I have a table called Products and I need to find the products with unique title for a particular category. Earlier we used to do with this query in entity framework core 2.2 :
currentContext.Products
.GroupBy(x => x.Title)
.Select(x => x.FirstOrDefault()))
.Select(x => new ProductViewModel
{
Id = x.Id,
Title = x.Title,
CategoryId= x.CategoryId
}).ToList();
But after upgrading to Entity Framework Core 5.0, we get an error for Groupby Shaker exception:
The LINQ expression 'GroupByShaperExpression:KeySelector: t.title, ElementSelector:EntityShaperExpression: EntityType: Project ValueBufferExpression: ProjectionBindingExpression: EmptyProjectionMember IsNullable: False .FirstOrDefault()' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or 'ToListAsync'.
I know there are multiple way to client projection but I am searching for most efficient way to search.
Most likely that LINQ query couldn't be translated in EF Core 2.2 either, because of some limitations that the GroupBy operator has.
From the docs:
Since no database structure can represent an IGrouping, GroupBy operators have no translation in most cases. When an aggregate operator is applied to each group, which returns a scalar, it can be translated to SQL GROUP BY in relational databases. The SQL GROUP BY is restrictive too. It requires you to group only by scalar values. The projection can only contain grouping key columns or any aggregate applied over a column.
What happened in EF Core 2.x is that whenever it couldn't translate an expression, it would automatically switch to client evaluation and give just a warning.
This is listed as the breaking change with highest impact when migrating to EF Core >= 3.x :
Old behavior
Before 3.0, when EF Core couldn't convert an expression that was part of a query to either SQL or a parameter, it automatically evaluated the expression on the client. By default, client evaluation of potentially expensive expressions only triggered a warning.
New behavior
Starting with 3.0, EF Core only allows expressions in the top-level projection (the last Select() call in the query) to be evaluated on the client. When expressions in any other part of the query can't be converted to either SQL or a parameter, an exception is thrown.
So if the performance of that expression was good enough when using EF Core 2.x, it will be as good as before if you decide to explicitly switch to client evaluation when using EF Core 5.x. That's because both are client evaluated, before and now, with the only difference being that you have to be explicit about it now. So the easy way out, if the performance was acceptable previously, would be to just client evaluate the last part of the query using .AsEnumerable() or .ToList().
If client evaluation performance is not acceptable (which will imply that it wasn't before the migration either) then you have to rewrite the query. There are a couple of answers by Ivan Stoev that might get you inspired.
I am a little confused by the description of what you want to achieve: I need to find the products with unique title for a particular category and the code you posted, since I believe it's not doing what you explained. In any case, I will provide possible solutions for both interpretations.
This is my attempt of writing a query to find the products with unique title for a particular category.
var uniqueProductTitlesForCategoryQueryable = currentContext.Products
.Where(x => x.CategoryId == categoryId)
.GroupBy(x => x.Title)
.Where(x => x.Count() == 1)
.Select(x => x.Key); // Key being the title
var productsWithUniqueTitleForCategory = currentContext.Products
.Where(x => x.CategoryId == categoryId)
.Where(x => uniqueProductTitlesForCategoryQueryable .Contains(x.Title))
.Select(x => new ProductViewModel
{
Id = x.Id,
Title = x.Title,
CategoryId= x.CategoryId
}).ToList();
And this is my attempt of rewriting the query you posted:
currentContext.Products
.Select(product => product.Title)
.Distinct()
.SelectMany(uniqueTitle => currentContext.Products.Where(product => product.Title == uniqueTitle ).Take(1))
.Select(product => new ProductViewModel
{
Id = product.Id,
Title = product.Title,
CategoryId= product.CategoryId
})
.ToList();
I am getting the distinct titles in the Product table and per each distinct title I get the first Product that matches it (that should be equivalent as GroupBy(x => x.Title)+ FirstOrDefault AFAIK). You could add some sorting before the Take(1) if needed.
