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How to GroupBy data from many tables in dotnet core Entity Framework

I'm building some marketplace web app, let's say something like e-bay. Typical scenario is:
User makes offer which consists of one or more items and those items are of certain type.After that other users are bidding on that offer.
Here is simplified diagram.
On SQL Fiddle (here) you can see both CREATE TABLE and INSERT INTO statements
Sample data:
There are two offers. On one offer (Id 1) which consists of one item which is type of "watch". There is another offer, (Id 2), which has one item which is of type "headphone".
On both offers there are bids. On watch, there are two bis; one bid with 100 dollars and another with 120. On headphones, there are bids with 50 and 80 dollars.
What I want to achieve is to have average bid per type. In this sample, that means i want to get 110 as average bid for watch and 65 as average bid for headphone. To achieve that using T-SQL, I would write query like this:
SELECT t.name,
avg(amount)
FROM bid b
LEFT JOIN offer o ON b.OfferId = o.id
LEFT JOIN offeritem oi ON o.id = oi.OfferId
LEFT JOIN itemType t ON oi.itemtypeid = t.Id
GROUP BY t.name
So, my question is - how to achieve that in dotnet core 3.0 EntityFramework
Using GroupBy, like this:
_context.Bids
.Include(b => b.Offer)
.ThenInclude(o => o.OfferItems)
.ThenInclude(os => os.ItemType)
.GroupBy(b => b.Offer.OfferItems.First().ItemType.Name);
gives exception:
Client side GroupBy is not supported.
. When I try with projection, like this:
_context.Bids
.Include(b => b.Offer)
.ThenInclude(o => o.OfferItems)
.ThenInclude(os => os.ItemType)
.GroupBy(b => b.Offer.OfferItems.First().ItemType.Name)
.Select(g => new
{
Key = g,
Value = g.Average(b => b.Amount)
});
i get exception again.
Processing of the LINQ .... failed. This may indicate either a bug or
a limitation in EF Core.
EDIT:
This approach
_context.Bids
.Include(b => b.Offer)
.ThenInclude(o => o.OfferItems)
.ThenInclude(os => os.ItemType)
.GroupBy(b => new { b.Offer.OfferItems.First().ItemType.Name}, b => b.Amount)
.Select(g => new
{
Key = g.Key.Code,
Value = g.Average()
});
also threw an exception, but this time:
Cannot use an aggregate or a subquery in an expression used for the
group by list of a GROUP BY clause.
...
So, is there a way to group that data (get simple Average) or should I make another query and iterate throught collection and make calculation myself? That would lower performance for sure (I was hoping I can do server grouping, but as you can see, i got into mentioned issues). Any ideas? Thanks in advance.

In your case it is hard to hide subquery from grouping
You can try it in such way
var joined =
context.Bid
.SelectMany(x =>
x.Offer.OfferItem
.Select(y => new
{
Amount = x.Amount,
Name = y.ItemType.Name
})
.Take(1));
var grouped = from i in joined
group i by i.Name into groups
select new
{
Key = groups.Key,
Amount = groups.Average(x => x.Amount)
};
it gives me a query
SELECT [t].[Name] AS [Key], AVG([t].[Amount]) AS [Amount]
FROM [Bid] AS [b]
INNER JOIN [Offer] AS [o] ON [b].[OfferId] = [o].[Id]
CROSS APPLY (
SELECT TOP(1) [b].[Amount], [i].[Name], [o0].[Id], [i].[Id] AS [Id0], [o0].[OfferId]
FROM [OfferItem] AS [o0]
INNER JOIN [ItemType] AS [i] ON [o0].[ItemTypeId] = [i].[Id]
WHERE [o].[Id] = [o0].[OfferId]
) AS [t]
GROUP BY [t].[Name]

EF Core 2.0 Group By other properties

I have 2 tables:
USERS
UserId
Name
Scores (collection of table Scores)
SCORES
UserId
CategoryId
Points
I need to show all the users and a SUM of their points, but also I need to show the name of the user. It can be filtered by CategoryId or not.
Context.Scores
.Where(p => p.CategoryId == categoryId) * OPTIONAL
.GroupBy(p => p.UserId)
.Select(p => new
{
UserId = p.Key,
Points = p.Sum(s => s.Points),
Name = p.Select(s => s.User.Name).FirstOrDefault()
}).OrderBy(p => p.Points).ToList();
The problem is that when I add the
Name = p.Select(s => s.User.Name).FirstOrDefault()
It takes so long. I don't know how to access the properties that are not inside the GroupBy or are a SUM. This example is very simple becaouse I don't have only the Name, but also other properties from User table.
How can I solve this?

