Given the following tables:
SCHOOL
schoolid (int PK)
name
TEACHER
teacherId (int PK)
name, homeRoomId (fk varchar10)
subjectId (fk varchar10)
schoolid (FK int)
HOMEROOM
homeRoomId (PK varchar10)
roomNumber
active
SUBJECT
subjectId (PK varchar10)
name
active
I am using EF6 in an MVC app. I have lazy loading enabled. I am trying to return a list of all teachers for a given SchoolId and I need to include homeroom and subject data for each teacher.
A school contains many teachers, a teacher works for only one school, a teacher has only one homeroom and teachs only one subject. The homeroom and subject ids are varchars because they are pre-existing ids and look like: SUBJECT: A03, Math.
My code to load all teachers with homeroom and subject for a single schoolid:
public List<TeacherModel> GetTeachersBySchool(int schoolId)
{
List<TeacherModel> teachers = new List<TeacherModel>();
using (var db = new myDBEntities())
{
var list = db.Teacher.Where(a => a.SchoolId == schoolId).ToList();
foreach ( var s in list)
{
TeacherModel teacher = new TeacherModel()
{
TeacherId = s.TeacherId,
Name = s.Name,
HomeRoomId = s.HomeRoomId,
HomeRoomNumber = s.HomeRoom.RoomNumber,
SubjectId = s.SubjectId,
SubjectName = s.Subject.Name
};
teachers.Add(teacher);
}
return teachers;
}
}
The homeroom entity is loading but the Subject entity is null even through a sql query in the database returns one row for this teacher. Due to the null Subject entity, the query errors out as object reference not set to blah blah.
I have found that the problem seems to be when the SubjectId contains alpha characters. A couple examples of a subjectid are: "A03" or "1001023". The second entity will load, the first will not. I assume that even though the datatype is string/varchar EF6 is pulling out the numeric values and passing those as the id, so if the ID has alphas, it fails.
Does this jibe? How do I fix it? As a last resort I can add a surrogate key (INT Identity 1,1) for use with these entities but I'm hoping there is another way.
I have two tables department and teacher like this:
Department table (DeptID is the primary key)
DeptID | DeptName
1 P
2 C
3 M
Teacher table (DeptID is a foreign key)
DeptID | TeacherName
1 ABC
1 PQR
2 XYZ
I have used database first approach to create a single model out of these two tables. I want to display both details in a single view like this:
TeacherName | DeptName
ABC P
PQR P
XYZ C
I tried to create controllers using scaffolding but it would provide views and CRUD operations for a single table in the model.
Is there any method using which I can map these two tables together in a single view ? or is it possible (easily achievable) when I use different models for each table in the database ?
You have to create Viewmodel.
public class DepartmentTeacher
{
public int DeptID {get;set;}
public string DeptName {get;set;}
public int TeachID {get;set;}
public string TeachName {get;set;}
}
using (var db = new SchoolContext())
{
var query = (from tc in db.Teacher
join dp in db.Department on tc.DeptID equals dp.DeptID
//where st.STUDENT_ID == Customer_Id maybe you need
select new
{
dp.DeptName,
tc.TeachName
});
foreach (var item in query)
{
DepartmentTeacher.DeptName = item.DeptName;
DepartmentTeacher.TeachName = item.TeachName;
}
}
return View(DepartmentTeacher);
You can use every process this viewmodel.However you have to description this Viewmodel on your view page.
I have "Products table" with following fields
PID int,
product_name varchar(),
product_price int
"Cart table" with following fields
cart_ID
user_id
PID
So I want to display cart items of logged in user
For example if user_ID=100 is logged in , then only his cart items should be displayed to him, with all the product details.
Am using asp.net with entity framework
public ActionResult Cart()
{
Products pro = new Products();
Cart cart =new Cart();
var productID = db.cartDetails.Where(pid => pid.productId == cart.productId && cart.user_id == session["user_ID"]);
return View(db.productsDetails.Where(pid => pid.productId == productID));
}
Now problem arises, ProductID being var type I cannot compare it with pid => pid.productid.
