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Get the id of the latest record in group by with linq

I'm trying to get the latest record for a group in linq but I want the id, not the date as sometimes the dates can be exactly the same as other records.
The following code gives me the key and the last date
var latestPositions = from bs in Scan
group bs by bs.Asset into op
select new
{
Asset = op.Key,
LastSeen = op.Max(x => x.LastSeen)
};
Returns something like this...
Asset LastSeen
BS1 2020-05-10
BS2 2020-07-10
Which is what I need, but I then need to get to the rest of the data from that table row, but if I join it on the two columns I can get multiple rows, is there a way for me to return the id column in the group by, so I can join on that?
Thanks

GroupBy cannot help here because of SQL limitation. But you can write workaround
var latestPositions = from bs in Scan
where !Scan.Any(s => s.Asset == bs.Asset && s.LastSeen > bs.LastSeen)
select bs;
But I have to mention that fastest way is using window functions which are not available in EF Core:
SELET
sc.Id
FROM (
SELECT
s.Id,
ROW_NUMBER() OVER (PARTITION BY s.Asset ORDER BY s.LastSeen DESC) AS RN
FROM Scan s
) sc
WHERE sc.RN == 1
But there is exists EF Core extension linq2db.EntityFrameworkCore which makes it possible via LINQ (I assume that Asset is just ID, not a navigation property)
var queryRn = from bs in Scan
select new
{
Entity = bs,
RN = Sql.Ext.RowNumber().Over()
.PartitionBy(bs.Asset).OrderByDesc(bs.LastSeen).ToValue()
};
// switch to alternative translator
queryRn = queryRn.ToLinqToDB();
var latestPositions = from q in queryRn
where q.RN == 1
select q.Entity;

I think I did what you wanted above and I wrote the full code on this link
If it's not what you want, can you write what you want a little more clearly.
var temp = from l in list
group l by l.Asset into lg
orderby lg.Key
select new { Asset = lg.Key, LastSeen = lg.Max(x => x.LastSeen), ID = lg.Where(x => x.LastSeen == lg.Max(y => y.LastSeen)).Single().ID };

So every Scan has a property Asset, a DateTime property LastSeen, and zero or more other properties.
Requirement: given a sequence of Scans, give me per Asset (all or some of the ) properties of the LastSeen Scan.
The following will do the trick:
var lastSeenScan = dbContext.Scans.GroupBy(scan => scan.Asset,
// parameter resultSelector: take every Asset, and all Scans that have this Asset value
// and order these Scans by descending value of lastSeen:
(asset, scansWithThisAssetValue) => scansWithThisAssetValue
.OrderByDescending(scan => scan.LastSeen)
// Select the scan properties that you plan to use.
// Not needed if you want the complete Scan
.Select(scan => new
{
Id = scan.Id,
Operator = scan.Operator,
...
})
// and keep only the First one, which is the Last seen one:
.FirstOrDefault();
In words: divide your table of of Scans into groups of scans that have the same value for property Asset. Order all scans in each group by descending value of LastSeen. This will make the Scan that has last been seen the first one.
From all scans in the group select only the properties that you plan to use, and take the first one.
Result: for every used Asset, you get the (selected properties of the) scan that has the highest value of LastSeen.

How do I convert this statement into linq?

How can I use Sum() in Linq?
select sum(op.Quantity),
sum(p.UnitPrice),
sum(p.Discount),
o.CreateAt
from OrderProduct op join
[Order] o on o.ID = op.OrderID join
Product p on p.ID = op.ProductID
group by o.CreateAt

