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LINQ getting field from another collection

I have 3 tables and I'm trying to get a combined result with a sum of one field of them.
I'm working with C#, .NET, Entity Framework 7 and SQL Server.
I need to get the city's Name of each result, but I store the idCity
Brand table:
public byte IdBrand { get; set; }
public string Name { get; set; } = null!;
Bundles table:
public int IdBundle { get; set; }
public short IdCity{ get; set; }
public short IdBrand { get; set; }
public decimal? Volume { get; set; }
Cities:
public short IdCity { get; set; }
public string Name { get; set; } = null!;
I've tried this linq query and got almost the result I want but the city field is failing and I got stuck...
var volume = context.Bundles
.GroupBy(city => city.IdCity)
.Select(cad => new
{
CITY = context.Cities.Local.ToList().ElementAt(cad.Key)!.Name,
BRAND1 = cad.Where(c => c.IdBrand == 1).Sum(c => c.Volume),
BRAND2 = cad.Where(c => c.IdBrand == 19).Sum(c => c.Volume)
}).ToList();
I get this result that I expect but the CITY is not correct, I think because the cad.Key is not the same than Cities Index
I also tried:
context.Cities.ToList()
.Where(i => context.Bundles.Any(a=> i.IdCity == a.IdCity))
.Select(x=> x.Name)
CITY
BRAND1
BRAND2
LONDON
10.2
12
MOSCOU
11.4
1
PARIS
9.1
0.4
I guess that the cad.Key is not what I need to use to get the ElementAt Cities but how can I get the city .Name from another table in the Select? Or what is the best way to perform this query?

Try the following query, it should have better performance:
var query =
from b in context.Bundles
group b by b.IdCity into g
select new
{
IdCity = g.Key,
BRAND1 = g.Sum(c => c.IdBrand == 1 ? c.Volume : 0),
BRAND2 = g.Sum(c => c.IdBrand == 19 ? c.Volume : 0)
} into agg
join city in context.Cities on agg.IdCity equals city.Id
select new
{
CITY = city.Name,
BRAND1 = agg.BRAND1,
BRAND2 = agg.BRAND2
};

C# How do you query an object with list properties by joining two tables

public class CategoryDomainModel
{
public string? _id { get; set; }
public string? RestaurantID { get; set; }
public string? CategoryName { get; set; }
public Enums.Status Status { get; set; }
}
public class MenuItemDomainModel
{
public string? _id { get; set; }
public string? CategoryID { get; set; }
public string? ImageLink { get; set; }
public string? ItemName { get; set; }
public string? ItemDescription { get; set; }
}
Imagine you have these two tables in mongodb and a category has many menus.
When you want to join the two tables and get all categories + menus by Restaurant ID with a result like this
public class CategoryAndMenusDomainModel
{
public string? _id { get; set; }
public string? RestaurantID { get; set; }
public string? CategoryName { get; set; }
public Enums.Status Status { get; set; }
public List<MenuItemDomainModel>? Menus { get; set; }
}
How do you go about it?
Ive tried:
var categoryCollection = database.GetCollection<CategoryDomainModel>("Categories");
var menuCollection = database.GetCollection<MenuItemDomainModel>("Menus");
var categoriesAndMenus = (from b in categoryCollection.AsQueryable()
join c in menuCollection.AsQueryable()
on b._id equals c.CategoryID
where b.RestaurantID == restautantID
select new CategoryAndMenusDomainModel
{
_id = b._id,
CategoryName = b.CategoryName,
RestaurantID = b.RestaurantID,
Menus = new List<MenuItemDomainModel>
{
new MenuItemDomainModel
{
_id = c._id,
ItemName = c.ItemName,
ItemDescription = c.ItemDescription
}
}
}).ToList();
But its throwing an exception:
"$project or $group does not support new List`1()