What I want to do is first get all the product_id's of user from cart table by comparing uid_id (Logged in user) with user_id in cart table, then displaying product details of those product_id's from product Table. So obviously I need to store multiple product_id's,so that i can populate their data on the cart page.
The LINQ expression db.cartDetails.Where(pid=>pid.productId==cart.productId && cart.user_id==session["user_ID"]) would return a collection of cartDetails and not the productId. You must use select to fetch the columns you need, something like this
var productIDs = db.cartDetails
.Where(pid => pid.productId == cart.productId && cart.user_id == session["user_ID"])
.Select(cd => cd.productId)
.ToList();
This would return you a List of productIds. (If you wish to get only one productId, you could use SingleOrDefault or FirstOrDefault depending on your scenario like db.cartDetails.SingleOrDefault(pid => ...).productId).
Also note that you could have used int type for productId instead of using var if you were expecting an integer. Now you are getting a collection type IQueryable<cardDetails> being assigned to productId.
Next you cannot use an equality operator for the returned List anymore, you should check if this list contains the productId from productDetails, something like this:
return View(db.productsDetails.Where(pid => productIDs.Contains(pid.productId)));
Couldn't test this code, but the basic idea is here.
One last thing, consider using a join between the two tables: https://msdn.microsoft.com/en-us/library/bb311040.aspx
Did this and now my cart is working fine
public ActionResult Cart()
List<int> productIDs = new List<int>();
productIDs = db.cartDetails.Where(ch => ch.userId ==12).Select(cd => cd.productId).ToList() ;
List<Products> pDetails = new List<Products>();
for(int i=0;i<productIDs.Count;i++)
{
pDetails.Add(db.productsDetails.Find(productIDs[i]));
}
return View(pDetails);
When I retrieve a list of items from a database including some children (via .Include), and order the randomly, EF gives me an unexpected result.. I creates/clones addition items..
To explain myself better, I've created a small and simple EF CodeFirst project to reproduce the problem.
First i shall give you the code for this project.
The project
Create a basic MVC3 project and add the EntityFramework.SqlServerCompact package via Nuget.
That adds the latest versions of the following packages:
EntityFramework v4.3.0
SqlServerCompact v4.0.8482.1
EntityFramework.SqlServerCompact v4.1.8482.2
WebActivator v1.5
The Models and DbContext
using System.Collections.Generic;
using System.Data.Entity;
namespace RandomWithInclude.Models
{
public class PeopleContext : DbContext
{
public DbSet<Person> Persons { get; set; }
public DbSet<Address> Addresses { get; set; }
}
public class Person
{
public int ID { get; set; }
public string Name { get; set; }
public virtual ICollection<Address> Addresses { get; set; }
}
public class Address
{
public int ID { get; set; }
public string AdressLine { get; set; }
public virtual Person Person { get; set; }
}
}
The DB Setup and Seed data: EF.SqlServerCompact.cs
using System.Collections.Generic;
using System.Data.Entity;
using System.Data.Entity.Infrastructure;
using RandomWithInclude.Models;
[assembly: WebActivator.PreApplicationStartMethod(typeof(RandomWithInclude.App_Start.EF), "Start")]
namespace RandomWithInclude.App_Start
{
public static class EF
{
public static void Start()
{
Database.DefaultConnectionFactory = new SqlCeConnectionFactory("System.Data.SqlServerCe.4.0");
Database.SetInitializer(new DbInitializer());
}
}
public class DbInitializer : DropCreateDatabaseAlways<PeopleContext>
{
protected override void Seed(PeopleContext context)
{
var address1 = new Address {AdressLine = "Street 1, City 1"};
var address2 = new Address {AdressLine = "Street 2, City 2"};
var address3 = new Address {AdressLine = "Street 3, City 3"};
var address4 = new Address {AdressLine = "Street 4, City 4"};
var address5 = new Address {AdressLine = "Street 5, City 5"};
context.Addresses.Add(address1);
context.Addresses.Add(address2);
context.Addresses.Add(address3);
context.Addresses.Add(address4);
context.Addresses.Add(address5);
var person1 = new Person {Name = "Person 1", Addresses = new List<Address> {address1, address2}};
var person2 = new Person {Name = "Person 2", Addresses = new List<Address> {address3}};
var person3 = new Person {Name = "Person 3", Addresses = new List<Address> {address4, address5}};
context.Persons.Add(person1);
context.Persons.Add(person2);
context.Persons.Add(person3);
}
}
}
The controller: HomeController.cs
using System;
using System.Data.Entity;
using System.Linq;
using System.Web.Mvc;
using RandomWithInclude.Models;
namespace RandomWithInclude.Controllers
{
public class HomeController : Controller
{
public ActionResult Index()
{
var db = new PeopleContext();
var persons = db.Persons
.Include(p => p.Addresses)
.OrderBy(p => Guid.NewGuid());
return View(persons.ToList());
}
}
}
The View: Index.cshtml
#using RandomWithInclude.Models
#model IList<Person>
<ul>
#foreach (var person in Model)
{
<li>
#person.Name
</li>
}
</ul>
this should be all, and you application should compile :)
The problem
As you can see, we have 2 straightforward models (Person and Address) and Person can have multiple Addresses.
