real scenario, where the number of records is way greater (something around a million records), it stops working.
I put some debug messages to test it, and everything works fine, but when it reaches the return View() statement, it simply stops, throwing me a MySQLException: Timeout expired. That let me wondering if the data I sent to the page is retrieved by the page itself (it only search for the displayed items in the database when the page itself needs it, or something like that).
All of my other pages use the same set of tools: MVCContrib Grid Helper, ADO.NET, Linq to SQL, MySQL, and everything else works alright.
You absolutely should paginate your data set before executing your query if you have millions of records. This could be done using the .Skip and .Take extension methods. And those should be called before running any query against your database.
Trying to fetch millions of records from a database without pagination would very likely cause a timeout at best.
Well, assuming information in this blog is correct, .AsPagination method requires you to sort your data by a particular column. It's possible that trying to do an OrderBy on a table with millions of records in it is just a time consuming operation and times out.
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'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.
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)