We Keep Coding

ASP.NET MVC C# Select and Where Statements

I'm having trouble understanding .Select and .Where statements. What I want to do is select a specific column with "where" criteria based on another column.
For example, what I have is this:
var engineers = db.engineers;
var managers = db.ManagersToEngineers;
List<ManagerToEngineer> matchedManager = null;
Engineer matchedEngineer = null;
if (this.User.Identity.IsAuthenticated)
{
var userEmail = this.User.Identity.Name;
matchedEngineer = engineers.Where(x => x.email == userEmail).FirstOrDefault();
matchedManager = managers.Select(x => x.ManagerId).Where(x => x.EngineerId == matchedEngineer.PersonId).ToList();
}
if (matchedEngineer != null)
{
ViewBag.EngineerId = new SelectList(new List<Engineer> { matchedEngineer }, "PersonId", "FullName");
ViewBag.ManagerId = new SelectList(matchedManager, "PersonId", "FullName");
}
What I'm trying to do above is select from a table that matches Managers to Engineers and select a list of managers based on the engineer's id. This isn't working and when I go like:
matchedManager = managers.Where(x => x.EngineerId == matchedEngineer.PersonId).ToList();
I don't get any errors but I'm not selecting the right column. In fact the moment I'm not sure what I'm selecting. Plus I get the error:
Non-static method requires a target.

if you want to to select the manager, then you need to use FirstOrDefault() as you used one line above, but if it is expected to have multiple managers returned, then you will need List<Manager>, try like:
Update:
so matchedManager is already List<T>, in the case it should be like:
matchedManager = managers.Where(x => x.EngineerId == matchedEngineer.PersonId).ToList();
when you put Select(x=>x.ManagerId) after the Where() now it will return Collection of int not Collection of that type, and as Where() is self descriptive, it filters the collection as in sql, and Select() projects the collection on the column you specify:
List<int> managerIds = managers.Where(x => x.EngineerId == matchedEngineer.PersonId)
.Select(x=>x.ManagerId).ToList();

The easiest way to remember what the methods do is to remember that this is being translated to SQL.
A .Where() method will filter the rows returned.
A .Select() method will filter the columns returned.
However, there are a few ways to do that with the way you should have your objects set up.
First, you could get the Engineer, and access its Managers:
var engineer = context.Engineers.Find(engineerId);
return engineer.Managers;
However, that will first pull the Engineer out of the database, and then go back for all of the Managers. The other way would be to go directly through the Managers.
return context.Managers.Where(manager => manager.EngineerId == engineerId).ToList();
Although, by the look of the code in your question, you may have a cross-reference table (many to many relationship) between Managers and Engineers. In that case, my second example probably wouldn't work. In that case, I would use the first example.

You want to filter data by matching person Id and then selecting manager Id, you need to do following:
matchedManager = managers.Where(x => x.EngineerId == matchedEngineer.PersonId).Select(x => x.ManagerId).ToList();
In your case, you are selecting the ManagerId first and so you have list of ints, instead of managers from which you can filter data
Update:
You also need to check matchedEngineer is not null before retrieving the associated manager. This might be cause of your error

You use "Select" lambda expression to get the field you want, you use "where" to filter results

Distinct rows from db table with unique IDs but same names

I am creating a videorental system and I'm told that there are to be multiple entries of the same movie, as in a real videostore, but they should all have unique IDs. I'm using LINQ to fetch the data, and currently my view shows me the entire table, so the same movie name gets repeated alot. I tried using the .Distinct extension method, but since the IDs are not the same, they are all distinct. I need to ignore the ID portion of the object, but can't figure it out.
public static List<Movie> GetAllMovies()
{
return Context.Movie.ToList();
}
public static List<Movie> GetAllMoviesDistinct()
{
return Context.Movie.Distinct().ToList();
}
These two methods do the exact same thing

You can do this with MoreLINQ:
var result = Context.Movie
.DistinctBy(m => new {
// take all properties except your ID
})
.ToList();

You can use GroupBy to get movies with for example unique name.
return Context.Movie.GroupBy(m => m.Name).Select(x => x.First()).ToList();

You'd need something like this:
public static List<string> GetAllMovieTitlesDistinct()
{
return Context.Movie.GroupBy(x => x.Title).Select(x => x.Key).ToList();
}
Since you have multiple entries for the same movie, it doesn't really make sense to return a random (or just the first) movie with a specific name. What I'd rather do, is get the unique movie titles, select one title and then list all the entries for that.
What the method above does is, it groups all the movies by their names and then just selects each unique movie title. Please note that it does not return a specific entry for each title, as that would be highly arbitrary and might lead to unwanted results.