We seed the generated database 3 persons and 5 addresses.
If we get all the persons from the database, including the addresses and randomize the results and just print out the names of those persons, that's where it all goes wrong.
As a result, i sometimes get 4 persons, sometimes 5 and sometimes 3, and i expect 3. Always.
e.g.:
Person 1
Person 3
Person 1
Person 3
Person 2
So.. it's copying/cloning data! And that's not cool..
It just seems that EF looses track of what addresses are a child of which person..
The generated SQL query is this:
SELECT
[Project1].[ID] AS [ID],
[Project1].[Name] AS [Name],
[Project1].[C2] AS [C1],
[Project1].[ID1] AS [ID1],
[Project1].[AdressLine] AS [AdressLine],
[Project1].[Person_ID] AS [Person_ID]
FROM ( SELECT
NEWID() AS [C1],
[Extent1].[ID] AS [ID],
[Extent1].[Name] AS [Name],
[Extent2].[ID] AS [ID1],
[Extent2].[AdressLine] AS [AdressLine],
[Extent2].[Person_ID] AS [Person_ID],
CASE WHEN ([Extent2].[ID] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C2]
FROM [People] AS [Extent1]
LEFT OUTER JOIN [Addresses] AS [Extent2] ON [Extent1].[ID] = [Extent2].[Person_ID]
) AS [Project1]
ORDER BY [Project1].[C1] ASC, [Project1].[ID] ASC, [Project1].[C2] ASC
Workarounds
If i remove the .Include(p =>p.Addresses) from the query, everything goes fine. but of course the addresses aren't loaded and accessing that collection will make a new call to the database every time.
I can first get the data from the database and randomize later by just adding a .ToList() before the .OrderBy.. like this: var persons = db.Persons.Include(p => p.Addresses).ToList().OrderBy(p => Guid.NewGuid());
Does anybody have any idea of why it is happening like this?
Might this be a bug in the SQL generation?
As one can sort it out by reading AakashM answer and Nicolae Dascalu answer, it strongly seems Linq OrderBy requires a stable ranking function, which NewID/Guid.NewGuid is not.
So we have to use another random generator that would be stable inside a single query.
To achieve this, before each querying, use a .Net Random generator to get a random number. Then combine this random number with a unique property of the entity to get randomly sorted. And to 'randomize' a bit the result, checksum it. (checksum is a SQL Server function that compute a hash; original idea founded on this blog.)
Assuming Person Id is an int, you could write your query this way :
// Random instances should be stored and reused, not instanciated at each usage.
// But beware, it is not thread safe. If you want to share it between threads, you
// would have to use locks, see its documentation.
// https://learn.microsoft.com/en-us/dotnet/api/system.random.
// But using locks is a bad idea for scalability, especially in a Web context.
var randomGenerator = new Random();
// ...
var rnd = randomGenerator.NextDouble();
var persons = db.Persons
.Include(p => p.Addresses)
.OrderBy(p => SqlFunctions.Checksum(p.Id * rnd));
Like the NewGuid hack, this is very probably not a good random generator with a good distribution and so on. But it does not cause entities to get duplicated in results.
