I need to execute a dynamic set type, here's what I'm trying to do (pseudo):
var type = GetSetType(); //System.Type
var set = context.Set(type);
var results = set.ToArray();
I know this can't work for sure, Enumerable ex. methods are only for generic IEnumerables, but I tried set.AsQueryable().Cast<object>().ToArray(), and a NotSupportedException was thrown: "LINQ to Entities only supports casting EDM primitive or enumeration types.".
Any way to execute a non-generic DbSet?
I should have deleted my question since the answer is too simple, but for the record, let's just keep it up.
namespace System.Linq
{
public static class EntityFrameworkExtensions
{
public static IEnumerable<object> AsEnumerable(this DbSet set)
{
foreach (var entity in set)
yield return entity;
}
}
}
Usage:
var collection = context.Set(entityType).AsEnumerable();
Beware that any filtering performed on the retuned collection will not happen on the SQL server, but on the collection enumerator, the entire table will be returned. Use only when loading all rows anyway.
Related
I'm trying to write an extension method to include a certain property (text element, themselves containing a collection of translations) that are present in many of my entity models.
I had no problem with the .Include function:
public static IIncludableQueryable<T, IEnumerable<Translation>> IncludeTextBitWithTranslations<T>(this IQueryable<T> source, Expression<Func<T, TextBit>> predicate) where T: class
{
var result = source.Include(predicate).ThenInclude(t => t.Translations);
return result;
}
And tests proved successful.
Now, in some cases, I have entities that have all their texts in a child - for example Article entity has an ArticleInfo property that contains a few text elements. So I figure I just needed to do another extension that was a ThenInclude instead. With a few differences I finally get this :
public static IIncludableQueryable<TEntity, ICollection<Translation>> ThenIncludeTextBitWithTranslations<TEntity, TPreviousProperty, TextBit>(this IIncludableQueryable<TEntity, TPreviousProperty> source, Expression<Func<TPreviousProperty, TextBit>> predicate) where TEntity: class
{
var result = source.ThenInclude(predicate)
.ThenInclude(t => t.Translations);
return result;
}
And now I get this error:
'TextBit' does not contain a definition for 'Translations' and no extension method 'Translations' accepting an argument of 'TextBit' type was found
This error appears on the last lambda expression t => t.Translations.
This error is extremely weird for me, I've been looking all over the internet for some help on the matter but I was unsuccessful.
I tried forcing the type to the ThenInclude by adding them manually :
var result = source.ThenInclude(predicate)
.ThenInclude<TEntity, TextBit, ICollection<Translation>>(t => t.Translations);
but without success.
Does anyone have some clues as to why?
I'm very much at a loss here
You have extra type parameter TextBit in second one (ThenIncludeTextBitWithTranslations<TEntity, TPreviousProperty, TextBit>), so it is considered as a generic type, not an actual one, remove it:
public static IIncludableQueryable<TEntity, ICollection<Translation>> ThenIncludeTextBitWithTranslations<TEntity, TPreviousProperty>(this IIncludableQueryable<TEntity, TPreviousProperty> source, Expression<Func<TPreviousProperty, TextBit>> predicate) where TEntity: class
{
var result = source.ThenInclude(predicate).ThenInclude(t => t.Translations);
return result;
}
I have a method that receives an IEnumerable<Guid> of IDs to objects I want to delete. One suggested method is as follows
foreach(Guid id in ids)
{
var tempInstance = new MyEntity { Id = id };
DataContext.Attach(tempInstance); // Exception here
DataContext.Remove(tempInstance);
}
This works fine if the objects aren't already loaded into memory. But my problem is that when they are already loaded then the Attach method throws an InvalidOperationException - The instance of entity type 'MyEntity' cannot be tracked because another instance with the key value 'Id:...' is already being tracked. The same happens if I use DataContext.Remove without calling Attach.
foreach(Guid id in ids)
{
var tempInstance = new MyEntity { Id = id };
DataContext.Remove(tempInstance); // Exception here
}
I don't want to use DataContext.Find to grab the instance of an already loaded object because that will load the object into memory if it isn't already loaded.
I cannot use DataContext.ChangeTracker to find already loaded objects because only objects with modified state appear in there and my objects might be loaded and unmodified.
