I'm transfering my .NET Framework (EF6) code to ASP.NET Core (EF Core), and I stumbled upon this issue. Here is some example code:
In EF6 I use Include() and Select() for eager-loading:
return _context.Post
.Include(p => p.PostAuthor.Select(pa => pa.Author).Select(a => a.Interests))
PostAuthor is a junction table and there is also a Junction table "AuthorInterest" which I didn't need to involve in EF6 (Select goes straight to a.Interests).
Anyway, I can see that in EF7 this is reworked, meaning that I should use ThenInclude() for nested queries now. However...
return _context.Post
.Include(p => p.PostAuthor)
.ThenInclude(pa => pa.Select(pa2 => pa2.Author))
...etc
The above code fails because of the Select() statement. The documentation on https://docs.efproject.net/en/latest/querying/related-data.html seems to suggest that I don't need it and I can access Author immediately, but I get an ICollection in the last lambda displayed, so I obviously need the Select(). I go through multiple junction tables further on in the query, but for simplicity's sake, let's just focus on the first one.
How do I make this work?
but I get an ICollection in the last lambda displayed, so I obviously need the Select()
No, you don't. EF Core Include / ThenInclude totally replace the need of Select / SelectMany used in EF6. Both they have separate overloads for collection and reference type navigation properties. If you use the overload with collection, ThenInclude operates on the type of the collection element, so at the end you always end up with a single entity type.
In your case, pa should resolve to your junction table element type, so Author should be directly accessible.
For instance the EF6 include chain:
.Include(p => p.PostAuthor.Select(pa => pa.Author).Select(a => a.Interests))
translates to EF Core:
.Include(p => p.PostAuthor).ThenInclude(pa => pa.Author).ThenInclude(a => a.Interests)
Related
I have a structure of 26 tables which I want to load data from the database and get them inside the code. For example I use the Include method to join the related data of an entity:
var Application = dbContext.Applications
.Include(a => a.PwEmployee)
.Include(a => a.Promoters).ThenInclude(f => f.Addresses)
.Include(a => a.Promoters).ThenInclude(f => f.People)
... and many more
I don't want to include the generated query since it is not that readable and also not the problem here. What I want to do is use the generated query in a stored procedure and do this:
List<Application> Applications = await dbContext.Applications.FromSqlRaw("Exec GetFullApplication {0}", uuid).ToListAsync();
When I do this with the generated SQL query from EF Core, I get an exception:
System.ArgumentException: An item with the same key has already been added. Key: ID
When it works from EF Core to the database, shouldn't it work also the other way around with said SQL statement that EF Core generated? Is there any performance improvement to expect when doing this?
I have an app using EF 6 and MVC 5 that works fine for inputting data, but now when I try to display some of it I'm having troubles. The basic layout of my entities can be seen in the following diagram:
The first part where I'm having trouble is in querying and filtering the data. I would like to return a list of premises and related data where a survey and signoff exist, but an approval does not. In straight SQL, the query that works now is:
SELECT *
FROM Premises p LEFT OUTER JOIN Approvals a ON a.Id = p.Id
JOIN Surveys s ON s.PremiseId = p.Id
JOIN SignOffs so ON so.Id = s.Id
WHERE a.ApprovedBy IS NULL
The code that I started with is like this:
var premises = Premises.Include(p => p.Approval)
.Include(p => p.Surveys)
.Include(p => p.Surveys.Select(s => s.SignOff));
This appears* to return all records including the child data, but when I try to filter it so I get only records that have a signoff record but do not have an approval, it doesn't work.
var premises = Premises.Include(p => p.Approval).Where(p => p.Approval.ApprovedBy == null)
.Include(p => p.Surveys)
.Include(p => p.Surveys.Select(s => s.SignOff).Where(s => s.Signature != null));
If I use this code, I get this error:
The Include path expression must refer to a navigation property defined on the type. Use dotted paths for reference navigation properties and the Select operator for collection navigation properties.
Parameter name: path
I've changed this query around a lot to try different things, so I'm not sure what all I have done, but I think the first Where statement might work by itself, but the second one definetly causes the error.
How do I need to structure my query to get it to return the requested data properly filtered?
Also, I put an asterisk above on stating that the query appears to return all the data and child data because I can't actually test it. When I'm trying to write my Razor CSHTML page for this, it's not giving me intellisense for the child and grandchild data, and if I enter what I think it should be I get errors. How do I need to reference this data on the page?
You cannot use Include() like this, it is only good for specifying to load a navigation property, not to specify to load an entity when a navigation property is something (not null, in your case).
To do the filtering, I suggest something like this:
var premises = Premises.Include(p => p.Approval).Include(p => p.Surveys).Include(p => p.Surveys.Select(s => s.SignOff))
.Where(p=>p.Approval.ApprovedBy!=null && p.Surveys.Any(s=>s.SignOff.Signature!=null));
So basically, the includes and the filtering have nothing to do with each other. With the includes, you only specify what to load, you can still use the filter on the original entity set.
