Whenever we want to use IEnumerable extension methods instead IQueryable version, we have use AsEnumerable() method.
var list = context.Details.AsEnumerable().Where(x => x.Code == "A1").ToList();
Why DbSet used IQueryable version of methods Like(Where , Select ,...) by default when other version available?
You'd usually want to use the IQueryable form, so that the filtering is done on the database instead of locally.
The code you've got will pull all records down to the client, and filter them there - that's extremely inefficient. You should only use AsEnumerable() when you're trying to do something that can't be performed in the database - usually after providing a server-side filter. (You may be able to do a coarse filter on the server, then a more fine-grained filter locally - but at least then you're not pulling all the records...)
Because IQuaryable methods are translated into SQL by Entity Framework.But IEnumerable method are not.So if you use IEnumearable all the data will be fetched from DB which is not always what you want.
Related
I'm running an EF statement where I need to calculate de deductibles. After long trying, I can't seem to add a custom function in a .Select() statement. Instead I'm trying to add the values after my .Select() statement.
The problem here is, in my CalculateDeductibles() I can't seem to add any values to item.Deductibles.
The GetDeductibles(item.RequestId) is a rather heavy funtion that does several extra queries, so I'm trying to prevent to convert my IQueryable to an IList object.
So there are actually 2 questions:
Can I have the GetDeductibles() function directly in my .Select() statement?
Can I somehow (with keeping an eye on performance) add the value after I did my .Select()
Code:
public IQueryable<ReinsuranceSlip> GetReinsuranceSlipsOverview(int userId, int companyId, string owner, string ownerCompany)
{
IQueryable<ReinsuranceSlip> model = null;
model = _context.Request
.Where(w => w.RequestGroup.ProgramData.MCContactId == userId)
.Select(x => new ReinsuranceSlip()
{
Id = x.Id,
RequestId = x.Id,
LocalPolicyNumber = x.LocalPolicyNumber,
BusinessLine = x.RequestGroup.ProgramData.BusinessLine.DisplayName,
BusinessLineId = x.RequestGroup.ProgramData.BusinessLine.Id,
ParentBroker = x.RequestGroup.ProgramData.Broker.Name,
LocalBroker = x.Broker.Name,
InceptionDate = x.InceptionDate,
RenewDate = x.RenewDate,
//Deductibles = CalculateDeductibles(x)
});
CalculateDeductibles(model);
return model;
}
private void CalculateDeductibles(IQueryable<ReinsuranceSlip> model)
{
//model.ForEach(m => m.Deductibles = GetDeductibles(m.RequestId));
foreach (var item in model)
{
item.Deductibles = GetDeductibles(item.RequestId);
}
}
Updated and Sorry for the first version of this answer. I didn't quite understand.
Answer 1: IQueryable is using to creating a complete SQL statement to call in SQL Server. So If you want to use IQueryable, your methods need to generate statements and return it. Your GetDetuctibles method get request Id argument but your queryable model object didn't collect any data from DB yet, and it didn't know x.Id value. Even more, your GetCarearDetuctiples get an argument so and with that argument generates a queryable object and after some calculations, it returns decimal. I mean yes you can use your methods in select statement but it's really complicated. You can use AsExpendable() LINQ method and re-write your methods return type Expression or Iqueryable.
For detailed info you should check. This:
Entity Navigation Property IQueryable cannot be translated into a store expression and this: http://www.albahari.com/nutshell/predicatebuilder.aspx
And you also should check this article to understand IQueryable interface: https://samueleresca.net/2015/03/the-difference-between-iqueryable-and-ienumerable/
Answer 2: You can use the IEnumerable interface instead IQueryable interface to achieve this. It will be easy to use in this case. You can make performance tests and improve your methods by time.
But if I were you, I'd consider using Stored Procedures for performance gain.
You'll have to understand the differences between an IEnumerable and an IQueryable.
An IEnumerable object holds everything to enumerate over the elements in the sequence that this object represents. You can ask for the first element, and once you've got it, you can repeatedly ask for the next element until there is no more next element.
An IQueryable works differently. An IQueryable holds an Expression and a Provider. The Expression is a generic description of what data should be selected. The Provider knows who has to execute the query (usually a database), and it knows how to translate the Expression into a format that the Provider understands.
There are two types of LINQ functions: the ones that return IQueryable<TResult> and the ones that return TResult. Functions form the first type do not execute the query, they will only change the expression. They use deferred execution. Functions of the second group will execute the query.
When the query must be executed, the Provider takes the Expression and tries to translate it into the format that the process that executes the query understand. If this process is a relational database management system this will usually be SQL.