Beware:
If your query ordering does not guarantees uniqueness of your entities ranking, you must complement it for guarantying it. By example, if you use a non-unique property of your entities for the checksum call, then add something like .ThenBy(p => p.Id) after the OrderBy.
If your ranking is not unique for your queried root entity, its included children may get mixed with children of other entities having the same ranking. And then the bug will stay here.
Note:
I would prefer use .Next() method to get an int then combine it through a xor (^) to an entity int unique property, rather than using a double and multiply it. But SqlFunctions.Checksum unfortunately does not provide an overload for int data type, though the SQL server function is supposed to support it. You may use a cast to overcome this, but for keeping it simple I finally had chosen to go with the multiply.
tl;dr: There's a leaky abstraction here. To us, Include is a simple instruction to stick a collection of things onto each single returned Person row. But EF's implementation of Include is done by returning a whole row for each Person-Address combo, and reassembling at the client. Ordering by a volatile value causes those rows to become shuffled, breaking apart the Person groups that EF is relying on.
When we have a look at ToTraceString() for this LINQ:
var people = c.People.Include("Addresses");
// Note: no OrderBy in sight!
we see
SELECT
[Project1].[Id] AS [Id],
[Project1].[Name] AS [Name],
[Project1].[C1] AS [C1],
[Project1].[Id1] AS [Id1],
[Project1].[Data] AS [Data],
[Project1].[PersonId] AS [PersonId]
FROM ( SELECT
[Extent1].[Id] AS [Id],
[Extent1].[Name] AS [Name],
[Extent2].[Id] AS [Id1],
[Extent2].[PersonId] AS [PersonId],
[Extent2].[Data] AS [Data],
CASE WHEN ([Extent2].[Id] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1]
FROM [Person] AS [Extent1]
LEFT OUTER JOIN [Address] AS [Extent2] ON [Extent1].[Id] = [Extent2].[PersonId]
) AS [Project1]
ORDER BY [Project1].[Id] ASC, [Project1].[C1] ASC
So we get n rows for each A, plus 1 row for each P without any As.
Adding an OrderBy clause, however, puts the thing-to-order-by at the start of the ordered columns:
var people = c.People.Include("Addresses").OrderBy(p => Guid.NewGuid());
gives
SELECT
[Project1].[Id] AS [Id],
[Project1].[Name] AS [Name],
[Project1].[C2] AS [C1],
[Project1].[Id1] AS [Id1],
[Project1].[Data] AS [Data],
[Project1].[PersonId] AS [PersonId]
FROM ( SELECT
NEWID() AS [C1],
[Extent1].[Id] AS [Id],
[Extent1].[Name] AS [Name],
[Extent2].[Id] AS [Id1],
[Extent2].[PersonId] AS [PersonId],
[Extent2].[Data] AS [Data],
CASE WHEN ([Extent2].[Id] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C2]
FROM [Person] AS [Extent1]
LEFT OUTER JOIN [Address] AS [Extent2] ON [Extent1].[Id] = [Extent2].[PersonId]
) AS [Project1]
ORDER BY [Project1].[C1] ASC, [Project1].[Id] ASC, [Project1].[C2] ASC
So in your case, where the ordered-by-thing is not a property of a P, but is instead volatile, and therefore can be different for different P-A records of the same P, the whole thing falls apart.
I'm not sure where on the working-as-intended ~~~ cast-iron bug continuum this behaviour falls. But at least now we know about it.
I dont think there is an issue in query generation, but there is definately an issue when EF tries to convert rows into object.
It looks like there is an inherent assumption here that data for the same person in a joined statement will be returned grouped together order by or not.
for example the result of a joined query will always be
P.Id P.Name A.Id A.StreetLine
1 Person 1 10 ---
1 Person 1 11
2 Person 2 12
3 Person 3 13
3 Person 3 14
even if you order by some other column, same person would always appear one after the other.
this assumption is mostly true for any joined query.
