在扩展方法中使用扩展方法进行 EF 查询

Question

I have table Requests, that has Approvals :

public class Request {
    public virtual List<Approval> Approvals { get; set; } 
}

Requests come from DB

public DbSet<Request> Request { get; set; }

I have extension method:

public static IQueryable<Request> HaveApprovals(this IQueryable<Request> requests) {
    return requests.Where(r => r.ActiveApprovals().Count() > 0).ToList();
}

And another extension method:

public static IEnumerable<Approval> ActiveApprovals(this Request request) {
     return request.Approvals.Where(a => !a.Deleted);
}

But getting error:

LINQ to Entities does not recognize the method 'System.Collections.Generic.IEnumerable`1[Domain.Approval] ActiveApprovals(Domain.Request)' method, and this method cannot be translated into a store expression.

For some reason one extension method is able to be translated to LINQ, but when using extension method inside another, then it fails to translate. Any suggestions?

Answer 1

Try to rewrite your HaveApprovals extension in this way:

public static IQueryable<Request> HaveApprovals(this IQueryable<Request> requests) {
    return requests.Where(r => r.Approvals.Any(a => !a.Deleted)).ToList();
}

Error occurs because EF tries to execute any operation in query on the server side ( SQL server). So SQL know nothing about your extension method and how to execute it.

UPDATE#1:

query below

var query = this.DataContext.Products.Where(x => x.Sales.Any(s => s.SectorId == 1));

will generate following SQL query

SELECT [t0].[Id], [t0].[Name], [t0].[Description]
FROM [dbo].[Products] AS [t0]
WHERE EXISTS(
    SELECT NULL AS [EMPTY]
    FROM [dbo].[Sales] AS [t1]
    WHERE ([t1].[SectorId] = @p0) AND ([t1].[ProductId] = [t0].[Id])
    )
}

It will not extract all records so it will not decrease performance.

Answer 2

Return a Iqueryable or Ienumerable and not a list. The query will be handled by sql so it cannot understand the returned list