You're confusing what the Include LINQ method does. It only tells EF to eagerly load that relationship, which is actually unnecessary if your query itself utilizes that relationship; EF will include the relationship by default in that case.
What it doesn't do is allow you filter those relationships. For example, in this portion of your code:
.Include(p => p.Surveys.Select(s => s.SignOff).Where(s => s.Signature != null));
The where clause is applied to Premises, not SignOff as you seem to think. In other words, Where filters the main table being queried, not the table you're including.
There's two paths forward here. You can simply filter Premises by the important parts, i.e.:
var premises = Premises.Where(p => p.Approval.ApprovedBy == null && p => p.Surveys.Any(s => s.SignOff.Signature != null));
That will return only premises where these conditions are true, but the included Surveys collection will contain all surveys related to each premise, not just the ones with null signoff signatures.
If you need to filter the related items as well, then you must explicitly load them:
foreach (premise in premises)
{
context.Entry(premise)
.Collection(p => p.Surveys)
.Query()
.Where(s => s.SignOff.Signature != null)
.Load();
}
Two things of note:
Because of the nature of how this query must be applied, there's no way to do it once for all premises. You'll have to iterate over the premises and explicitly load the Surveys collection for each.
Since this will issue a new query, you want to avoid loading the Surveys collection either lazily or eagerly before this explict load. Otherwise, you're querying the same information twice, which is very inefficient. The easiest way to ensure that is to remove the virtual keyword from the collection property. However, if you do that, then you will have to eager or explicitly load the collection or it will be null. For more information, see: https://msdn.microsoft.com/en-us/library/jj574232(v=vs.113).aspx
I'm transfering my .NET Framework (EF6) code to ASP.NET Core (EF Core), and I stumbled upon this issue. Here is some example code:
In EF6 I use Include() and Select() for eager-loading:
return _context.Post
.Include(p => p.PostAuthor.Select(pa => pa.Author).Select(a => a.Interests))
PostAuthor is a junction table and there is also a Junction table "AuthorInterest" which I didn't need to involve in EF6 (Select goes straight to a.Interests).
Anyway, I can see that in EF7 this is reworked, meaning that I should use ThenInclude() for nested queries now. However...
return _context.Post
.Include(p => p.PostAuthor)
.ThenInclude(pa => pa.Select(pa2 => pa2.Author))
...etc
The above code fails because of the Select() statement. The documentation on https://docs.efproject.net/en/latest/querying/related-data.html seems to suggest that I don't need it and I can access Author immediately, but I get an ICollection in the last lambda displayed, so I obviously need the Select(). I go through multiple junction tables further on in the query, but for simplicity's sake, let's just focus on the first one.
How do I make this work?
but I get an ICollection in the last lambda displayed, so I obviously need the Select()
No, you don't. EF Core Include / ThenInclude totally replace the need of Select / SelectMany used in EF6. Both they have separate overloads for collection and reference type navigation properties. If you use the overload with collection, ThenInclude operates on the type of the collection element, so at the end you always end up with a single entity type.
In your case, pa should resolve to your junction table element type, so Author should be directly accessible.
For instance the EF6 include chain:
.Include(p => p.PostAuthor.Select(pa => pa.Author).Select(a => a.Interests))
translates to EF Core:
.Include(p => p.PostAuthor).ThenInclude(pa => pa.Author).ThenInclude(a => a.Interests)
I'm new to Fluent API. In my scenario, a Student can be in one Grade and a Grade can have many Students. Then, these two statements accomplish the same thing:
modelBuilder
.Entity<Student>()
.HasRequired<Grade>(s => s.Grade)
.WithMany(s => s.Students);
And:
modelBuilder
.Entity<Grade>()
.HasMany<Student>(s => s.Students)
.WithRequired(s => s.Grade);
My question is - how should I choose one statement over the other? Or do I need both statements?
For bidirectional relationship like yours (i.e. when both ends have navigation properties), it doesn't really matter, you can use one or the another (you can also use both, but it's not recommended because it's redundant and may lead to out of sync between the two).
It really matters when you have unidirectional relationship because only With methods have parameterless overloads.
Imagine you don't have Grade.Students property. Then you can use only:
modelBuilder.Entity<Student>()
.HasRequired(s => s.Grade)
.WithMany();
and if you don't have Student.Grade property, then you can use only:
modelBuilder.Entity<Grade>()
.HasMany(s => s.Students)
.WithRequired();
You just need one.This is more than enough for 1 : M relationship.
modelBuilder.Entity<Student>()
.HasRequired<Grade>(s => s.Grade) //Student entity requires Grade
.WithMany(s => s.Students); //Grade entity includes many Students entities
I've been looking at Applying filters when explicitly loading related entities and could not get it to work for a many-to-many relationship.
I created a simple model:
Brief description:
A Student can take many Courses and a Course can have many Students.