The following approach throws the same InvalidOperationException when setting EntityEntry.State, even when I override GetHashCode and Equals on MyEntity to ensure dictionary lookups see them as the same object.
foreach(Guid id in ids)
{
var tempInstance = new MyEntity { Id = id };
EntityEntry entry = DataContext.Entry(tempInstance);
entry.State == EntityState.Deleted; // Exception here
}
The only way so far I have found that I can achieve deleting objects by ID without knowing if the object is the following:
foreach(Guid id in ids)
{
var tempInstance = new MyEntity { Id = id };
try
{
DataContext.Attach(tempInstance); // Exception here
}
catch (InvalidOperationException)
{
}
DataContext.Remove(tempInstance);
}
It's odd that I am able to call DataContext.Remove(tempInstance) without error after experiencing an exception trying to Attach it, but at this point it does work without an exception and also deletes the correct rows from the database when DataContext.SaveChanges is executed.
I don't like catching the exception. Is there a "good" way of achieving what I want?
Note: If the class has a self-reference then you need to load the objects into memory so EntityFrameworkCore can determine in which order to delete the objects.
Strangely, although this is a quite common exception in EF6 and EF Core, neither of them expose publicly a method for programmatically detecting the already tracked entity instance with the same key. Note that overriding GetHashCode and Equals doesn't help since EF is using reference equality for tracking entity instances.
Of course it can be obtained from the DbSet<T>.Local property, but it would not be as efficient as the internal EF mechanism used by Find and the methods throwing the aforementioned exception. All we need is the first part of the Find method and returning null when not found instead of loading from the database.
Luckily, for EF Core the method that we need can be implemented relatively easily by using some of the EF Core internals (under the standard This API supports the Entity Framework Core infrastructure and is not intended to be used directly from your code. This API may change or be removed in future releases. policy). Here is the sample implementation, tested on EF Core 2.0.1:
using Microsoft.EntityFrameworkCore.Internal;
namespace Microsoft.EntityFrameworkCore
{
public static partial class CustomExtensions
{
public static TEntity FindTracked<TEntity>(this DbContext context, params object[] keyValues)
where TEntity : class
{
var entityType = context.Model.FindEntityType(typeof(TEntity));
var key = entityType.FindPrimaryKey();
var stateManager = context.GetDependencies().StateManager;
var entry = stateManager.TryGetEntry(key, keyValues);
return entry?.Entity as TEntity;
}
}
}
Now you can use simply:
foreach (var id in ids)
DataContext.Remove(DataContext.FindTracked<MyEntity>(id) ?? new MyEntity { Id = id }));
or
DataContext.RemoveRange(ids.Select(id =>
DataContext.FindTracked<MyEntity>(id) ?? new MyEntity { Id = id }));
I'm a SQL guy who's tinkering with Web API and Entity Framework 6 and I keep receiving the error "The operation cannot be completed because the DbContext has been disposed" when I my code is:
namespace DataAccessLayer.Controllers
{
public class CommonController : ApiController
{
[Route("CorrespondenceTypes")]
[HttpGet]
public IQueryable GetCorrespondenceTypes()
{
using (var coreDB = new coreEntities())
{
var correspondenceType = coreDB.tblCorrespondenceTypes.Select(cor => new { cor.CorrespondenceTypeName });
return correspondenceType;
}
}
}
}
But if change my code around a little and try this it works:
namespace DataAccessLayer.Controllers
{
public class CommonController : ApiController
{
readonly coreEntities coreDB = new coreEntities();
[Route("CorrespondenceTypes")]
[HttpGet]
public IQueryable GetCorrespondenceTypes()
{
var correspondenceType = coreDB.tblCorrespondenceTypes.Select(cor => new { cor.CorrespondenceTypeName });
return correspondenceType;
}
}
}
My question is why does the second one work but not the first? Is it better practice to have a global connection string or call DBContext explicitly each time?
Your are getting error because you are returning the IQueryable for which Entity framework has yet not executed the query and DbContext has been disposed when that query needs to be executed.
Remember Entity framework will not execute query until collection is initialized or any method that does not support deferred execution. Visit this link for list of Linq deferred execution supported method.
why does the second one work but not the first?
In first code snippet you are returning an instance of IQuerable which has not executed DbQuery and then after it just fires dispose on your context (coreDB). So then after whenever your code iterate over the collection it tries to fire DbQuery but finds that context has already been destroyed so you are getting an error.
In second case when ever you are iterating over the collection coreDB context must be alive so you are not getting an error.
