I've seen this question asked a million times, but every single suggestion I've encountered I already seem to have covered.
I have entity framework in my MVC solution, and I have a 'repository' which attempts to retrieve a collection of MyObjects:
public static List<MyObject> GetMyObjects()
{
using (var context = new MyEntities())
{
return context.MyObjects.Include("Contact").OrderByDescending(o => o.Date).ToList();
}
}
I call this method as part of a controller action which attempts to serialize it:
public JsonResult All()
{
return Json(MyRepository.GetMyObjects(), JsonRequestBehavior.AllowGet);
}
And I get the following error:
The ObjectContext instance has been disposed and can no longer be used for operations that require a connection.
I don't know where to turn on this one, I appreciate entity framework 'lazy-loads' relational data sets if and when they're needed, and I appreciate attempting to serialize the whole collection would indeed attempt to load these (outside of the using statement), but I have the 'Include' there.
What I've Tried
I've tried the 'include', I've also ensured no other associations are part of the MyObject class (i.e. I have no other Include()s to write).
To avoid this you have some options.Don't declare your navigation properties as virtual or disable Lazy Loading behavior on your context. Lazy loading is enable by default and is achieved by creating instances of derived proxy types and then overriding virtual properties to add the loading hook. So, if you want to work with a serializer I recommend you turn off lazy loading:
public class YourContext : DbContext
{
public YourContext()
{
this.Configuration.LazyLoadingEnabled = false;
}
}
These links can help you to understand better what I explain in my answer:
Loading Related Entities
Requirements for Creating POCO Proxies
If you remove the virtual keyword from your navigation properties, the POCO entity not meet the requirements described in the second link, so, EF won't create a proxy class to lazy load your navigation properties. But if you disabled lazy loading, even when your navigation properties are virtual, they won't be loaded in any entity. It's good idea disable lazy loading when you are using a serializer. Most serializers work by accessing each property on an instance of a type.
As a third option you could use the JsonIgnore attribute on your navigation properties that you don't want to be serialized as part of your entity, but as I said before, the best option is disable lazy loading.
When you use Include and using Lazy loading, and wrap the dbContext in a Using statement, then once it tries to get the linked objects, the dbContext is already disposed.
You can try eager loading of the navigation property like this:
IQueryable<MyObjects> query = db.MyObjects.Include(m => m.Contact);
Or you could take out the Using statement, as it is limiting your lazy loading...
I had the same problem and solved like below;
I created a new object and put the values I am gonna use after getting object from db.
Example Code:
var listFromDb= db.XTable.Where(x => x.Id > 5).ToList();
var list = new List<Package>();
foreach (var item in listFromDb)
{
var a = new Package()
{
AccountNo = item.AccountNo,
CreateDate = item.CreateDate,
IsResultCreated = item.IsResultCreated,
};
list.Add(a);
}
Related
I'm using EF6 with my web application, and just trying to shed light on how exactly eager loading / lazy loading works. From my research, it seems that if I want to have dynamic proxies, I would have to mark properties as "virtual". Also it seems that dynamic proxies are enabled by default. I have not disabled lazy loading or dynamic proxy loading in my db context. I have an example:
//action method which is passed in a projectId
var project = dbContext.Projects.SingleOrDefault(p => p.Id == projectId);
//inspect project by setting debug point here.
public class Project{
public Client Client{get;set;} // this has a dynamic proxy created when loading
public System System{get;set;} // this property is just null
}
Usually I have to get around this by going:
dbContext.Projects.Include(p => p.System).SingleOrDefault(p => p.Id == projectId);
What gives? Can someone demystify this for me?
To ensure lazy loading proxies are enabled you need to declare the properties as virtual and ensure that lazy loading hasn't been disabled on the DbContext.
The behaviour you are likely seeing is due to the dbContext already having fetched to one of the related entities and associating it automatically to your requested related entity.
Let's use an example with a Project (ID#1) with a Client (ID#1)
If you do something like:
using ( var context = new MyDbContext())
{
var project = context.Projects.Single(x => x.Id == 1);
Console.WriteLine("Has Client: " + (project.Client != null).ToString());
}
Without virtual you would get "Has Client: False". With virtual that console statement would trigger a 2nd query to the DB and then return "Has Client: True".
Now, where things get interesting:
using ( var context = new MyDbContext())
{
var tempClient = context.Clients.Single(x => x.Id == 1);
var project = context.Projects.Single(x => x.Id == 1);
Console.WriteLine("Has Client: " + (project.Client != null).ToString());
}
In this case our context simply loads a reference to Client ID #1. We don't do anything else or associate it to our project reference, we simply load the project same as before. In this case the output would be "Has Client: True", even though we don't eager load it, and it isn't marked as virtual. Since Project #1 has a reference to Client #1 and Client #1 is already tracked by the DbContext, the reference is included when we request Project #1.
