I've learned about lazy loading, eager loading with .include and explicit loading with .load(), but something that confuses me is when you project in a query and explicitly request a navigation property like this:
var address = from a in context.Addresses
select {a, Name = a.Contact.Name}
Here Contact is a navigation property in Addresses that links to a Contact entity.
I tried with both lazy loading on and off and it works both ways. I wonder when I request my data like this, am I eager loading or deferred loading? My understanding that only one query will be made to the database which means it's eager loading, except in this case only the "Name" property of the Contact entity will be loaded, as opposed to the entire Contact entity if I were to use context.Addresses.include("Contact")? Does it make a query like this more efficient than eager loading with .include() ?
Some clarifications will be appreciated.
Lazy loading, eager loading and explicit loading works in the most common scenarios. Projection is replacement for eager loading because it can also load related entities with single query. Using projection make sense if you need to load entities with some complex query because eager loading doesn't work in these cases. Use projection if you need:
Any kind of join or aggregation in the linq query. As I know Include is ignored once you start using manual joins.
Any kind of filtering or sorting in navigation property. Include can only load all related entities from navigation property without any sorting. Once you need to apply any where condition or order by on related entities you can't use Include and you must use projection.
It's lazy evaulation as it won't execute until something enumerates over address.
Eager loading is normally used to describe an entity object's navigation properties being pre-loaded but in this case you are not directly materializing any entity objects as you are projecting onto an anonymous type.
If you access a.Contact.Name rather than Name you'll most likely cause another database hit as you haven't eager loaded the Contact object of Address, you specifically selected and projected the Name property onto an anonymous object.
Related
I tried searching but couldn't find an exact answer to this on docs.microsoft at: https://learn.microsoft.com/en-us/ef/core/querying/related-data#eager-loading
I have this bit of code:
await MyDbContext.Users.Where(u => u.Id == Id).Include(u => u.UserContacts).ThenInclude(uC => uC.Contact);
The return type is of type User and Contact is also of type User.
When I debug and step through, it seems like all the Contacts of the required User are loaded, as well as all their Contacts, and then all their Contacts, and so on.
Question:
Is this expected behavior for EF and if so, doesn’t this impact performance, having to look up the DB to such depths?
Is there a way for me to specify a kind of 'max depth'?
The closest match to what I'm asking here that I cam across was probably this SO thread:
How does Entity Framework work with recursive hierarchies? Include() seems not to work with it
You can navigate to entities that your eager loading query doesn't specify in two scenarios:
Navigation Property Fix-UP
If a query, or a previous query on the same DbContext has loaded an Entity that is the target of a Navigation Property, the change tracker will "fix up" the navigation property.
Lazy Loading
Obviously if Lazy Loading is enabled, Navigation Properties will be loaded as you access them.
I have read about lazy loading from this web site.
Enable or disable LazyLoading
"If we request a list of Students with LazyLoading enabled, the data provider will get all of our students from the DB but each StudentAddress property won’t be loaded until the property will be explicitly accessed."
This statement says that when I set Lazy Loading Enabled = true the related data won't be loaded. However
List<Students> stdList = Datacontext.Students.ToList();
if I set lazy loading enabled = true the above code returns all stundents with their teachers and address. What is the point that I am missing here? Please could someone explain it?
No matter what setting you have, if you use .ToList() it will "enumerate the enumerable". This is very significant, and this phrase should become common knowledge to you.
When .ToList() is used, many things occur. Enumerating the enumerable means that the previous set of how to enumerate the set is now being used to actually iterate through the set and populate the data. What that means is that the previous enumerator (which was stored internally as an Expression Tree) is now going to be sent from Entity Framework to your SQLProvider Factory. That will then convert the object graph from the Expression Tree into SQL and execute the query on the server, thus returning the data and populating your list.
Lazy loading instead of using ToList() would be if you had this IQueryable enumerable, and then iterated that manually loading each element in the set, or only partial elements in the set.