Consider this workaround:
var categoriesAndMenus = (from b in categoryCollection.AsQueryable()
join c in menuCollection
on b._id equals c.CategoryID
where b.RestaurantID == restautantID
select new
{
_id = b._id,
CategoryName = b.CategoryName,
RestaurantID = b.RestaurantID,
C_id = c._id,
C_CategoryName = c.ItemName,
C_ItemDescription = c.ItemDescription
}
)
.ToList()
// below step is client side tranformation, but all required data is already received from the server, so it's just minor projection
.Select(i => new CategoryAndMenusDomainModel
{
_id = i._id,
CategoryName = i.CategoryName,
RestaurantID = i.RestaurantID,
Menus = new List<MenuItemDomainModel>
{
new MenuItemDomainModel
{
_id = i._id,
ItemName = i.CategoryName,
ItemDescription = i.C_ItemDescription
}
}
});
The generated MQL query for this case will be:
{
"aggregate": "Categories",
"pipeline": [
{
"$lookup":
{
"from": "Menus",
"localField": "_id",
"foreignField": "CategoryID",
"as": "c"
}
},
{ "$unwind": "$c" },
{ "$match": { "RestaurantID": "1" } },
{
"$project":
{
"_id": "$_id",
"CategoryName": "$CategoryName",
"RestaurantID": "$RestaurantID",
"C_id": "$c._id",
"C_CategoryName": "$c.ItemName",
"C_ItemDescription": "$c.ItemDescription"
}
}
]
}
I'm 99% confident that client side projection can be moved to the server side, for example via a raw MQL query with $map, see MongoDB project into an array, but it will require more investigation

Linq translates the C# code into SQL. SQL doesn't know about C# lists, so it doesn't know how to format that. You need to do this in two steps: 1) get the data from the database in flat rows, then 2) in C#, put in into the structure you want.
Here I've used the standard Northwind database, but you should be able to adapt it to your data.
var rows = (from c in Categories
join p in Products on c.CategoryID equals p.CategoryID
//where c.CategoryID == 1
select new
{
_id = c.CategoryID,
CategoryName = c.CategoryName,
RestaurantID = c.CategoryID,
menu_id = p.ProductID,
ItemName = p.ProductName,
ItemDescription = p.ProductName
}).ToList();
var categoriesAndMenus =
(from r in rows
group r by r._id into grp
let cat = grp.First()
select new
{
_id = cat._id,
cat.CategoryName,
cat.RestaurantID,
Menu = grp.Select(g =>new
{
_id = g.menu_id,
g.ItemName,
g.ItemDescription
} )
}).ToList();

Groupby, select EF Ccore 3.1

I have three tables like this:
public partial class PriceSite
{
public DateTime ValidFromGmtDtm { get; set; }
public long PriceSiteId { get; set; }
public virtual ICollection<PriceSiteProduct> PriceSiteProduct { get; set; }
}
public partial class PriceSiteProduct
{
public long PriceSiteId { get; set; }
public long ProductId { get; set; }
public virtual Product Product { get; set; }
}
public partial class Product
{
public string ProductCd { get; set; }
public string ProductNm { get; set; }
public long ProductId { get; set; }
public virtual ICollection<PriceSiteProduct> PriceSiteProduct { get; set; }
}
We need to group the data by Productname and choose the product with highest ValidFromGmtDtm (Pricesite).
I tried this query
using (var dbContext = _dbContextProvider.DbContext)
{
var data = from t1 in dbContext.PriceSite
join t2 in dbContext.PriceSiteProduct
on t1.PriceSiteId equals t2.PriceSiteId
join t3 in dbContext.Product
on t2.ProductId equals t3.ProductId
where t1.ValidFromGmtDtm > shellSiteNotification.ValidFromDtm
select new
{
PriceSiteId = t1.PriceSiteId,
ValidFrom = t1.ValidFromGmtDtm,
ProductId = t3.ProductId,
ProductNm = t3.ProductNm,
ProductValues = t3.PumpPriceProduct
};
var res1 = data.ToList();
var data2 = from element in res1
group element by element.ProductNm into groups
select groups.OrderByDescending(p => p.ValidFrom).FirstOrDefault();
}
I am getting timeout error by running this query.
I also tried this query:
var grouped = from priceSite in _dbContextProvider.DbContext.PriceSite
where priceSite.ValidFromGmtDtm < shellSiteNotification.ValidFromDtm
from priceSiteProduct in priceSite.PriceSiteProduct
group priceSiteProduct by priceSiteProduct.Product.ProductNm into produtGroup
select new
{
ProductNamKey = produtGroup.Key,
produtGroup = produtGroup.OrderByDescending(x => x.PriceSite.ValidFromGmtDtm)
};
This code throws a runtime error as we cannot access the navigation property in select in EF Core.
I also tried another query - but this also results in a timeout
var result = from s in _dbContextProvider.FreshReadOnlyPricingDbContext.PriceSite
from r in s.PriceSiteProduct
let p = r.Product
select new
{
PriceSiteId = s.PriceSiteId,
ValidFrom = s.ValidFromGmtDtm,
ProductId = p.ProductId,
ProductNm = p.ProductNm,
ProductValues = p.PumpPriceProduct
};
var list = result.ToList();