Instead of just writing a SQL statement, it would be more helpful to define your goal. You have some tables and you want to extract information from it. Which information do you want? Now I have to guess your structure.
It seems you have Orders and Products and OrderProducts.
Every Order has zero or more OrderProducts; every OrderProduct belongs to exactly one Order. Every Order also has a CreateAt property.
Every Product has zero or more OderProducts; every OrderProduct belongs to exactly one Product. Every Product also has a UnitPrice and a Discount.
Finally every OrderProduct has a property Quantity
You didn't mention whether you are using entity framework or not. But even if not, you have access to the three tables using IQueryables. In Entity Framework this is done via the DbSets:
IQueryable<Product> products = ...
IQueryable<Order> orders = ...
IQueryable<OrderProduct> orderProducts = ...
The query in baby steps:
var joinedItems = from product in Products
join orderProduct in orderProducts on oderProduct.ProductId == product.Id
join order in orders on order.Id == orderProduct,OrderId
select new
{
Quantity = orderProduct.Quantity,
UnitPrice = product.UnitPrice,
Discount = product.Discount,
CreateAt = order.CreateAt,
};
Multiple joins is the only time I use query syntax instead of method syntax.
Click here to see the method syntax for multiple joins
Because every product has only one UnitPrice, I assume you don't want the sum of all UnitPrice of one Product, but you want the sum of all Unitprices of items created on CreateAt, and similarly the sum of all Discounts and Quantities.
So we first group all joinedItems into joinedItems with the same CreateAt:
var itemsGroupedByCreateAt = joinedItems
.GroupBy(joinedItem => joinedItem.CreateAt);
In words: take all joinedItems and group them into groups where all joinedItems in the group have the same value for CreateAt.
The result is a sequence of Groups. Every group has joinedItems. All joinedItems in the group have the same value for CreateAt. this common value is the Key of the group.
So all you have to do is for every group to sum the UnitPrices of all joinedItems in the group. Do the same for the Quantity and Discount. Note that every group will create one sumResult:
var sumResults = itemsGroupedByCreateAt
.Select(group => new
{
TotalQuantity = group
.Select(groupElement => groupElement.Quantity)
.Sum(),
TotalUnitPrice = group
.Select(groupElement => groupElement.UnitPrice)
.Sum(),
Totaldiscount = group
.Select(groupElement => groupElement.UnitPrice)
.Sum(),
// only if you need it in your result:
CreateAt = group.Key,
});
In words: take all your groups of joinedItems. From every group, take the joinedItems in the group. From these joinedItems take only the Quantities, and Sum them. Put the result in totalQuantity.
Do the same for the UnitPrices and the Discounts
Of course you can do this all in one big linq statement. Not sure whether this would improve readability and maintainability.

Custom Transformer for raw SQL query using NHibernate

I have a table of customerorders that are tied to a table of purchases. Every order can have multiple purchases, it's peculiar but imagine it as if you paid for each item separately at checkout. Every order has a customer name and date, every purchase has a payment type and total on them.
I have a nifty query that provided you know the name of the customer, you can find their most recent unique purchase types.
For example:
Customer A made 3 orders total, 2 via credit card and 1 via cash.
I ask the database "what's the newest unique orders by payment type for A?" Database returns 2 results - the most recent credit card order & the 1 cash order.
This is query:
String sqlQueryStr = $#"SELECT ee.PaymentType, ee.Total
FROM
(
SELECT e.CustomerName, ee.PaymentType, ee.Total, MAX(e.Date) as MaxTimestamp
FROM customerorders ee
INNER JOIN purchases e ON e.Id=ee.OrderId WHERE CustomerName='{customerName}'
GROUP BY 1, 2, 3
) AS sub
INNER JOIN purchases e ON e.Date = sub.MaxTimestamp AND e.CustomerName = sub.CustomerName
INNER JOIN customerorders ee ON ee.OrderId=e.Id AND ee.PaymentType = sub.PaymentType;"
ISQLQuery query = session.CreateSQLQuery(sqlQueryStr);
return query.SetResultTransformer(new AliasToBeanResultTransformer(typeof(Purchase)))
.List<Purchase>();
This works great on its own. The result would be as follows for Customer A, and I have what I want:
'PaymentType: Credit Total:100'
'PaymentType: Cash Total:50'
However, now I want to do something where I don't provide 'customerName'. I want to get everyone's in the same way.
Now if you recall what I said before, Purchase does not have a CustomerName. So I can't just remove the WHERE and use the transformer anymore!
When I remove the 'WHERE' and add an additional SELECT for e.CustomerName, I get this output in MySQL using the query:
'CustomerName: A PaymentType: Credit Total:100'
'CustomerName: A PaymentType: Cash Total:50'
'CustomerName: B PaymentType: Credit Total:20'
'CustomerName: C PaymentType: Credit Total:15'
I was thinking of making a custom transformer, but I'm not sure what kind of transformer will allow me to process this kind of result. It's now not not just a Purchase, it's got the Name also. And I would probably want them grouped together right (not on different rows)?

public sealed class CustomTransformer: IResultTransformer
{
public IList TransformList(IList collection)
{
return collection;
}
public object TransformTuple(object[] tuple, string[] aliases)
{
return new UniqueCustomerPurchase()
{
CustomerName = (string)tuple[0],
PaymentType = (string)tuple[1],
Total = (uint)tuple[2]
};
}
}
This is what I have at the moment. This seems to work well, however, I wish there was a way to group the CustomerName to a list of Purchases(paymentType & total) instead of having to do this. I end up having to iterate over the collection a second time to group them like so:
ISQLQuery query = session.CreateSQLQuery(sqlQueryStr);
return query.SetResultTransformer(new CustomTransformer())
.List<UniqueCustomerPurchase>()
.GroupBy(cp => cp.CustomerName)
.Select(g => new KeyValuePair(g.Key, g.Select(cp => cp));

Unable to retrieve Distinct record from database?