It takes so long because the query is causing client evaluation. See Client evaluation performance issues and how to use Client evaluation logging to identify related issues.
If you are really on EF Core 2.0, there is nothing you can do than upgrading to v2.1 which contains improved LINQ GroupBy translation. Even with it the solution is not straight forward - the query still uses client evaluation. But it could be rewritten by separating the GroupBy part into subquery and joining it to the Users table to get the additional information needed.
Something like this:
var scores = db.Scores.AsQueryable();
// Optional
// scores = scores.Where(p => p.CategoryId == categoryId);
var points = scores
.GroupBy(s => s.UserId)
.Select(g => new
{
UserId = g.Key,
Points = g.Sum(s => s.Points),
});
var result = db.Users
.Join(points, u => u.UserId, p => p.UserId, (u, p) => new
{
u.UserId,
u.Name,
p.Points
})
.OrderBy(p => p.Points)
.ToList();
This still produces a warning
The LINQ expression 'orderby [p].Points asc' could not be translated and will be evaluated locally.
but at least the query is translated and executes as single SQL:
SELECT [t].[UserId], [t].[Points], [u].[UserId] AS [UserId0], [u].[Name]
FROM [Users] AS [u]
INNER JOIN (
SELECT [s].[UserId], SUM([s].[Points]) AS [Points]
FROM [Scores] AS [s]
GROUP BY [s].[UserId]
) AS [t] ON [u].[UserId] = [t].[UserId]

LINQ: Select the Min and Max values from a collection on an entity after grouping

The goal is to get the first DateTime and Last DateTime from a collection on an Entity (Foreign Key). My Entity is an organization and my collection are Invoices. I'm grouping results since Organizations unfortunately are not Unique. I'm dealing with duplicate data and cannot assume my organizations are unique so I'm grouping by a Number field on my Entity.
I'm using .NET Core 2.1.2 with Entity Framework.
I'm trying to get the following query generated from LINQ:
SELECT MIN([organization].[Id]) AS Id, MIN([organization].[Name]) AS Name,
MIN([organization].[Number]) AS Number, MIN([invoice].[Date])
AS First, MAX([invoice].[Date]) AS Last
FROM [organization]
INNER JOIN [invoice] ON [invoice].[OrganizationId] = [organization].[Id]
GROUP BY [organization].[Number], [organization].[Name]
ORDER BY [organization].[Name]
However I have no idea how to get to write the LINQ query to get it to generate this result.
I got as far as:
await _context
.Organization
.Where(z => z.Invoices.Any())
.GroupBy(organization => new
{
organization.Number,
organization.Name
})
.Select(grouping => new
{
Id = grouping.Min(organization => organization.Id),
Name = grouping.Min(organization => organization.Name),
Number= grouping.Min(organization => organization.Number),
//First = ?,
//Last = ?
})
.OrderBy(z => z.Name)
.ToListAsync();
I have no clue how to write the LINQ query in such a way that it generates the above.
I have a couple questions still:
Are the Min statements for Id, Name and Number correct ways of getting the
first element in the grouping?
Do I need a join statement or is "WHERE EXISTS" better (this got generated before I changed the code)?
Does anyone know how to finish writing the LINQ statement? Because I have to get the first and last Date from the Invoices Collection on my Organization Entity:
organization.Invoices.Min(invoice => invoice.Date)
organization.Invoices.Max(invoice => invoice.Date)