It is known EF and SQL limitation, you cannot get items after grouping. Only Key and aggregation result is supported. So basically EF proposes to put ToList() before grouping and it means that you will load several million records to the client.
Without third party extensions which can do that effectively, you can use the following workaround:
using (var dbContext = _dbContextProvider.DbContext)
{
var dataQuery =
from t1 in dbContext.PriceSite
join t2 in dbContext.PriceSiteProduct
on t1.PriceSiteId equals t2.PriceSiteId
join t3 in dbContext.Product
on t2.ProductId equals t3.ProductId
where t1.ValidFromGmtDtm > shellSiteNotification.ValidFromDtm
select new
{
PriceSiteId = t1.PriceSiteId,
ValidFrom = t1.ValidFromGmtDtm,
ProductId = t3.ProductId,
ProductNm = t3.ProductNm,
ProductValues = t3.PumpPriceProduct
};
var query =
from key in dataQuery.Select(d => new { d.ProductNm }).Distinct()
from d in dataQuery.Where(d => d.ProductNm == key.ProductNm)
.OrderByDescending(d => d.ValidFrom)
.Take(1)
select d;
var result = query.ToList();
}

How to use List.Distinct to populate a custom class?

I have the following two classes (in C#)
public class courseList
{
public string MajorName { get; set; }
public string MajorNameID { get; set; }
public string CourseID { get; set; }
public string CourseName { get; set; }
}
public class CourceNames
{
[DataMember]
public string CourseID { get; set; }
[DataMember]
public string CourseName { get; set; }
}
public class Courses
{
[DataMember]
public string MajorNameID { get; set; }
[DataMember]
public string MajorName { get; set; }
[DataMember]
public List<CourceNames> CourseNames { get; set; }
public Courses()
{
Course = new List<CourceNames>();
}
}
I am reading two tables from MYSQL database using SQLreader to
List<courseList> courseList
class.
MY result record is as follows :
MajorNameID MajorName CourseName CourseID
100000 Physics Thermodynamic PHY101
100000 Physics Quantum PHY200
100000 Physics Relativity PHY300
200000 Chemistry Gases CHM300
200000 Chemistry Oreganic CHM500
200000 Chemistry Inroganic CHM120
300000 Mathematics Pure MAT100
300000 Mathematics Applied MAT300
As u could see, I want to populate Courses class. I am not sure how I could do this using Linq.
I recently learnt the following method but it's not working correctly.
List<Courses> courses = courseList .GroupBy(
d => new { d.MajorNameID , d.MajorName },
d => d.MajorName,
(key, g) => new courses
{
MajorNameID = key.MajorNameID,
MajorName = key.MajorName,
CourseNames = g.Distinct().ToList()
}
).ToList();
I get the following error :
> Severity Code Description Project File Line Suppression State
> Error CS0029 Cannot implicitly convert type
> 'System.Collections.Generic.List<string>' to
> 'System.Collections.Generic.List<CourceNames>'...........
I do not fully understand the above. Could someone help me how I could set it up correctly?
I want to load couses per each major in the list.