var groupesList = Context.Movie.GroupBy(x => x.Name,
(key, val) => new {Key = key, Value = val}).ToList();
then you can call Key(unuque) or Value by key for all inform for example all ID

LINQ Query - Only get Order and MAX Date from Child Collection

I'm trying to get a list that displays 2 values in a label from a parent and child (1-*) entity collection model.
I have 3 entities:
[Customer]: CustomerId, Name, Address, ...
[Order]: OrderId, OrderDate, EmployeeId, Total, ...
[OrderStatus]: OrderStatusId, StatusLevel, StatusDate, ...
A Customer can have MANY Order, which in turn an Order can have MANY OrderStatus, i.e.
[Customer] 1--* [Order] 1--* [OrderStatus]
Given a CustomerId, I want to get all of the Orders (just OrderId) and the LATEST (MAX?) OrderStatus.StatusDate for that Order.
I've tried a couple of attempts, but can seem to get the results I want.
private IQueryable<Customer> GetOrderData(string customerId)
{
var ordersWithLatestStatusDate = Context.Customers
// Note: I am not sure if I should add the .Expand() extension methods here for the other two entity collections since I want these queries to be as performant as possible and since I am projecting below (only need to display 2 fields for each record in the IQueryable<T>, but thinking I should now after some contemplation.
.Where(x => x.CustomerId == SelectedCustomer.CustomerId)
.Select(x => new Custom
{
CustomerId = x.CustomerId,
...
// I would like to project my Child and GrandChild Collections, i.e. Orders and OrderStatuses here but don't know how to do that. I learned that by projecting, one does not need to "Include/Expand" these extension methods.
});
return ordersWithLatestStatusDate ;
}
---- UPDATE 1 ----
After the great solution from User: lazyberezovsky, I tried the following:
var query = Context.Customers
.Where(c => c.CustomerId == SelectedCustomer.CustomerId)
.Select(o => new Customer
{
Name = c.Name,
LatestOrderDate = o.OrderStatus.Max(s => s.StatusDate)
});
In my hastiness from my initial posting, I didn't paste everything in correctly since it was mostly from memory and didn't have the exact code for reference at the time. My method is a strongly-typed IQueryabled where I need it to return a collection of items of type T due to a constraint within a rigid API that I have to go through that has an IQueryable query as one of its parameters. I am aware I can add other entities/attributes by either using the extension methods .Expand() and/or .Select(). One will notice that my latest UPDATED query above has an added "new Customer" within the .Select() where it was once anonymous. I'm positive that is why the query failed b/c it couldn't be turn into a valid Uri due to LatestOrderDate not being a property of Customer at the Server level. FYI, upon seeing the first answer below, I had added that property to my client-side Customer class with simple { get; set; }. So given this, can I somehow still have a Customer collection with the only bringing back those 2 fields from 2 different entities? The solution below looked so promising and ingenious!
---- END UPDATE 1 ----
FYI, the technologies I'm using are OData (WCF), Silverlight, C#.
Any tips/links will be appreciated.

This will give you list of { OrderId, LatestDate } objects
var query = Context.Customers
.Where(c => c.CustomerId == SelectedCustomer.CustomerId)
.SelectMany(c => c.Orders)
.Select(o => new {
OrderId = o.OrderId,
LatestDate = o.Statuses.Max(s => s.StatusDate) });
.
UPDATE construct objects in-memory
var query = Context.Customers
.Where(c => c.CustomerId == SelectedCustomer.CustomerId)
.SelectMany(c => c.Orders)
.AsEnumerable() // goes in-memory
.Select(o => new {
OrderId = o.OrderId,
LatestDate = o.Statuses.Max(s => s.StatusDate) });
Also grouping could help here.

If I read this correctly you want a Customer entity and then a single value computed from its Orders property. Currently this is not supported in OData. OData doesn't support computed values in the queries. So no expressions in the projections, no aggregates and so on.
Unfortunately even with two queries this is currently not possible since OData doesn't support any way of expressing the MAX functionality.
If you have control over the service, you could write a server side function/service operation to execute this kind of query.

