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Model item passed into the dictionary is of type 'Entity.Infrastructure.DbQuery`1[N]', but this dictionary requires a model item of type 'N' [duplicate]

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The model item passed into the dictionary is of type .. but this dictionary requires a model item of type
(7 answers)
Closed 5 years ago.
var model = from m in db.Customers
orderby m.a descending
select new Get_Complaint_Form_Sub
{
CustomerFullName = m.a,
Email = m.Email_Address,
PhoneNumber = m.Telephone_No
};
I am using this LINQ query to retrieve selected columns in db. It fetches the last record in the table in descending order

It sounds like the problem here is downstream, i.e. something that is consuming model. In particular, it sounds like it is expecting one N, not a query of them. So... you might want to add .SingleOrDefault(), .FirstOrDefault(), .Single() or .First() to your query (depending on what you intended). For example:
var query = from m in db.Customers
orderby m.a descending
select new Get_Complaint_Form_Sub
{
CustomerFullName = m.a,
Email = m.Email_Address,
PhoneNumber = m.Telephone_No
};
var model = query.FirstOrDefault();
As a more general note: an IQueryable<T> is usually a pending query - it hasn't been executed yet. Most times that you are talking about a model, you'd want to be talking about rigid data; so if you were trying to represent some number of N, you should probably use .ToList() or .ToArray() to evaluate the query and populate a list/array.

As suggested by Marc, the problem is not with your Linq query, but rather it is in your MVC view. The error message you posted is telling you that your view (i.e. your .cshtml file) is expecting one type but you are passing into it another. It looks like you have made the common mistake of passing in an enumeration of values instead of a single value. Your solution is either:
Pass in a single value to your view by using Single(), `First() etc.
Fix your view so it takes IEnumerable<T> instead of just T, and update the code within to deal with that accordingly.

LINQ to SQL, condition on foreign entities

In SQL, it'd be done as such:
SELECT * FROM Student
WHERE SchoolId IN
(SELECT id FROM School WHERE Name LIKE '%elementary%')
How do I implement this with LINQ? I've tried the following:
var list = context.Students.Where(x => context.Schools.Where(r => r.Name.Contains("elementary").Select(r => r.Id).Contains(x.SchoolId))
but it's not giving me what I want, unfortunately...
I know it's possible to retrieve all the Ids from the School table first, but I think it'd take a heavy toll on the performance. Preferably I'd like LINQ to SQL to handle everything; I can't do this using vanilla SQL because I need stuff to be dynamic and currently LINQ is the best solution for me.
The code above is all for illustration purposes; what I'm doing is a tad different (but more or less the same). I really do need some help on this; if you need any more information just feel free to ask.
EDIT: My bad, I missed out a field. It works, but the results didn't show up because I was missing that field... So sorry...

Try this:
var result = from st in context.Student
from sc in context.Schools
where sc.Name.Contains("elementary") && sc.SchoolId == st.SchoolId
select st;
I am a bit hazy on the syntax, pardon me. But this should point you to the right direction.

Something like this should work. The first use of Contains is on a string object to see if the string contains the substring "elementary". The second use of Contains is on a list and checks to see if the first result list contains SchoolId.
var sublist = from s in context.Schools
where s.Name.Contains("elementary")
select id;
var list = from s in context.Students
where sublist.Contains(s.SchoolId)
select s;

How do I apply the LINQ to SQL Distinct() operator to a List<T>?