Try simplifying your group by to just grouping by key and using Select expression on each item in group.
This final trick is that you have to cast the course names to the correct type. The compiler should give you enough hints, but Linq GroupBy is VERY different to MySQL/TSQL GroupBy statements so they can be tricky to master at first.
List<Courses> courses = courseList.GroupBy(
d => new { d.MajorNameID, d.MajorName }
).Select(g => new Courses
{
MajorNameID = g.Key.MajorNameID,
MajorName = g.Key.MajorName,
CourseNames = g.Distinct().Select(c =>
new CourceNames { CourseID = c.CourseID, CourseName = c.CourseName }).ToList()
}
).ToList();
[UPDATE: I don't normally use element projection syntax, so I had to compile this to check]
Using the same GroupBy overload as your question (element projection), this is the same query, note that the element selector expression now selects the elements, the original post has a key expression in there instead:
List<Courses> courses2 = courseList.GroupBy(
d => new { d.MajorNameID, d.MajorName }, // key selector
c => new CourceNames { CourseID = c.CourseID, CourseName = c.CourseName }, // element selector
(key, g) => new Courses
{
MajorNameID = key.MajorNameID,
MajorName = key.MajorName,
CourseNames = g.Distinct().ToList()
}
).ToList();

The lambda variable g is a group, you cannot directly do a .Distinct on it
List<Courses> courses = courseList .GroupBy(
d => new { d.MajorNameID , d.MajorName },
d => d.MajorName,
(key, g) => new Courses
{
MajorNameID = key.MajorNameID,
MajorName = key.MajorName,
CourseNames = g.Distinct().ToList()
}).ToList();
You need to project the course name from the group g and then do a .Distinct on it. Which will be like:
List<Courses> courses = courseList .GroupBy(
d => new { d.MajorNameID , d.MajorName },
d => d.MajorName,
(key, g) => new Courses
{
MajorNameID = key.MajorNameID,
MajorName = key.MajorName,
CourseNames = g.Select(x => x.CourseName).Distinct().ToList()
}).ToList();

How can I select from an included entity in LINQ and get a flat list similar to a SQL Join would give?

I have two classes:
public class Topic
{
public Topic()
{
this.SubTopics = new HashSet<SubTopic>();
}
public int TopicId { get; set; }
public string Name { get; set; }
public virtual ICollection<SubTopic> SubTopics { get; set; }
}
public class SubTopic
public int SubTopicId { get; set; }
public int Number { get; set; }
public int TopicId { get; set; }
public string Name { get; set; }
public virtual Topic Topic { get; set; }
}
What I would like to do is to get a Data Transfer Object output from LINQ that will show me. I do want to see the TopicId repeated if there is more than one SubTopic inside that topic:
TopicId Name SubTopicId Name
1 Topic1 1 SubTopic1
1 Topic1 2 SubTopic2
1 Topic1 3 SubTopic3
2 Topic2 4 SubTopic4
I tried to code a Linq statement like this:
var r = context.Topics
.Select ( s => new {
id = s.TopicId,
name = s.Name,
sid = s.SubTopics.Select( st => st.SubTopicId),
sidname = s.SubTopics.Select ( st => st.Name)
}).
ToList();
But this does not really work as it returns sid and sidname as lists.
How will it be possible for me to get a flat output showing what I need?

You need SelectMany to expand a nested collection, along these lines
var r = context.Topics.SelectMany(t => t.SubTopics
.Select(st => new
{
TopicID = t.TopicId,
TopicName = t.Name,
SubTopicID = st.SubTopicId,
SubTopicName = st.Name
}));

try this :
var r = context.Topics
.Select ( s => new {
id = s.TopicId,
name = s.Name,
sid = s.SubTopics.Where(st=>st.TopicId==s.TopicId).Select( st => st.SubTopicId ),
sidname = s.SubTopics..Where(st=>st.TopicId==s.TopicId).Select ( st => st.Name)
}).
ToList();
Hope it will help

#Sweko provided an answer that satisfies the exact output that you requested. However, this can be even simpler if you just return the subtopic intact. It may run a bit quicker as well, since you don't need to create a new object for each element in the result.
Lastly, it looks like you wanted your result set ordered. For completeness, I've added those clauses as well.
var r = context.Topics
.SelectMany( topic => topic.SubTopics )
.OrderBy(sub => sub.TopicId)
.ThenBy(sub => sub.SubTopicId);