This is a consequence of using long running DbContexts that can lead to fairly unpredictable behaviour in applications that is completely situational. You need to take care with long running, even request bounded DbContext instances because this can result in getting incomplete pictures of your data. For instance if you have a parent with children references where Parent #1 has 3 children. (#1, #2, and #3) If your context for any reason has loaded and is tracking child #1 and #2, and you later load Parent #1 without eager loading the children, that parent's Children collection will just list 2 of the 3 children (#1 and #2) which can give your client an incomplete and inaccurate view of the data. (Fun bugs to track down when clients "sometimes" see incomplete data for no apparent reason.)
In general it is advisable to ensure all references are marked as virtual and keep DbContext lifespans as short as possible. I recommend using a unit of work pattern which can be injected and lifetime scoped to the request, but responsible to create tighter lifetime scopes for the DbContexts themselves similar to using (var context = new MyDbContext()) without binding your code to the DbContext.
According to this tutorial, dynamic proxies are created iff:
The POCO class is declared with public access.
The POCO class is not sealed.
The POCO class is not abstract.
Each navigation property is declared as public, virtual.
Each collection property is of type
ICollection <T>.
The ProxyCreationEnabled option is enabled in the
context class.
Are all those conditions met for your System class?
I have Backpack and Book entities. Book references Backpack (one to many).
I am creating an instance of Backpack and bunch of Books. So in this case backpack has bunch of books. I am saving those entities to the db. I am verifying that those got saved to the db. When I try to load backpack it loads fine and all the properties are set except the navigation properties. I am also checking that LazyLoading is not disabled. My navigation properties has the virtual keyword.
I am not sure what I am doing wrong. If I try to Load the backpack with Include() it loads the books:
dbContext.Backpacks.Where(b=>b.Name!="").Include("Books").FirstOrDefault()
I am trying to figure out why it is not loading the books lazily? I have the same problem with loading the book. When I load the book, it doesn't have the backpack attached. I see that the BackpackId is there.
In my property getter/setter I have some logic that will be fired, but I am not sure how that could be a problem.
With the limited information at hand, I can see the following explanations for your problem.
Lazy Loading or Proxy Creation are disabled
Make sure that lazy loading and proxy creation are enabled, the first doesn't work without the latter.
dbContext.Configuration.ProxyCreationEnabled = true;
dbContext.Configuration.LazyLoadingEnabled = true;
(see this SO post for details)
Accessing the entities after disposing the context
Simplified, Lazy Loading works like this:
Whenever you retrieve an entity from the context, you actually get an object of an automatically created subclass of the class you expected that overrides your virtual navigation properties, which is the proxy.
Whenever you then access said navigation properties, the proxy will access the database and load the linked entities when needed.
This last step is, of course, only possible if the entity/proxy is still attached to the context and can therefore query the database to retrieve said objects.
using( var dbContext = new MyContext() )
{
dbContext.Configuration.ProxyCreationEnabled = true;
dbContext.Configuration.LazyLoadingEnabled = true;
// Note: You can modify or derive from `MyContext` so that this is
// done automatically whenever a new `MyContext` is instantiated
var backpack = dbContext.Backpacks.Where(b=>b.Name!="").FirstOrDefault();
// This should work
var firstBook = backpack.Books.FirstOrDefault();
}
// This will probably not, because the context was already disposed
var firstDrink = backpack.Drinks.FirstOrDefault();
I hope this helps, but feel free to provide more information if it doesn't
After a several days of debugging, finally figured out the problem. As people mentioned above, you have to enable the LazyLoading and ProxyCreating. I had the issues even after having the enabling the LazyLoading and ProxyCreating. Also make sure you declare your navigation properties as virtual, otherwise EF will not be able to load entities lazily.
So the issue I had was, EF wasn't crating Proxies because of my entity didn't have a public or protected constructor with NO parameters. After creating public (in my case protected) constructor without parameter it worked.
NOTE: Not having public/protected constructor without parameters will not affect the eager loading.
Here is a link that explains the requirements for the LazyLoading
Eager loading is achieved using the Include() method and as a result you are forcing eager loading by using Include("Books").
Change this:
dbContext.Backpacks.Where(b=>b.Name!="").Include("Books").FirstOrDefault()
to this:
dbContext.Backpacks.Where(b=>b.Name!="").FirstOrDefault()
You should now see that the Books are no longer being loaded eagerly.
Reference:
http://www.entityframeworktutorial.net/EntityFramework4.3/eager-loading-with-dbcontext.aspx
https://msdn.microsoft.com/en-us/library/jj574232(v=vs.113).aspx
Steps I had to do using .NET Core 3.1 and Microsoft.EntityFrameworkCore 3.1.5...
1) Add the Microsoft.EntityFrameworkCore.Proxies NuGet package
2) Configure your DbContext to UseLazyLoadingProxies (in Startup.cs)
public void ConfigureServices(IServiceCollection services)
{
...
services.AddDbContext<DataContext>(optionsBuilder =>
{
optionsBuilder
.UseLazyLoadingProxies() // <--- You need this bit
.UseSqlServer(Configuration.GetConnectionString("DefaultConnection"));
});
...