I have a serious(it's getting me crazy) problem with LINQ to SQL. I am developing an ASP.NET MVC3 application using c# and Razor in Visual Studio 2010.
I have two database tables, Product and Categories:
Product(Prod_Id[primary key], other attributes)
Categories((Dept_Id, Prod_Id) [primary keys], other attributes)
Obviously Prod_Id in Categories is a foreign key. Both classes are mapped using the Entity Framework (EF). I do not mention the context of the application for simplicity.
In Categories there are multiple rows containing the Prod_Id. I want to make a projection of all Distinct Prod_Id in Categories. I did it using plain (T)SQL in SQL Server MGMT Studio according to this (really simple) query:
SELECT DISTINCT Prod_Id
FROM Categories
and the result is correct. Now I need to make this query in my application so I used:
var query = _StoreDB.Categories.Select(m => m.Prod_Id).Distinct();
I go to check the result of my query by using:
query.Select(m => m.Prod_Id);
or
foreach(var item in query)
{
item.Prod_Id;
//other instructions
}
and it does not work. First of all the Intellisense when I attempt to write query.Select(m => m. or item.shows just suggestions about methods (such as Equals, etc...) and not properties. I thought that maybe there was something wrong with Intellisense (I guess most of you many times hoped that Intellisense was wrong :-D) but when I launch the application I receive an error at runtime.
Before giving your answer keep in mind that;
I checked many forums, I tried the normal LINQ to SQL (without using lambdas) but it does not work. The fact that it works in (T)SQL means that there is something wrong with the LINQ to SQL instruction (other queries in my application work perfectly).
For application related reasons, I used a List<T> variable instead of _StoreDB.Categories and I thought that was the problem. If you can offer me a solution without using a List<T> is appreciated as well.

This line:
var query = _StoreDB.Categories.Select(m => m.Prod_Id).Distinct();
Your LINQ query most likely returns IEnumerable... of ints (judging by Select(m => m.Prod_Id)). You have list of integers, not list of entity objects. Try to print them and see what you got.

Calling _StoreDB.Categories.Select(m => m.Prod_Id) means that query will contain Prod_Id values only, not the entire entity. It would be roughly equivalent to this SQL, which selects only one column (instead of the entire row):
SELECT Prod_Id FROM Categories;
So when you iterate through query using foreach (var item in query), the type of item is probably int (or whatever your Prod_Id column is), not your entity. That's why Intellisense doesn't show the entity properties that you expect when you type "item."...
If you want all of the columns in Categories to be included in query, you don't even need to use .Select(m => m). You can just do this:
var query = _StoreDB.Categories.Distinct();
Note that if you don't explicitly pass an IEqualityComparer<T> to Distinct(), EqualityComparer<T>.Default will be used (which may or may not behave the way you want it to, depending on the type of T, whether or not it implements System.IEquatable<T>, etc.).
For more info on getting Distinct to work in situations similar to yours, take a look at this question or this question and the related discussions.

As has been explained by the other answers, the error that the OP ran into was because the result of his code was a collection of ints, not a collection of Categories.
What hasn't been answered was his question about how to use the collection of ints in a join or something in order to get at some useful data. I will attempt to do that here.
Now, I'm not really sure why the OP wanted to get a distinct list of Prod_Ids from Categories, rather than just getting the Prod_Ids from Projects. Perhaps he wanted to find out what Products are related to one or more Categories, thus any uncategorized Products would be excluded from the results. I'll assume this is the case and that the desired result is a collection of distinct Products that have associated Categories. I'll first answer the question about what to do with the Prod_Ids first, and then offer some alternatives.
We can take the collection of Prod_Ids exactly as they were created in the question as a query:
var query = _StoreDB.Categories.Select(m => m.Prod_Id).Distinct();
Then we would use join, like so:
var products = query.Join(_StoreDB.Products, id => id, p => p.Prod_Id,
(id,p) => p);
This takes the query, joins it with the Products table, specifies the keys to use, and finally says to return the Product entity from each matching set. Because we know that the Prod_Ids in query are unique (because of Distinct()) and the Prod_Ids in Products are unique (by definition because it is the primary key), we know that the results will be unique without having to call Distinct().
Now, the above will get the desired results, but it's definitely not the cleanest or simplest way to do it. If the Category entities are defined with a relational property that returns the related record from Products (which would likely be called Product), the simplest way to do what we're trying to do would be the following:
var products = _StoreDB.Categories.Select(c => c.Product).Distinct();
This gets the Product from each Category and returns a distinct collection of them.
If the Category entity doesn't have the Product relational property, then we can go back to using the Join function to get our Products.
var products = _StoreDB.Categories.Join(_StoreDB.Products, c => c.Prod_Id,
p => p.Prod_Id, (c,p) => p).Distinct();
Finally, if we aren't just wanting a simple collection of Products, then some more though would have to go into this and perhaps the simplest thing would be to handle that when iterating through the Products. Another example would be for getting a count for the number of Categories each Product belongs to. If that's the case, I would reverse the logic and start with Products, like so:
var productsWithCount = _StoreDB.Products.Select(p => new { Product = p,
NumberOfCategories = _StoreDB.Categories.Count(c => c.Prod_Id == p.Prod_Id)});
This would result in a collection of anonymous typed objects that reference the Product and the NumberOfCategories related to that Product. If we still needed to exclude any uncatorized Products, we could append .Where(r => r.NumberOfCategories > 0) before the semicolon. Of course, if the Product entity is defined with a relational property for the related Categories, you wouldn't need this because you could just take any Product and do the following:
int NumberOfCategories = product.Categories.Count();
Anyway, sorry for rambling on. I hope this proves helpful to anyone else that runs into a similar issue. ;)