This translation is the reason that you can't add your own functionality: the Expression must be translatable to SQL, and the only thing that your functions may do is call functions that will change the Expression to something that can be translated into SQL.
In fact, even entity framework does not support all LINQ functionalities. There is a list of Supported and Unsupported LINQ methods
Back to your questions
Can I have GetDeductibles directly in my query?
No you can't, unless you can make it thus simple that it will only change the Expression using only supporte LINQ methods. You'll have to write this in the format of an extension function. See extension methods demystified
Your GetDeductibles should have an IQueryable<TSource> as input, and return an IQueryable<TResult> as output:
static class QueryableExtensions
{
public static IQueryable<TResult> ToDeductibles<TSource, TResult, ...>(
this IQueryable<TSource> source,
... other input parameters, keySelectors, resultSelectors, etc)
{
IQueryable<TResult> result = source... // use only supported LINQ methods
return result;
}
}
If you really need to call other local functions, consider calling AsEnumerable just before calling the local functions. The advantage above ToList is that smart IQueryable providers, like the one in Entity Framework will not fetch all items but the items per page. So if you only need a few ones, you won't have transported all data to your local process. Make sure you throw away all data you don't need anymore before calling AsEnumerable, thus limiting the amount of transported data.
Can I somehow add the value after I did my .Select()
LINQ is meant to query data, not to change it. Before you can change the data you'll have to materialize it before changing it. In case of a database query, this means that you have a copy of the archived data, not the original. So if you make changes, you'll change the copies, not the originals.
When using entity framework, you'll have to fetch every item that you want to update / remove. Make sure you do not select values, but select the original items.
NOT:
var schoolToUpdate = schoolDbContext.Schools.Where(schoolId = 10)
.Select(school = new
{
... // you get a copy of the values: fast, but not suitable for updates
})
.FirstOrDefault();
BUT:
School schoolToUpdate = schoolDbContext.Schools.Where(schoolId = 10)
.FirstOrDefault()
Now your DbContext has the original School in its ChangeTracker. If you change the SchoolToUpdate, and call SaveChanges, your SchoolToUpdate is compared with the original School, to check if the School must be updated.
If you want, you can bypass this mechanism, by Attaching a new School directly to the ChangeTracker, or call a Stored procedure.
I've got a class with a bool method, e.g.:
public bool IsInFuture()
{ return this.Date > DateTime.Now; }
And I store it in a database using EF Code First. If I'll try to use that predicate in Linq operations, I'll get an exception, as it can not be translated into SQL:
await context.Where(order => order.IsInFuture()).ToListAsync();
That predicate's logic can be rather complicated and I wouldn't like to duplicate it in my code. It there any way I can "inline" it's code into Linq operations? I'm pretty sure, this problem has a solution.
Thanks in advance!
You can't use the predicate directly. How should EF know how to convert it into an SQL WHERE statement?
You have a few alternatives:
Rephrase the query to use standard operations, e.g. smaller-than. This will duplicate the logic from the predicate, though.
Use a raw SQL query. This will also duplicate the logic, but in SQL.
Load the entire list of entities and then filter in-memory (i.e. Where() after ToList()). Like this, you can reuse the predicate. Note that this only a good idea for very small data sets.
I'm trying to get rows based on a WHERE clause in a DbSet object. I have this:
dbContext.Workers
I can get a list like this:
workers = m.Workers.Where(w => w.BranchId == curUser.BranchId).ToList<Worker>();
But as you can see, it returns a List<Worker> and I can't use methods like workers.Find(WorkerId) with it.
Basically, I'm trying to return a DBSet based on some filter I mean I want to use LINQ on a DBSet class. I want this because I need to use workers.Find(WorkerId) also maybe I will need to update this model. So, I will get a list based on where clause, I will change some values and will use dbContext.SaveChanges(). Is that possible?
Thanks
Where(...) returns an IQueryable, which you can manipulate BEFORE the query runs. ToList() will force execution of the query and put the objects into memory. If you want to 'find' an item AFTER the query then you could do something like this with your List:
workers.SingleOrDerfault(x => x.WorkerId == WorkerId);
If you have them all in memory like this, and make changes, then you will persist those changes with a call to .SaveChanges().
However, if you need to apply more filtering to your IQueryable BEFORE the query hits the database, then you'll want to manipulate the IQueryable BEFORE the call to ToList().
I'd like to know whether, when defining my interface, I should prefer IQueryable over List for groups of objects. Or perhaps IEnumerable is better, since IEnumerable types can be cast to IQueryable to be used with LINQ.
I was looking at a course online and it was dealing with EntityFramework, for which it is best to use IQueryable since LINQ uses it (okay there's more to it than that but probably not important right now).