You can use Join for this query as below :
currentContext.Products
.GroupBy(x => x.Title)
.Select(x => new ProductViewModel()
{
Title = x.Key,
Id = x.Min(b => b.Id)
})
.Join(currentContext.Products, a => a.Id, b => b.Id,
(a, b) => new ProductViewModel()
{
Id = a.Id,
Title = a.Title,
CategoryId = b.CategoryId
}).ToList();
If you watch or log translated SQL query, it would be as below:
SELECT [t].[Title], [t].[c] AS [Id], [p0].[CategoryId] AS [CategoryId]
FROM (
SELECT [p].[Title], MIN([p].[Id]) AS [c]
FROM [Product].[Products] AS [p]
GROUP BY [p].[Title]
) AS [t]
INNER JOIN [Product].[Products] AS [p0] ON [t].[c] = [p0].[Id]
As you can see, the entire query is translated into one SQL query and it is highly efficient because GroupBy operation is being performed in database and no additional record is fetched by the client.
As mentioned by Ivan Stoev, EFC 2.x just silently loads full table to the client side and then apply needed logic for extracting needed result. It is resource consuming way and thanks that EFC team uncovered such potential harmful queries.
Most effective way is already known - raw SQL and window functions. SO is full of answers like this.
SELECT
s.Id,
s.Title,
s.CategoryId
FROM
(SELECT
ROW_NUMBER() OVER (PARTITION BY p.Title ORDER BY p.Id) AS RN,
p.*
FROM Products p) s
WHERE s.RN = 1
Not sure that EFC team will invent universal algorithm for generating such SQL in nearest future, but for special edge cases it is doable and maybe it is their plan to do that for EFC 6.0
Anyway if performance and LINQ is priority for such question, I suggest to try our adaptation of linq2db ORM for EF Core projects: linq2db.EntityFrameworkCore
And you can get desired result without leaving LINQ:
urrentContext.Products
.Select(x => new
{
Product = x,
RN = Sql.Ext.RowNumber().Over()
.PartitionBy(x.Title)
.OrderBy(x.Id)
.ToValue()
})
.Where(x => x.RN == 1)
.Select(x => x.Product)
.Select(x => new ProductViewModel
{
Id = x.Id,
Title = x.Title,
CategoryId = x.CategoryId
})
.ToLinqToDB()
.ToList();
Short answer is you deal with breaking changes in EF Core versions.
You should consider the total API and behavior changes for migration from 2.2 to 5.0 as I provided bellow:
Breaking changes included in EF Core 3.x
Breaking changes in EF Core 5.0
You may face other problems to write valid expressions using the newer version. In my opinion, upgrading to a newer version is not important itself. This is important to know how to work with a specific version.
You should use .GroupBy() AFTER materialization. Unfortunately, EF core doesn't support GROUP BY. In version 3 they introduced strict queries which means you can not execute IQeuriables that can't be converted to SQL unless you disable this configuration (which is not recommended). Also, I'm not sure what are you trying to get with GroupBy() and how it will influence your final result. Anyway, I suggest you upgrade your query like this:
currentContext.Products
.Select(x=> new {
x.Id,
x.Title,
x.Category
})
.ToList()
.GroupBy(x=> x.Title)
.Select(x => new Wrapper
{
ProductsTitle = x.Key,
Products = x.Select(p=> new ProductViewModel{
Id = p.Id,
Title = p.Title,
CategoryId= p.CategoryId
}).ToList()
}).ToList();
I have the following EF code
Func<CxForumArticle, bool> whereClause = a => a.CreatedBy == authorId;
IEnumerable<CxForumArticle> articlesCol = ctx.Articles
.Where(whereClause)
.Where(a => a.PublishingStatus == EnPublishStatus.PUBLISHED)
.OrderByDescending(a => a.ModifiedOn).Skip(offset).Take(pageSize);
It produces the following SQL
SELECT
[Extent1].[ArticleId] AS [ArticleId],
[Extent1].[Alias] AS [Alias],
[Extent1].[MigratedId] AS [MigratedId],
[Extent1].[Title] AS [Title],
[Extent1].[Teaser] AS [Teaser],
[Extent1].[ClobId] AS [ClobId],
[Extent1].[UnifiedContentId] AS [UnifiedContentId],
[Extent1].[EditorComments] AS [EditorComments],
[Extent1].[CreatedOn] AS [CreatedOn],
[Extent1].[CreatedBy] AS [CreatedBy],
[Extent1].[ModifiedOn] AS [ModifiedOn],
[Extent1].[ModifiedBy] AS [ModifiedBy],
[Extent1].[PublishingStatus] AS [PublishingStatus]
FROM [dbo].[ForumArticle] AS [Extent1]
As you see, there is no ordering and paging in this SQL. So EF orders and pages data in memory.