Dynamically Modifying a LINQ to SQL Select Statement's Columns

I'm trying to build a REST-ful API for my app. Currently I have something like this:
www.example.com/submissions/?format=json
This will return latest ten submissions in JSON. Each object has its details, such as submission name, date created, user, body, etc.
I'd like to do something such as:
www.example.com/submissions/?format=json&filter=name,user
The filter should make the request to return the same result but to only include the details mentioned, i.e. each object will only have a name and user.
This is fairly straightforward in terms of the JSON output. I can load all the columns from the database and create and serialize an object that will only include the columns in the filter. However, I do not want to load all the columns in the database - I want to bother my database with only the columns that I will include in the response.
I want to do something like this:
var result = from record in Submissions
select
{
Name,
Date,
User,
Body
};
Now I have the result object, which is IQueryable, so no call to database made yet.
Then, I should examine the filter querystring and exclude the columns that are not mentioned.
Finally, I can execute the select statement with something like
JavaScript.Serialize(result.ToList());
Is this possible with LINQ to SQL?

An alternative to building your Select expression tree by hand is Dynamic LINQ, which provides a Select method that takes a string:
var filter = "name,user";
var result = Submissions.Select("new(" + filter + ")");
The string is then translated into an expression tree and passed on to your query provider.

Yes. You are going to want to research Modifying Expression Trees. Specifically the MemberInit Expression.

Using [0] or First() with LINQ OrderBy

If I have a structure like this
Albums
- Album
- Discs
- Tracks
and I want to order a collection of albums by the title of the first track on the first disc.
Is there something similar to the following I could do (keeping in mind I need to use the OrderBy extension method that accepts a string)?
albums.OrderBy("Discs[0].Tracks[0].Title")
I need to be able to sort using a string expression thus the need to use the OrderBy method i.e. albums.OrderBy("Track[0].Title"). The reason for this is our custom framework uses a sort expression (e.g. "Title") passed back from a GridView which is looked up in a dictionary (e.g. "Track[0].Title") to get the correct order by clause. That is, the field and direction of sorting is dynamically determined at runtime.
or
albums.OrderBy("Discs.First().Tracks.First().Title")

Untested, but how about:
var query = from album in albums
let disc = album.Discs.First()
let track = disc.Tracks.First()
orderby track.Title
select album;

LINQ has two ways to query "from . in .." and Lambda expressions. They way you were almost writing it looked Lambda-ish. Here would be the Lambda expression:
albums.OrderBy(a=>a.Discs.First().Tracks.First().Title)
I used variable 'a' to indicate album but you can use any variable, this is identical to the first expression:
albums.OrderBy(album=>album.Discs.First().Tracks.First().Title)
or you can use the from obj in obj form as mention in the other answers.

How about this, in order to satisfy your need for an initial query that does not perform the sorting? This uses anonymous types to store the album information, plus the name of the first track so you can sort on it later.
var query = from album in albums
let disc = album.Discs.First()
let track = disc.Tracks.First()
select new { Album = album, FirstTrack = track.Title };
var sortedQuery = from album in query
order by album.FirstTrack
select album.Album;

Sorry people,
It looks like the OrderBy method that I am asking about and trying to use is specific to the ORM (genom-e) that we are using and is not reflected on the .net Queryable or IEnumerable classes (unlike the majority of genom-e's LINQ functionality). There is no OrderBy overload that accepts a string in .net, this is specific to genom-e.
Those of you using .net encountering a similar problem should probably give either of the above two answers a try.