But there is a deeper issue here i think. OrderBy is for when you want data in certain order ( as opposite to random), so that assumption does seem reasonable.
i think you should really get data out and then randomize it according to some other means in your code
From theory:
To sort a list of items, the compare function should be stable relative to items; this means that for any 2 items x, y the result of x< y should be the same as many time is queried(called).
I think the issue is related to misunderstanding of specification(documentation) of OrderBy method:
keySelector - A function to extract a key from an element.
EF didn't mention explicitly if the provided function should return the same value for same object as many times is called (in your case returns different/random values), but I think the "key" term that they used in documentation implicitly suggested this.
When you define a query path to define the query results, (use Include), the query path is only valid on the returned instance of ObjectQuery. Other instances of ObjectQuery and the object context itself are not affected. This functionality lets you chain multiple "Includes" for eager loading.
Therefor, Your statement translates into
from person in db.Persons.Include(p => p.Addresses).OrderBy(p => Guid.NewGuid())
select person
instead of what you intended.
from person in db.Persons.Include(p => p.Addresses)
select person
.OrderBy(p => Guid.NewGuid())
Hence your second workaround works fine :)
Reference: Loading Related Objects While Querying A Conceptual Model in Entity
Framework - http://msdn.microsoft.com/en-us/library/bb896272.aspx
I also ran into this problem, and solved it by adding a Randomizer Guid property to the main class I was fetching. I then set the column's default value to NEWID() like this (using EF Core 2)
builder.Entity<MainClass>()
.Property(m => m.Randomizer)
.HasDefaultValueSql("NEWID()");
When fetching, it gets a bit more complicated. I created two random integers to function as my order-by indexes, then ran the query like this
var rand = new Random();
var randomIndex1 = rand.Next(0, 31);
var randomIndex2 = rand.Next(0, 31);
var taskSet = await DbContext.MainClasses
.Include(m => m.SubClass1)
.ThenInclude(s => s.SubClass2)
.OrderBy(m => m.Randomizer.ToString().Replace("-", "")[randomIndex1])
.ThenBy(m => m.Randomizer.ToString().Replace("-", "")[randomIndex2])
.FirstOrDefaultAsync();
This seems to be working well enough, and should provide enough entropy for even a large dataset to be fairly randomized.
how can you do multiple "group by's" in linq to sql?
Can you please show me in both linq query syntax and linq method syntax.
Thanks
Edit.
I am talking about multiple parameters say grouping by "sex" and "age".
Also I forgot to mention how would I say add up all the ages before I group them.
If i had this example how would I do this
Table Product
ProductId
ProductName
ProductQty
ProductPrice
Now imagine for whatever reason I had tons of rows each with the same ProductName, different ProductQty and ProductPrice.
How would I groupt hem up by Product Name and add together ProductQty and ProductPrice?
I know in this example it probably makes no sense why there would row after row with the same product name but in my database it makes sense(it is not products).
To group by multiple properties, you need to create a new object to group by:
var groupedResult = from person in db.People
group by new { person.Sex, person.Age } into personGroup
select new
{
personGroup.Key.Sex,
personGroup.Key.Age,
NumberInGroup = personGroup.Count()
}
Apologies, I didn't see your final edit. I may be misunderstanding, but if you sum the age, you can't group by it. You could group by sex, sum or average the age...but you couldn't group by sex and summed age at the same time in a single statement. It might be possible to use a nested LINQ query to get the summed or average age for any given sex...bit more complex though.
EDIT:
To solve your specific problem, it should be pretty simple and straightforward. You are grouping only by name, so the rest is elementary (example updated with service and concrete dto type):
class ProductInventoryInfo
{
public string Name { get; set; }
public decimal Total { get; set; }
}
class ProductService: IProductService
{
public IList<ProductInventoryInfo> GetProductInventory()
{
// ...
var groupedResult = from product in db.Products
group by product.ProductName into productGroup
select new ProductInventoryInfo
{
Name = productGroup.Key,
Total = productGroup.Sum(p => p.ProductCost * p.ProductQty)
}
return groupedResult.ToList();
}
}