Quick question regarding this issue.
I have an error unable to get the data from database,
Is there something issue with EF Core or my linq query?
var query = _postRepository.Table
.Where(x => x.Approval == true)
.Include(p => p.Industry)
.GroupBy(grp => new { grp.Industry.Id, grp.Industry.Name })
.Select(s => new JobPostStatistics
{
IndustryId = s.Key.Id,
IndustryName = s.Key.Name,
TotalJobPost = s.Count()
});
return query.Take(6).ToList();
InvalidOperationException: Sequence contains more than one element
Seems in EF RC1 GroupBy cause error, here is reported bug:
GroupBy query cause error Sequence contains more than one element
EF Core 2.1 will not only work, but it will also be capable of translating GroupBy to SQL GROUP BY. The expected release date is Q4 2017. Previous versions also support GroupBy but execute it on the client side, which can be a huge problem if you're using it to retrieve aggregates, like SUM, from the database. Imagine retrieving 2 million rows from the server to calculate the sum on the client side, instead of getting it directly from the database server.
In the meantime, you can always write your own SQL query and map it to an entity or anonymous type, using DbSet.FromSql, i.e. invoking FromSql as a member of an entity collection of your context.
Here you have enabled eager loading by using include keyword. You try with disable lazy loading
Server-side GroupBy-support of EF Core has been postponed to after the release: https://github.com/aspnet/EntityFramework/issues/2341
When you log the queries EF creates you can see that it does not create GROUP BY in the SQL.
Here somebody else posted the same error with another GroupBy query.
https://github.com/aspnet/EntityFramework/issues/4775
I had another error with GroupBy and Include(). As EF does not support GroupBy in the SQL and the grouping is done on the client side, I split my query into a server side part and a client side part. The first part queries from the database with the Include() into an anonymous type. Then I do the grouping on the client side.
var event2Country = DbContext.Event2Countries
.Include(j => j.Location)
.Select(j => new { j.Location.Id, j.Location.Name })
.AsNoTracking();
viewModel.MostUsedCountries = event2Country
.GroupBy(j => new { j.Id, j.Name })
.OrderByDescending(j => j.Count())
.ThenBy(j => j.Key.Id)
.Select(j => new SimpleListCountViewModel()
{
Id = j.Key.Id,
Name = j.Key.Name,
Count = j.Count()
})
.Take(15)
.ToList();
Currently you cannot use GroupBy in EF Core if you have a lot of data. A workaround would be to use FromSql. See issue 4959 on Github.
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?
The property is defined as virtual. But before I access the order property, the order entity's data has been loaded, why?
Full source code:
A couple of things:
When you state: doesn't work, you are getting an Order back, but the $ figure is 0.0 when you expect a different value? You appear have 2x order records, but based on what is coming back you're expecting a non-zero figure, are both records non-zero? In your debug view, expand the "Orders" in the pop-up context menu, this will reveal what order details EF has loaded.
Firstly you should be careful around the use of "OrDefault" renditions of the methods. Your code is assuming that a value is returned. In these cases you're better off using Single() or First() as applicable.
Additionally, when using First you should be specifying an OrderBy clause to ensure that there is a reliable ordering.
SaveChanges Should only be called if you modify data.
Lastly, lazy loading is an enabler for loading infrequently used data on demand. You should largely work to avoid relying on lazy-load calls. If you need the entire entities and you know you are going to use orders, then eager-load them.
I.e.
using (var context = new EfContext())
{
var customer = context.Customers
.Include(c => c.Orders)
.Single(c => c.CustomerId = customerId);
// Do stuff...
}
If you want just 1 applicable order for a given employee then consider using Select to retrieve it:
I.e.
using (var context = new EfContext())
{
var data = context.Customers.Where(c => c.CustomerId = customerId)
.Select(c => new { Customer = c, FirstOrder = c.Orders.OrderBy(o => o.OrderDate).FirstOrDefault()})
.Single();
// Do Stuff...
}
This will give you an anonymous type containing the Customer (without eager loading the orders) and the matching first order.
Better is just to use Select to retrieve the specific fields you need from customer and order(s). This reduces the amount of data (rows and columns) pulled back from the database.
sorry,It looks like the problem of visual studio，When the call statement is commented,the order table's sql querie no longer being executed,but when the call statement is included,although the breakpoint is set,the order table's sql querie still be executed.
visual studio debug execute the Expression automatically
comment the staement
sql profiler
include the statement
sql profiler
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 am trying to make a left join with Linq to Entities in EF and eager load a collection in one of the entities.
The left join works with this code:
var joinedResult = context.Set<Client>()
.GroupJoin(
context.Set<Project>(),
client => client.Id,
project => project.ClientId,
(client, projects) => new
{
Client = client,
Project = projects.FirstOrDefault()
})
.ToList();
Now the Client entity has a Categories collection I want to load. Since this is a projection, I cannot include the collection before performing the group join, neither after the join since I am creating an anonymous type.
I tried to add the collection in the anonymous type as suggested by this anwser to a similar question:
ClientCategories = client.Categories.Select(s => s.Name),
but i keep getting an ObjectDisposedException when accessing the collection from the Client object outside the business layer.
Accessing the field directly after the query before returning from the business layer like this:
joinedResult.ForEach(result => result.Client.Categories.Count() /* NOP */);
works, but of course it results in a new database query per result.
How should I proceed to load this collection ?
Simple question, is it possible to achieve this query this with Entity Framework when updating one entity?
update test set value = value + 1 where id = 10
Use the Batch Update feature of the Entity Framework Extended Library, like this:
dbContext.Tests.Update(t => t.Id == 10, t => new Test() { Value = t.Value + 1 });
Not really under this form no.
You will have to select all entities that match your criteria, foreach over them and update them.
If you are looking for something that will do it right in the DB because your set could be huge, you will have to use SQL directly. (I don't remember if EF has a way to execute UPDATE queries directly the way Linq To SQL does).
It should be, it will just be a little bit more constrained generally.
var myEntity = context.First(item => item.id == 10);
myEntity.value += 1;
context.SaveChanges();
Should produce similar SQL, you can watch the profiler to see what SQL is actually being generated, but it should be very similar to your statement.