Get a distinct list

I want to select a distinct list.
The following code is not working:
public IQueryable<BusinessObjects.Order> GetByBusinessId(Guid Id)
{
rentalEntities db = DataContextFactory.CreateContext();
List<Rental.BusinessObjects.Order> transformedList = new List<BusinessObjects.Order>();
foreach (Rental.DataObjects.EntityModel.Order item in db.Orders.Where(x => x.BusinessID == BusinessId).ToList())
{
transformedList.Add(OrderMappers.ToBusinessObject(item));
}
return( transformedList.AsQueryable()).Distinct();
}

Try this:
return Rental.DataObjects.EntityModel.Order item in db.Orders
.Where(x => x.BusinessID == BusinessId)
.Distinct()
.Select(item => OrderMappers.ToBusinessObject(item));
This should move the distinct operation to the underlying database call as it's applied before the query is materialized - this is more efficient as the duplicate rows aren't retrieved from the database server. If for some reason you don't want to do that, then check your equals implementation (as mentioned by Sorin)

You may want to check how your business objects implement Equals(), my guess is they are are different even if they have (let's say) the same ID.

You might like to try the DistinctBy() extension method from the MoreLinq library. This lets you easily control the exact semantics of how two objects are compared for distinctness. For instance:
return transformedList.AsQueryable().DistinctBy(orderBO => orderBO.OrderId);
http://morelinq.googlecode.com/files/morelinq-1.0-beta.zip

Get distinct records using linq to entity

Hi I'm using linq to entity in my application. I need to get distinct records based on one column value "Name"
So I have a table similar like you can see below:
(User)
ID
Name
Country
DateCreated
I need to select all this items but uniques based on Name (unique). Is it possible to accomplish using linq, if so please show me how.
var items = (from i in user select new {i.id, i.name, i.country, i.datecreated}).Distinct();

The Distinct() method doesn't perform well because it doesn't send the DISTINCT SQL predicate to the database. Use group instead:
var distinctResult = from c in result
group c by c.Id into uniqueIds
select uniqueIds.FirstOrDefault();
LINQ's group actually creates subgroups of entities keyed by the property you indicate:
Smith
John
Mary
Ed
Jones
Jerry
Bob
Sally
The syntax above returns the keys, resulting in a distinct list. More information here:
http://imar.spaanjaars.com/546/using-grouping-instead-of-distinct-in-entity-framework-to-optimize-performance

The purely LINQ way that occurs is to group by name, select distinct groups by key, then select based on that.
from i in user
group new {i.ID, i.Country, i.DateRecord} by i.Name into byNmGp
select byNmGp.First();
Edit: Entity Framework is of course a very popular linq provider, but it doesn't handle First() well here, though the logically equivalent (in this case) FirstOrDefault() will work fine. I prefer First() when not forced into FirstOrDefault() by EF's limitations, because its meaning better matches what is sought here.
Another way is to define a helper class:
private class MyRecord : IEquatable<MyRecord>
{
public int ID;
public string Name;
public string Country;
public DateTime DateCreated;
public bool Equals(MyRecord other)
{
return Name.Equals(other.Name);
}
public override bool Equals(object obj)
{
return obj is MyRecord && Equals((MyRecord)obj);
}
public override int GetHashCode()
{
return Name.GetHashCode();
}
}
/*...*/
var items = (from i in user select new MyRecord {i.ID, i.Name, i.Country, i.DateRecord}).Distinct();
This simply defines distinct differently. Performance will differ by whether the query provider can interpret that definition of equality or not. Convenience will differ based on whether you've similar LINQ queries doing much the same thing or not.

You can use something like this:
var distinctReports = reports.Select(c => c.CompanyCode)
.Distinct()
.Select(c => reports.FirstOrDefault(r => r.CompanyCode == c))
.ToList();

Here's another variation I ended up using which was based off the response from Svetlana. Shows an example of populating a GridView control with unique values. Thanks!
dataGridView_AnalyzeTestSuites.DataSource = (
from tr in _db.TestResults
where tr.TaskId == taskId
select new { TestSuiteName = tr.Test.TestSuite.Name }
).Distinct().ToList();

Hi here is how you can select distinct records with inner join. Hope it helps
var distinctrecords =
(entity.Table.Join(entity.Table2, x => x.Column, y => y.Column, (x, y) => new {x, y})
.Select(#t => new {#t.x.Column2, #t.y.Column3}))
.GroupBy(t => t.Column2)
.Select(g => g.FirstOrDefault());