}
3) Mark all appropriate properties (those you want lazy loaded) as virtual
public class MyEntity
{
public virtual OtherEntity? { get; set; } // Lazy loaded coz `virtual`
public ICollection<OtherEntity> { get; set; } // NOT lazy loaded coz not `virtual`
}
I have two objects that have a many to many relationship with each other.
I'm using Entity Framework in a Database First approach.
My Database Profiler is showing that everytime I fetch one set of objects A, it loads the other set of object B for each element in A. I assumed that with lazy loading, this wouldn't happen, or that B would be fetched when accessing via the navigation property.
The code to access the objects uses a generic approach, where entities is my DbContext
public virtual IQueryable<T> GetAll()
{
IQueryable<T> query = entities.Set<T>();
return query;
}
Both navigation properties are implemented as virtual ICollection<T> and I have Configuration.LazyLoadingEnabled = true explicitly set in my DbContext constructor.
Am I missing something, or approaching this the wrong way? Should I just remove the navigation properties and explicitly load what I need via other queries?
In the presence of the experts I am reluctant to ask this :) but would setting ProxyCreationEnabled=false on the DbContext not help in this case? Sorry if this is too "newbish" of a point
Please help me, I am new to EF.Lazy loading for POCO objects doesn't seem to be working.
My POCO classes are in a sepearte assembly, other than the one one for Data access(i.e DAL)
The Data Acess layer simply wraps the calls made to the EF's object context. Please see the code below
public FilterMaster GetFilter(long ID)
{
FilterMaster entity = new FilterMaster();
try
{
using (var context = new RadarEntities())
{
//context.ContextOptions.LazyLoadingEnabled = false;
//context.ContextOptions.ProxyCreationEnabled = true;
entity = context.FilterMasters.SingleOrDefault(filter => filter.ID == ID);
//context.FilterMasters.Include(
context.LoadProperty(entity, "SQLQuery");
}
}
When DAL call is completed, the ObjectContext is lost, and when I tried to fetch the related child objects of the Root POCO class, I get null.
I've tried explicitly enabling ProxyCreation, EnabledLazyLoading, checked the proxy clases generated are not sealed and all the related properties are marked virtual (as suggested on some other links).
-As the lazy load was not working, I thought of eagerly loading all the related POCO objects, so tried invoking LoadProperty method, which works.
Q1: Am I Imissing something the lazy loading isn't working?
Q2: If I want to expelictly load all the related child objects the will have to call the LoadProperty method for all properties or there is any simpler way?
You are disposing your ObjectContext. This is what is preventing you from using LazyLoading. If you need LazyLoading, the class containing GetFilter should create an ObjectContext when it is created, implement IDisposable, and dispose of the ObjectContext when it is disposed.
Q1: Am I Imissing something the lazy loading isn't working?
It's working but there is no magic involved - underneath a proxy is created for you which will try to retrieve the property value from the database for you on the first access.
For EF the database connection is represented by the context, which you currently dispose automatically at the end of your using block. Without database connection EF cannot lazily retrieve the properties and hence lazy loading won't work.
You will have to keep the context alive until you have accessed all the properties you need to access for lazy loading, or alternatively eagerly load those properties.
Q2: If I want to explicitly load all the related child objects the
will have to call the LoadProperty method for all properties or there
is any simpler way?
Yes, you can specify an Include() query to eagerly retrieve properties, in your case that would be:
entity = context.FilterMasters
.Include("SQLQuery")
.SingleOrDefault(filter => filter.ID == ID);
I'm using EF 4.1 and code-first in an MVC project, and AutoMapper to map entities to view models.
Prior to using code-first I was able to exclude navigation properties in order to prevent anything from being loaded that wasn't already. I'm using .Include() in my queries to include the references that I need in order to avoid additional database round-trips.
However, with code-first my entity only exposes an entity property (or ICollection if there are more than one). How can I know whether it has been loaded without triggering the load?
Assuming this can be done, is there a way to make this the default behavior for AutoMapper, so that I do not have to explicitly exclude members on every single entity?
You can check whether a reference or collection navigation property of an entity has been loaded by:
bool isLoaded1 = dbContext.Entry(entity).Reference(e => e.MyReferenceProperty)
.IsLoaded();
bool isLoaded2 = dbContext.Entry(entity).Collection(e => e.MyCollectionProperty)
.IsLoaded();
EF Code First does lazy loading only for properties marked as virtual (it can override those and place DynamicProxy instead of it). If you don't make your property virtual, you will turn off lazy loading for that property.
You should be able to explicitly load them by turning off lazy-loading:
using(var context = new FooBarEntities())
{
context.ContextOptions.LazyLoadingEnabled = false;
Foo foo = context.Foo.Where(x => x.Id == myId).Single();
...
if(!foo.Bars.IsLoaded)
{
foo.Bars.Load();
}
//do something with foo.Bars here
}