Here is the trick.
To make inner join by using collection navigation property simple use SelectMany and project all primitive properties that you need later (this is important for the current EF Core query translator). Then perform the GroupBy and project the key properties / aggregates. Finally do the ordering.
So
var query = _context
.Organization
.SelectMany(organization => organization.Invoices, (organization, invoice) => new
{
organization.Id,
organization.Number,
organization.Name,
invoice.Date
})
.GroupBy(e => new
{
e.Number,
e.Name
})
.Select(g => new
{
Id = g.Min(e => e.Id),
Name = g.Key.Name,
Number = g.Key.Number,
First = g.Min(e => e.Date),
Last = g.Max(e => e.Date),
})
.OrderBy(e => e.Name);
is translated to
SELECT MIN([organization].[Id]) AS [Id], [organization].[Name], [organization].[Number],
MIN([organization.Invoice].[Date]) AS [First], MAX([organization.Invoice].[Date]) AS [Last]
FROM [Organization] AS [organization]
INNER JOIN [Invoice] AS [organization.Invoice] ON [organization].[Id] = [organization.Invoice].[OrganizationId]
GROUP BY [organization].[Number], [organization].[Name]
ORDER BY [organization].[Name]

Convert SQL query with multiple GroupBy columns to LINQ

SELECT
[TimeStampDate]
,[User]
,count(*) as [Usage]
FROM [EFDP_Dev].[Admin].[AuditLog]
WHERE [target] = '995fc819-954a-49af-b056-387e11a8875d'
GROUP BY [Target], [User] ,[TimeStampDate]
ORDER BY [Target]
My database table has the columns User, TimeStampDate, and Target (which is a GUID).
I want to retrieve all items for each date for each user and display count of entries.
The above SQL query works. How can I convert it into LINQ to SQL? Am using EF 6.1 and my entity class in C# has all the above columns.
Create Filter basically returns an IQueryable of the entire AuditLogSet :
using (var filter = auditLogRepository.CreateFilter())
{
var query = filter.All
.Where(it => it.Target == '995fc819-954a-49af-b056-387e11a8875d')
.GroupBy(i => i.Target, i => i.User, i => i.TimeStamp);
audits = query.ToList();
}
Am not being allowed to group by on 3 columns in LINQ and I am also not sure how to select like the above SQL query with count. Fairly new to LINQ.

You need to specify the group by columns in an anonymous type like this:-
var query = filter.All
.Where(it => it.Target == '995fc819-954a-49af-b056-387e11a8875d')
.GroupBy(x => new { x.User, x.TimeStampDate })
.Select(x => new
{
TimeStampDate= x.Key.TimeStampDate,
User = x.Key.User,
Usage = x.Count()
}).ToList();

Many people find query syntax simpler and easier to read (this might not be the case, I don't know), here's the query syntax version anyway.
var res=(from it in filter.All
where it.Target=="995fc819-954a-49af-b056-387e11a8875d"
group it by new {it.Target, it.User, it.TimeStampDate} into g
orderby g.Key.Target
select new
{
TimeStampDate= g.Key.TimeStampDate,
User=g.Key.User,
Usage=g.Count()
});
EDIT: By the way you don't need to group by Target neither OrderBy, since is already filtered, I'm leaving the exact translation of the query though.

To use GroupBy you need to create an anonymous object like this:
filter.All
.Where(it => it.Target == '995fc819-954a-49af-b056-387e11a8875d')
.GroupBy(i => new { i.Target, i.User, i.TimeStamp });

It is unnecessary to group by target in your original SQL.
filter.All.Where( d => d.Target == "995fc819-954a-49af-b056-387e11a8875d")
.GroupBy(d => new {d.User ,d.TimeStampDate} )
.Select(d => new {
User = d.Key.User,
TimeStampDate = d.Key.TimeStampDate,
Usage = d.Count()
} );

Linq select with join

I have a basic selection task to be carried out
var query = db.Candidate.Where(.....
but I want the where to be on another table called Tag
in sql it would be
select * from Candidate
join Tag on Tag.candidateId = Candidate.tagId and Tag.tagId = 7
Thus getting all Candidates with tag 7
Im trying to do this directly with a Where, is this possible or do I need to use Linq to SQL.

You can use a Join in LINQ to do that:
var candidates = from candidate in db.Candidates
join on tag in db.Tags
where candidate.CandidateId == tag.TagId
select candidate

Try doing it with .Join as shown below :-
var query = db.Candidate.Join(db.Tags, r => r.tagId , p => p.tagId , (r,p) => new{r.Name});

You didn't show the relations, but it must be possible to do something like:
var query = db.Candidate
.Include(c => c.Tags)
.Where(c => c.Tags.Any(t => t.ID == 7))
Just query the candidates where a candidate has any tag that has the ID you're looking for. This will be written out more or less as the SQL you show.
You can omit the Include() if you have lazy loading enabled.