This doesn't seem to be a good thing to do.
I read an article, claiming that I have to use expression in OrderBy clause. I did that
Func<CxForumArticle, bool> whereClause = a => a.CreatedBy == authorId;
Expression<Func<CxForumArticle, DateTime>> orderByFunc = a => a.ModifiedOn;
IEnumerable<CxForumArticle> articlesCol = ctx.Articles
.Where(whereClause)
.Where(a => a.PublishingStatus == EnPublishStatus.PUBLISHED)
.OrderByDescending(orderByFunc.Compile()).Skip(offset).Take(pageSize)
;
But I got the same result. Any ideas how can I force EF to sort and page the data in DB?
I know this is pretty old, but you actually need to have the whereClause use an expression as well. Entity Framework only knows how to convert expressions to SQL. If you use a delegate instead of an expression, it is going to query the entire table and then then filter using LINQ to Objects.
I think I understand from LinqPad doc that it uses a different linq to tsql translator than Entity Framework. LinqPad's is actually more efficient in at least one case! Here's the details:
LinqPad generates the following simple sql query:
SELECT [t0].[personId]
FROM [Person] AS [t0]
WHERE (NOT (([t0].[deleted]) = 1)) AND ([t0].[masterPersonId] = #p0)
ORDER BY [t0].[personId] DESC
For this C# linq code:
int? state = null;
string realmId = null;
int? reviewerId = null;
bool? deleted = false;
long? parentPersonId = 1275660779659;
var query = from person in Persons
where
(!deleted.HasValue || person.Deleted == deleted) &&
(!state.HasValue || person.personState == state) &&
(!parentPersonId.HasValue || person.masterPersonId == parentPersonId) &&
(realmId == null || person.realmId == realmId) &&
(reviewerId == null ||(person.reviewerId == reviewerId ))
orderby person.personId descending
select person.personId;
So you can see LinqPad translates the linq statement above and removes extraneous sql when a parameter value is null. Nice!
EF however always generates this regardless of null parameters:
SELECT
[Extent1].[personId] AS [personId]
FROM [dbo].[Person] AS [Extent1]
WHERE (#p__linq__0 IS NULL OR [Extent1].[deleted] = #p__linq__1) AND
(#p__linq__2 IS NULL OR [Extent1].[personState] = #p__linq__3) AND
(#p__linq__4 IS NULL OR [Extent1].[masterPersonId] = #p__linq__5) AND
(#p__linq__6 IS NULL OR [Extent1].[realmId] = #p__linq__7) AND
((#p__linq__8 IS NULL) OR ([Extent1].[reviewerId] = #p__linq__9))
It makes for a slower query. We were hoping to use LinqPad to evaluate generated sql for EF, but obviously not if the results will be different. It looks like we can target a custom assembly for an EF connection in LinqPad. I'll play around with that to see if we can at least bring the sql queries together.
Anyone ever travel this road or know of an EF setting we can take advantage of? We're running EF4.
Thanks in advance.
I found this excellent webcast from the author of LinqPad where he mentions this very issue. http://oreilly.com/pub/e/1295
I was using Linq To SQL translation not EF's.
I had a LINQ query that loads a hierarchy of objects like the following.
Query #1
var result = db.Orders
.Include("Customer")
// many other .Include() here
.FirstOrDefault(x => x.Customer.CustomerId == 1 &&
x.OrderId == orderId);
I was having MAJOR performance problem with it.
The CPU usage was near 100% and memory usage was very high.