A Student can make many Presentation, but a Presentation can be made by only one Student.
So what we have is a many-to-many relationship between Students and Courses, as well as a one-to-many relationship between Student and Presentations.
I've also added one Student, one Course and one Presentation related to each other.
Here is the code I am running:
class Program
{
static void Main()
{
using (var context = new SportsModelContainer())
{
context.Configuration.LazyLoadingEnabled = false;
context.Configuration.ProxyCreationEnabled = false;
Student student = context.Students.Find(1);
context.
Entry(student).
Collection(s => s.Presentations).
Query().
Where(p => p.Id == 1).
Load();
context.
Entry(student).
Collection(s => s.Courses).
Query().
Where(c => c.Id == 1).
Load();
// Trying to run Load without calling Query() first
context.Entry(student).Collection(s => s.Courses).Load();
}
}
}
After loading the presentations I see that the count for Presentations changed from 0 to 1: . However, after doing the same with Courses nothing changes:
So I try to load the courses without calling Query and it works as expected:
(I removed the Where clause to further highlight the point - the last two loading attempts only differ by the "Query()" call)
Now, the only difference I see is that one relationship is one-to-many while the other one is many-to-many. Is this an EF bug, or am I missing something?
And btw, I checked the SQL calls for the last two Course-loading attempts, and they are 100% identical, so it seems that it's EF that fails to populate the collection.
I could reproduce exactly the behaviour you describe. What I got working is this:
context.Entry(student)
.Collection(s => s.Courses)
.Query()
.Include(c => c.Students)
.Where(c => c.Id == 1)
.Load();
I don't know why we should be forced also to load the other side of the many-to-many relationship (Include(...)) when we only want to load one collection. For me it feels indeed like a bug unless I missed some hidden reason for this requirement which is documented somewhere or not.
Edit
Another result: Your original query (without Include) ...
context.Entry(student)
.Collection(s => s.Courses)
.Query()
.Where(c => c.Id == 1)
.Load();
... actually loads the courses into the DbContext as ...
var localCollection = context.Courses.Local;
... shows. The course with Id 1 is indeed in this collection which means: loaded into the context. But it's not in the child collection of the student object.
Edit 2
Perhaps it is not a bug.
First of all: We are using here two different versions of Load:
DbCollectionEntry<TEntity, TElement>.Load()
Intellisense says:
Loads the collection of entities from
the database. Note that entities that
already exist in the context are not
overwritten with values from the
database.
For the other version (extension method of IQueryable) ...
DbExtensions.Load(this IQueryable source);
... Intellisense says:
Enumerates the query such that for
server queries such as those of
System.Data.Entity.DbSet,
System.Data.Objects.ObjectSet,
System.Data.Objects.ObjectQuery,
and others the results of the query
will be loaded into the associated
System.Data.Entity.DbContext,
System.Data.Objects.ObjectContext or
other cache on the client. This is
equivalent to calling ToList and then
throwing away the list without the
overhead of actually creating the
list.
So, in this version it is not guaranteed that the child collection is populated, only that the objects are loaded into the context.
The question remains: Why gets the Presentations collection populated but not the Courses collection. And I think the answer is: Because of Relationship Span.
Relationship Span is a feature in EF which fixes automatically relationships between objects which are in the context or which are just loaded into the context. But this doesn't happen for all types of relationships. It happens only if the multiplicity is 0 or 1 on one end.
In our example it means: When we load the Presentations into the context (by our filtered explicit query), EF also loads the foreign key of the Presentation entites to the Student entity - "transparently", which means, no matter if the FK is exposed as property in the model of not. This loaded FK allows EF to recognize that the loaded Presentations belong to the Student entity which is already in the context.
But this is not the case for the Courses collection. A course does not have a foreign key to the Student entity. There is the many-to-many join-table in between. So, when we load the Courses EF does not recognize that those courses belong to the Student which is in the context, and therefore doesn't fix the navigation collection in the Student entity.
EF does this automatic fixup only for references (not collections) for performance reasons:
To fix relationship, EF transparently
rewrites the query to bring
relationship info for all relations
which has multiplicity of 0..1 or1 on
the other end; in other words
navigation properties that are entity
reference. If an entity has
relationship with multiplicity of
greater then 1, EF will not bring back
the relationship info because it could
be performance hit and as compared to
bringing a single foreign along with
rest of the record. Bringing
relationship info means retrieving all
the foreign keys the records has.
Quote from page 128 of Zeeshan Hirani's in depth guide to EF.
It is based on EF 4 and ObjectContext but I think this is still valid in EF 4.1 as DbContext is mainly a wrapper around ObjectContext.
Unfortunately rather complex stuff to keep in mind when using Load.
And another Edit
So, what can we do when we want to explicitely load one filtered side of a many-to-many relationship? Perhaps only this:
student.Courses = context.Entry(student)
.Collection(s => s.Courses)
.Query()
.Where(c => c.Id == 1)
.ToList();