Once you have the list of elements returned, lazy loading will only come in to play if there are navigational properties. If there were related properties, for example if you have an Invoice and you want to get the related Customer information from the customer table. The relation will not explicitly be returned at first, only the invoices. So to get the customer data you could then (while the context was still open, i.e. not disposed) access that via the .Customer reference on your object and it would load. Conversely, to load all the customers during the original enumeration, you could use the .Include() functionality on your queryable, and that would then tell the sql provider factory to use a join when issuing the query.
In your specific example,
List<Students> stdList = Datacontext.Students.ToList();
This will actually not load all of the Teachers and Addresses regardless of if lazy loading is enabled or not. It will only load the students. If you want to lazy load a Teacher, while the Datacontext is still not disposed, you could then use
var firstStudent = stdList.First();
var teacher = firstStudent.Teacher;
//and at this point lazy loading will fetch the teacher
//by issuing **another** query (round trip) to the database
That would only be possible if lazy loading were enabled.
The alternative to this is to eager load, which would include the teachers and addresses. That would look like this
List<Students> stdList = Datacontext.Students
.Include( s => s.Teacher )
.Include( s => s.Address ).ToList();
And then later on if you were to try to access a teacher the context could be disposed and access would still be possible because the data was already loaded.
var firstStudent = stdList.First();
var teacher = firstStudent.Teacher;
//and at this point the teacher was already
//loaded and as a result no additional round trip is required
How was that you noticed that property was loaded, using the debugger? If that is the case, then you already have the answer. With the debugger you are accessing to that property too, so, that also triggers lazy loading.
How this works?
If your entities meets these requirements, then EF will create a proxy class for each of your entities that support change tracking or lazy loading. This way you can load the related entities only when these are accessed. As I explained earlier, the debugger will also trigger lazy loading.
Now, be careful with lazy loading, once you context has been disposed, you will get an exception when you try to get access to one of the related properties. So, I would suggest to use eager loading in that case.
When I use Entity Framework, I want to query out a record in a context and add it to another context with the same schema, after query out the record, I detach it from the context, but the related entities are all away, is there any way to solve it?
Thanks in advance!
This is "by design". EF can detach entities only one by one but in the same time EF doesn't support object graphs composed of attached and detached entities. Because of that when you detach entity it will break all relations to the rest of attached object graph. Detaching whole object graph is currently not supported but you can vote for this feature on Data UserVoice.
As a workaround you can turn off lazy loading on your context, use eager loading described by #CodeWarrior to load exactly data you need to pass to other context. Once you have data loaded serialize them to stream and immediately deserialize them to the new instance of the object graph. This is the way how to make deep clone of entity graph which is detached but has all relations intact (turning lazy loading off is needed otherwise serialization will load all other navigation properties as well which can result in much bigger object graph then expected). The only requirement is that your entities must be serializable by serializer of your choice (be aware of circular references which usually require some special handling or additional attributes on your entities).
Are you asking how to load the child entities? If so, you can do eager loading with the .Include method. Given a Person class and a PhoneNumber class where Person has a collection of PhoneNumber, you could do the following:
List<Person> People = db.People.Where(p => p.Name = "Henry")
.Include("PhoneNumbers")
.ToList();
Or you can do what is called explicit loading where you load your entities and call the .Load method on the collections of child and related entities that you want to load. Generally you do this when you do not have LazyLoading enabled (and LazyLoading is enabled by default in 4.0+ don't recall in previous versions).
Regardless of how you query and load them, you will have to detach entities that you want to attach to a different context.
Here is a link to a pretty good MSDN article on loading entities.
I've been using Entity Framework Profiler to test my data access in an MVC project and have come accross several pages where I'm making far more db queries than I need to because of N+1 problems.