Is it better practice to have a global connection string or call DBContext explicitly each time?
Answer to this question is based on developers taste or his own comforts. You can use your context wrapped within using statements as below:
public IList GetCorrespondenceTypes()
{
using (var coreDB = new coreEntities())
{
var correspondenceType = coreDB.tblCorrespondenceTypes.Select(cor => new { cor.CorrespondenceTypeName });
return correspondenceType.ToList();
}
}
As shown in above code snippet if you would use ToList before returning it would execute query before your coreDB got destroyed. In this case you will have to make sure that you returned materialized response (i.e. returned response after executing the DbQuery).
Note: I have noticed most of the people choose the second way. Which targets context as an instance field or property.
I have a question about caching with Entity Framework code first.
I need to cache my query's results and I came about something that I didn't know.
Func<T, bool> predicate does not work when filtering whilst Expression<Func<T, bool>> predicate does.
Maybe I'm missing the obvious.
Here is my scenario:
Whenever I call a method, e.g."GetOrders", I use a method called "GetCachedModels" internally to get the cached version.
When subsequently many calls are made with
"GetOrders(customerNo)" it checks the cache and the get it from there if it's there. That is the theory.
However when using Func predicate it does not find the item, but when using the Expression version it does?
My question is how do you use "Expressions" with a Where clause on a list?
The other solution would be to have a method for each search, e.g. "myservice.GetCustomer(etc..) or myservice.GetOrders(etc..) instead of generic
myservice.GetAll(); Which means adding many methods to the interface.
mycode:
public interface IGenericRepository
{
IList<T> GetAll<T>() where T : class;
IList<T> Find<T>(Func<T, bool> predicate) where T : class; //I use this so that I could use the predicate in an where clause against a list.
etc....
}
In my repository I have something like:
public IList<T> Find<T>(Func<T, bool> predicate) where T : class
{
List<T> models = GetCachedModels<T>().ToList();
var result= models.Where(predicate).ToList(); --Does not work!!! tried also with(expression.Compile()).ToList(); !!still no results!!!!
return result;
}
internal IList<T> GetCachedModels<T>() where T : class
{
IList<T> models;
Type typeParameterType = typeof(T);
string cacheName = string.Format("{0}Cache", typeParameterType.Name);
object cacheView = DbCache.Get(cacheName);
if (cacheView == null)
{
models = Set<T>().ToList();
DbCache.Add(cacheName, models, DateTime.Now.AddHours(1));
}
else
{
models = (IList<T>)cacheView;
}
return models;
}
//below method works but cannot use cache!!!!
public IList<T> Find<T>(Expression<Func<T, bool>> predicate) where T : class
{
return Set<T>().Where(predicate).ToList();
}
Expression predicates works only for IQueryable interface. List doesn't inherit it, so if you want to use this expression, you need to return IQueryable in GetCachedModels method, and return Set, so it can query this data. And then you can place it.
Otherwise if you want to cache all items from Set, then you need to pass Func instead of Expression, and then use it in Where extension method, like this - http://dotnetfiddle.net/5YsIy3
I'm trying to load employees using Entity Framework.
The method is supposed to return employee list.
It' s giving this error:
Cannot implicit convert....<Class names and methods>.... An Explicit conversion exists.
I think the problem is related to casting.
Please check below code.
public List<Employee> LoadEmployees()
{
try
{
EMployeeDB1Entities EE = new EMployeeDB1Entities();
var Employees = EE.Employees.Where(p => p.Name.StartsWith("T"));
return Employees;
}
catch
{
return null;
}
}
var Employees = EE.Employees.Where(p => p.Name.StartsWith("T")).ToList();
Update your code to:
return Employees.ToList();
Also do note that this is the ToList() method that actually triggers the database query.
EE.Employees.Where(....) doesn't query the database. The DB is queried when the result of the Where() is enumerated, which is what .ToList() does.
Thanks it works...one more issue, suppose if I want to bind above list
to grid then how can I bind ?
Assuming you're using WPF or Silverlight:
To bind the result of your query on a datagrid, you could expose a public property of type ObservableCollection.
This collection accepts an IEnumerable<T> object as constructor.
You can write:
var myCollection = new ObservableCollection<Employee>(this.LoadEmployees());
Then bind the ItemSource property of your datagrid to your collection.
If you have more problems using bindings, I recommend you to ask another question, because the subject is quite different.