I have a Database table like below,
ID Author Book
1 Kate Smuggler
2 Harper Smuggler
3 Slater Smuggler
4 Kate Katy
5 Smitha Katy
i want to retrieve all Book names(Distinct) based on the author name and display them on list view, i wrote the code below to Retrieve distinct values
int UserID = Convert.ToInt32(Session["User"]);
var Li = (from s in Data.TableName where s.Author == UserID select s.Book).Distinct().OrderBy(ID=>ID);
ListViewChatList.DataSourceID = "";
ListViewChatList.DataSource = Li;
ListViewChatList.DataBind();
but i get the following error 'System.String' does not contain a property with the name 'ID'.
How to solve this ?

This part:
select s.Book
is selecting just the book (title) part of the record. You can't get back from that to the ID... there can easily be multiple IDs for the same title, as per your sample data.
It's not clear what you'd want that query to return anyway, given that you're getting distinct book titles - ignoring the author, would you want IDs 1 and 4? 2 and 5? 3 and 5? Both represent the same set of distinct titles (Smuggler and Katy).
You could potentially group by name, and then do whatever you want with the IDs for the books with the shared name - but you really need to consider what your distinctness criterion means for the results.
Here's a query which will give you the lowest ID for each name:
var query = from s in Data.TableName
where s.Author == UserID
group s.ID by s.Book into g
select g.OrderBy(id => id).First();
It's not clear whether that's what you really need though.

var Li = from s in Data.TableName
where s.Author == User
orderby s.ID
select s.Book;

You're only selecting the Book column (a string), and then trying to order by ID, which you no longer have access to, and which certainly is not a property of the Book string.
Assume this is your data:
ID Author Book
1 Kate Smuggler
2 Kate Smuggler2
3 Kate Smuggler
4 Frey Katy
5 Smitha Katy
If you do the following, you'll get the first three books listed above:
(from s in Data.TableName where s.Author == "Kate" select s.Book);
Then you do a distinct, which reduces the books to the two unique ones (Smuggler and Smuggler2):
(...).Distinct();
Then you try to order by the ID, but you didn't originally select it so you don't have access to it. And you've taken a Distinct on the results, so you've lost some of the detail.
Since the two "Smuggler" books have been reduced to one, but their IDs were 1 and 3 (in my example), what should ordering them do? Should the single "Smuggler" book appear before "Smuggler2" or after?

Get the first record of a group in LINQ? [duplicate]

This question already has answers here:
How to get first record in each group using Linq
(7 answers)
Closed 5 years ago.
Summary: How to get top 1 element in ordered groups of data
I am trying to group by a CarId field , and then within each group, I want to sort on a DateTimeStamp field descending. The desired data would be for each Car give me the latest DateTimeStamp and only that 1 in the group.
I can get to this point, but having problems taking the top 1 off of the group and ordering the group by DateTimeStamp desc.
Here is what I have after the first group operation:
group 1
------------------------
CarId DateTimeStamp
1 1/1/2010
1 1/3/2010
1 3/4/2010
group 2
------------------------
CarId DateTimeStamp
2 10/1/2009
2 1/3/2010
2 9/4/2010
what I would desire only the top 1 in an ordered group
group 1
------------------------
CarId DateTimeStamp
1 3/4/2010
group 2
------------------------
CarId DateTimeStamp
2 9/4/2010
Brickwall: Where I get stopped, is needing the CarId and DateTimeStamp in the group by clause, in order to later sort by the DateTimeStamp. Maybe the sorting of the date should be done in a separate function, not sure.

data
.GroupBy(
x => x.CardId,
(x, y) => new {
Key = x,
Value = y.OrderByDescending(z => z.DateTimeStamp).FirstOrDefault()
}
);
This will group all the elements by CardId, then order the elements of each group by DateTimeStamp (descending), then pare down each group to only contain the first element. Finally, it returns an enumerable of "groups" (with the scare quotes since they're actually an anonymous type instead of an IGrouping) where each group has the one item you're seeking.

I find the query expression syntax easier to understand myself:
from x in data
group x by x.CarId into grp
select new {
CarId = x.CarId,
Record = (from x in grp orderby z.DateTimeStamp descending).FirstOrDefault()
};