Here is a simple example to show my problem:
var club = this.ActiveClub; // ActiveClub uses code similar to context.Clubs.First()
var members = club.Members.ToList();
return View("MembersWithAddress", members);
The view loops through Members and then follows a navigion property on each member to also show their address. Each of the address requests results in an extra db query.
One way to solve this would be to use Include to make sure the extra tables I need are queried up front. However, I only seem to be able to do this on the ObjectSet of Clubs attached directly to the context. In this case the ActiveClub property is shared by lots of controllers and I don't always want to query the Member and address table up front.
I'd like to be able to use something like:
var members = club.Members.Include("Address").ToList();
But, Members is an EntityCollection and that doesn't have the Include method on it.
Is there a way to force a load on the Members EntityCollection and ask EF to also load their Addresses?
Or, is using EntityCollection navigation properties on an entity in this way, just a really bad idea; and you should know what you're loading when you get it from the context?
If your entities inherits from EntityObject try to use this:
var members = club.Members.CreateSourceQuery()
.Include("Address")
.ToList();
If you use POCOs with lazy loading proxies try to use this:
var members = ((EntityCollection<Club>)club.Members).CreateSourceQuery()
.Include("Address")
.ToList();
Obviously second version is not very nice because POCOs are used to remove dependency on EF but now you need to convert the collection to EF class. Another problem is that the query will be executed twice. Lazy loading will trigger for Members once to access the property and then second query will be executed when you call ToList. This can be solved by turning off lazy loading prior to running the query.
When you say ActiveClub is shared I believe it means something like it is property in base controller used in derived controllers. In such case you can still use different code in different controller to fill the property.
Is it possible to load an entity excluding some properties? One of this entity's properties is expensive to select. I would like to lazy load this property. Is that possible?
Now that you have read everyone's reply, I will give you the correct answer. EF does not support lazy loading of properties. However it does support a much powerful concept then this. It's called table splitting where you can map a table to two entities. Say a product table in the the database can be mapped to product entity and ProductDetail entity. You can then move the expensive fields to the ProductDetail entity and then create a 1..1 association between prodcut and productdetail entity. You can then lazy load the productdetail association only when you need it.
In my performance chapter of my book, I have a recipe called.
13-9. Moving an Expensive Property to Another Entity
Hope that helps!
Julie Lerman has an article on how to split a table
With a scalar property, the only way to selectively not load a certain property is to project in ESQL or L2E:
var q = from p in Context.People
select new
{
Id = p.Id,
Name = p.Name // note no Biography
};
+1 to Dan; doing this lazily is worse than loading it up-front. If you want to control loading, be explicit.
stimms is correct, but be careful while using lazy loading. You may have performance issues and not realize the property is getting loaded at a specific location in your code. This is because it loads the data when you use the property
I prefer to use explicit loading. This way you know when they get loaded and where. Here's a link that gives an example for the LoadProperty http://sankarsan.wordpress.com/2010/05/09/ado-net-entity-framework-data-loading-part-2/
You can also you Eager Loading by using the Include method. Example here:http://wildermuth.com/2008/12/28/Caution_when_Eager_Loading_in_the_Entity_Framework
Given a query over an EntityFramework DbSet, where the targeted entity contains a BigProperty and a SmallProperty,
When you're trying to only access the SmallProperty without loading the BigProperty in memory :
//this query loads the entire entity returned by FirstOrDefault() in memory
//the execution is deferred during Where; the execution happens at FirstOrDefault
db.BigEntities.Where(filter).FirstOrDefault()?.SmallProperty;
//this query only loads the SmallProperty in memory
//the execution is still deferred during Select; the execution happens at FirstOrDefault
//a subset of properties can be selected from the entity, and only those will be loaded in memory
db.BigEntities.Where(filter).Select(e=>e.SmallProperty).FirstOrDefault();
Therefore you could exploit this behaviour to only query the BigProperty where you actually need it, and use select statements to explicitly filter it out everywhere else.
I tested this with the Memory Usage functionality from the Visual Studio debug Diagnostic Tools.