I need to create a generic method (ContainsLambda) that takes collection and a lambda expression (a single property) and would check if the given property contains values in the given collection.
Here is my method
public static TModel[] ContainsLambda<TModel, TKey>(IEnumerable<TKey> keys, Func<TModel, TKey> property)
{
DbSet<TModel> repository = DbContext.Set<TModel>();
return repository.Where(x => keys.Contains(property.Invoke(x)))
.ToArray();
}
Then I would call it using something like this ContainsLambda<Customer, int>(new List<int> {10, 20, 30}, p => p.Age)
The above code throws the following runtime error.
LINQ to Entities does not recognize the method 'Int32
Invoke(Customer)' method, and this method cannot be translated into a
store expression.
How can I call .Invoke() on a lambda that would be used in LINQ which would then be translated into SQL expression?
Try making your repository an enumerable. This bypasses Linq to Entities, and causes your code to run in Linq to Objects.
return repository.AsEnumerable()
.Where(x => keys.Contains(property.Invoke(x)))
.ToArray();
If you don't want all of the records in your repository to travel across the wire (i.e. you need the filtering to run on the server), you will have to write a custom SQL query to retrieve the data.
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 have a generic repository using EF6. The issue has to do with association properties requiring an "Include" even though it shouldn't. The following works:
IQueryable<User> dbQuery = _db.Set<User>();
return dbQuery.Where(x => x.Child.Name == "Foo").ToList();
However, the following does not work:
Func<User, bool> filter = x => x.Child.Name == "Foo";
IQueryable<User> dbQuery = _db.Set<User>();
return dbQuery.Where(filter).ToList();
It throws an "Object Reference not set..." exception on Child.
The following resolves it:
Func<User, bool> filter = x => x.Child.Name == "Foo";
IQueryable<User> dbQuery = _db.Set<User>();
dbQuery = dbQuery.Include(x => x.Child);
return dbQuery.Where(filter).ToList();
I don't understand why this is necessary though. Anyone know a way to resolve this without using the "Include"?
You are should use Expression to let EF provider parse your query.
Change the Func<User, bool> to Expression<Func<User, bool>>
The first snippet is providing an Expression to Where, which is being translated into SQL, and is doing the entire operation in the database. The latter two are passing a compiled method to Where, which it can't translate into SQL, which means that the entire database table is being pulled down into memory, and the entire operation is run in your application. When you pull down the whole table it doesn't pull down related records unless you Include them.
The solution is not to pull down both the entire table and also all of the data from all of the related records; the solution is to do the filtering in the database rather than in your application.
I have a Service Repository pattern built on top of Entity Framework.
The service has methods such as Find(IQuery query) that return IEnumerable.
The IQuery object is our own query object type where we convert strings to an IQueryable expression that the repository, which exposes IQueryable, can use.
What I'd like to do is be able to write a queryable on the client side and pass that over the service so that we can take advantage of the static typing and linq style queries instead of building our own query object in formation.
In other words I want to be able to do something like:
var query = new List<Type>().Where(x => x.Property == "argument").AsQueryable();
service.find(query);
Then I would pass the queryable or the expression it creates to my repository and work like that.
Is this sort of thing possible, or would I have to build the expression from scratch? It seems like this should be possible, but I really don't know where to begin or see examples of how to share an expression like this.
If you call AsQueryable() on an IEnumerable that doesn't already implement IQueryable, then that whole enumerable is just stuffed into a ConstantExpression, and any previous LINQ operations are not expressed as expression trees.
If you call AsQueryable() right on the source, then the LINQ operations will return an IQueryable with the proper query expression tree:
var query = new List<Type>().AsQueryable().Where(x => x.Property == "argument");
I have an extension method on an Enum called GetName which returns a string. I'm using it in linq to entity framework to select rows with a specific product name. However, when the code is getting executed it's throwing a NotSupportedException
LINQ to Entities does not recognize the method System.String
GetName(Tool.ViewModels.Product) method, and this method cannot be
translated into a store expression.
Here is the code I am executing:
try
{
//Linq to Entity Framework
var contextRow = Contexts.Data.Source.SingleOrDefault(p => p.Product == Product.ProductOne.GetName());
}
catch (Exception e)
{
throw e;
}
Does Linq not recognize extension methods in its evaluations? Or is there something more going on?
Under the hood the Entity Framework is mapping the code you type into SQL queries. When it sees the call to GetName it sees a call into a user defined function. It has no power to translate, what essentially amounts to raw IL, into a well formed SQL query. This is why you are getting that exception
when using LINQ, you have to define a context and within that context perform operations on the database, for example the code you want to run, you can be this way
using (ModelLinq _context = new ModelLinq()){
var rows= _context.anyTable.Where(p=> p.anyColumn == value);
}
And Entity Framework is a mapping of your data base for you to execute SQL queries with lenguale c #
regards!
Why am I getting:
The method 'Where' cannot follow the method 'Select' or is not
supported. Try writing the query in terms of supported methods or call
the 'AsEnumerable' or 'ToList' method before calling unsupported
methods.
...when using the WHERE clause, like when calling:
XrmServiceContext.CreateQuery<Contact>().Project().To<Person>().Where(p => p.FirstName == "John").First();
?
This works:
XrmServiceContext.CreateQuery<Contact>().Project().To<Person>().First();
Also this works:
XrmServiceContext.CreateQuery<Contact>().Where(p => p.FirstName == "John").First();
I'm using AutoMapper QueryableExtension.
Additional info:
I don't want to call ToList() before the Where clause. I know it will works that way.
CreateQuery<TEntity>() returns IQueryable<TEntity>.
It's because whatever query provider you are using isn't able to handle this. It's not invalid in the general case; in fact most query providers do support filtering after projecting. Certain query providers simply aren't as robust as others, or they are representing a query model that is less flexible/powerful than the LINQ interface (or both). As a result, LINQ operations that are correct from the C# compiler's point of view might still not be translatable by the query provider, so the best it can do is throw an exception at runtime.
Why don't you just move the where so it is before the projection? It will result in a single query being executed which filters and projects:
XrmServiceContext.CreateQuery<Contact>().Where(p => p.FirstName == "John").Project().To<Person>().First();
Looking at AutoMapper's instructions for the QueryableExtensions it has an example showing the Where clause before the projection. You need to refactor your code to support this model, as opposed to placing the Where clause after the projection.
public List GetLinesForOrder(int orderId)
{
Mapper.CreateMap()
.ForMember(dto => dto.Item, conf => conf.MapFrom(ol => ol.Item.Name);
using (var context = new orderEntities())
{
return context.OrderLines.Where(ol => ol.OrderId == orderId)
.Project().To().ToList();
}
}
Given the limitations of Dynamic CRM's LINQ provider you should not expect AutoMapper to necessarily get the LINQ query correct.
There is actually a logic behind this design. As the developer you create a working Where clause. You then let AutoMapper's Project().To() define the select statement. Since CRM's LINQ provider has support for anonymous types in it should work correctly. The purpose of projection in AutoMapper is to limit the data retrieved from each class to only that needed for the projected to class. It is not intended to write a Where clause based on the projected to class.