The code below was used, and it got me thinking if I should be specifying IQueryable instead of List for my groups of objects.
namespace eManager.Domain
{
public interface IDepartmentDataSource
{
IQueryable<Employee> Employees { get; }
IQueryable<Department> Departments { get; }
}
If I were building an interface for a service that calls the repository to get Employees, I would usually specify
List <Employees>
but is this best practice? Would IQueryable give more flexibility to classes that implement my interface? What about the overhead of having to import LINQ if they don't need it (say they only want a List)? Should I use IEnumerable over both of these?
The IQueryable interface is in effect a query on Entity Framework that is not yet executed. When you call ToList(), First() or FirstOrDefault() etc. on IQueryable Entity Framework will construct the SQL query and query the database.
IEnumerable on the other hand is 'just' an enumerator on a collection. You can use it to filter the collection but you'd use LINQ to Objects. Entity Framework doesn't come into play here.
To answer your question: it depends. If you want the clients of your repository to be able to further customize the queries they can execute you should expose IQueryable, but if you want full control in your repository on how the database is queried you could use IEnumerable.
I prefer to use IEnumerable, because that doesn't leak the use of Entity Framework throughout the application. The repository is responsible for database access. It is also easier to make LINQ to EF optimizations, because the queries are only in the repository.
What I usually do is make the repositories return IQueryable. Then in the BL I specify either IEnumerable or IQueryble. It is important to know the main differences between IQueryble and IEnumerable.
Lets say you fetch data into IEnumerable
IEnumerable employees=this.repository.GetAll();
Now let's say this specific function require only employees with age over 21 and the others are not needed at all.
You would do:
employees.Where(a=>a.Age>21)
In this case the original query will be loaded in the memory and then the Where will be applied.
Now lets say you change the function to fetch the data into IQueryable
IQueryable employees=this.repository.GetAll();
employees.Where(a=>a.Age>21)
This time when you modify the query with the Where clause, the whole query will be executed in the database (if possible) and you will only get employees with age over 21 from the database.
In the IEnumerable case you will get all the employees from the database and then they will get filtered in memory to satisfy the where condition.
Use IEnumerable, IList or something else?
If you know what operations will be executed on the collection you can easily choose which interface to use. Basically if you will only iterate over the collection you would use IEnumerable. If you would do more operations you need to choose the appropriate interface. There are good videos of .NET collections in pluralsight.
So I have the following scenario and I am not sure how to approach it.
In the app that we are building we have a ReferenceDataService which is used to load data via RIA serices. A query is built up on the client as an IQuerable and submitted to the server for retrieval. Now most of the reference queries that are submitted are identical and called multiple times, therefore I would like to service these queries from the entity cache instead.
Below is an example of the query expression that is generated and I have highlighted the area where the query changes. There are some scenarios where the query has additonal criteria but the majority probably dont.
Chime.DataModel.classification[]
.Where(c => ((value(Chime.Modules.Reference.Client.Agent.ReferenceDataLoader+<>c__DisplayClass20).includeSchema
*AndAlso c.class_code.StartsWith(value(Chime.Modules.Reference.Client.Agent.ReferenceDataLoader+<>c__DisplayClass20).schema))*
OrElse (Not(value(Chime.Modules.Reference.Client.Agent.ReferenceDataLoader+<>c__DisplayClass20).includeSchema)
AndAlso *c.parent_code.StartsWith(value(Chime.Modules.Reference.Client.Agent.ReferenceDataLoader<>c__DisplayClass20).schema))))*
.OrderByDescending(c => c.value_scheme_ind)
.ThenBy(c => c.sequence_number)
.ThenBy(c => c.class_label)
So what I would like to do is actually cache the query and if another comes in with identical criteria I can hit the cache. The problem I have is that the literals used in the query do not seem to be available therefore I cannot determin if one query is different from another. The schema code for example is nowhere to be found. It must be sent the the server at some point but I am unsure how I can get at it.
Does anyone know a way to do this or come across this before?
Dave
If you were using WCF Data Services, I would just say call ToString() on your IQueryable and it would return an exact URI of the query to be executed against the Data Service...and you could simply cache based on that string value.
As far as I know, RIA doesn't expose such hooks, so you'll have to rely on more traditional methods.
Implement an ExpressionVisitor that visits your IQueryable.Expression and computes a hash code for it.
You mentioned you can't see your constant string values in your expression tree. That's because they are not inlined yet into the expression tree and thus still exist as MemberAccessExpressions, not ConstantExpressions. To fix this, you can use a partial evaluating ExpressionVisitor. The source for such is given here
http://msdn.microsoft.com/en-us/library/bb546158.aspx
Look at the Evaluator class.