And I tweaked it to the following and the performance problem was fixed.
Query #2
var result = db.Orders
.Include("Customer")
// many other .Include() here
.Where(x => x.Customer.CustomerId == 1 &&
x.OrderId == orderId)
.FirstOrDefault();
I just want to confirm my suspicion.
Query #1 is probably looping through all my records in memory looking for a matching record
vs
Query #2 filters the records on the Database and then getting the first record only.
Is that why the Query #1 has performance problems?
Just to be safe, do I need to use the .Select(x => x) before the .FirstOrDefault()?
Query #3
var result = db.Orders
.Include("Customer")
// many other .Include() here
.Where(x => x.Customer.CustomerId == 1 &&
x.OrderId == orderId)
.Select(x => x)
.FirstOrDefault();
No, they both should result in a same SQL query when being executed. You can prove it by looking into SQL Profiler and see what is the exact SQL being submitted from EF in both cases. Your performance optimization should have been caused by some other factors. Here is why:
Include method returns an ObjectQuery<T>:
public class ObjectQuery<T> : ObjectQuery, IOrderedQueryable<T>,
IQueryable<T>, IEnumerable<T>,
IOrderedQueryable, IQueryable,
IEnumerable, IListSource
Which means its FirstOrDefault method comes with 2 overloads:
// Defined by Enumerable:
FirstOrDefault(Func<T, Boolean>)
// Defined by Queryable:
FirstOrDefault(Expression<Func<T, Boolean>>)
When you code .FirstOrDefault(x => x.Customer.CustomerId == 1 compiler will go into a process called Overload Resolution to infer the type of the lambda expression x => x.Customer.CustomerId == 1 since it is convertible to the type of both overload's parameter types.
Compiler will use an algorithm (that I am still trying to find in C# Language Specification!), figure out that converting the lambda to the Expression<Func<T, Boolean> is a better conversion than to Func<T, Boolean> so pick the IQueryable overload.
Therefore, you'll see the predicate in the generated SQL when observing it in the SQL Profiler.
I found the culprit. It's the SQL query generated by Entity Framework.
I have a complicated Schema with a lot of many-to-many relationships.
Entity Framework was generating a 32,000 line long SQL string :'(
Now, I am changing my code to load the hierarchy manually for some part.
Please let me know if anyone knows some good articles to read about Eager Loading and Many-to-Many relationships.
I think best would be to use ...Where(condition).Take(1).FirstOrDefault() because Take(1) can be easily translated to SQL as a TOP clause. Anybody with me?
I have a problem with many-to-many relation on EF4 and npgsql provider.
I have 3 tables: Order, OrderStatus and OrderStatusDict (dictionary). OrderStatus is some kind of changeLog - every order's status change is added to the OrderStatus table with its actual date and new status. I need to get all orders, which status.OrderStatusDict.OrderStatusName == "Pending", so im doing this this way:
var lst = (from o in db.Order
where o.OrderStatus
.OrderByDescending(s => s.Date)
.FirstOrDefault()
.OrdereStatusDict.OrderStatusName == "Pending"
select o).ToList();
And I get an exception:
An error occurred while preparing the
command definition. See the inner
exception for details.
Data:{System.Collections.ListDictionaryInternal}
Inner exception: {"The method or
operation is not implemented."}
And it looks that OrderByDescending kills my query, cause if I comment
.OrderByDescending(s => s.Date)
Everything works fine, but I get the oldest OrderStatus from db :/
Can u help me? Is this a provider's cause or EF problem? Do you have any ideas I could gain this other way?
I wouldn't write the query that way.
Try:
var lst = (from o in db.Order
let maxDate = o.OrderStatus.Max(s => s.Date)
where o.OrderStatus.Any(s => s.Date == MaxDate
&& s.OrdereStatusDict.OrderStatusName.Equals("Pending", StringComparison.OrdinalIgnoreCase))
select o).ToList();
I'm guessing the status comparison should be case-insensitive.
Note that my query behaves differently than yours if the max date is not unique. I'm guessing it's either unique or that my way is right.