How can I use IEnumerable.Where().Count() in LINQ query? - asp.net-mvc

Question

In Enitity Framework I'd like to use a linq query in the where clause of another linq query.
public class A {
}
public class B {
public A A { get; set; }
}
public class AB {
public A A { get; set; }
public B B { get; set; }
}
IEnumerable<MassiveObject> filteredData = ... // MassiveObject contains A and some more attributes
var query = from b in db.Bs
.Include("A")
where filteredData.Where(x => x.A.equals(b.A)).Count() > 0 // filteredAs contains an object that contains the same A as the b's A
select new AB {
A = filteredData.Where(a => a.equals(b.A)),
B = b
};
It looks like LINQ doesn't support that. "Unable to create a constant value of type 'MassiveObject'. Only primitive types or enumeration types are supported in this context." Neither the first nor the second use of the nested linq.
Is there another way to this without looping through the query's results after the query was executed?

Answer 1

filteredData is a collection that exists inside the memory of your process. On the other hand, the EF query is translated into SQL that runs on the SQL server.
The SQL server cannot know what data exists in the memory of your process, and even if it could see that memory SQL still has no concept of object instances or iteration. So what you are trying to do is fundamentally impossible.

Answer 2

Linq does not support using an enumeration of an object in a SQL construct because there is no way to make that a SQL stement to start with.
Pull the b's and then filter in that enumeration.

Answer 3

You can't mix in-memory and SQL LINQ queries like this. Instead you need to pull the SQL data into memory before doing the join.
Try something like this instead:
var query =
from b in db.Bs.Include("A").ToArray()
join a in filteredData.Select(x => x.A) on b.A equals a into gas
where gas.Any() // or use gas.Count() > 0
select new AB
{
A = gas.First(),
B = b,
};
Note the .ToArray() call that brings the SQL data into memory.

Answer 4

It isn't very clear what you are trying to achieve, however, previous queries can be used in the current query in various ways e.g.
query2 = source.Where(s => query1.Contains(s));
or using LINQ joins.
In your case itlooks like youare trying to reuse a filter,not a query.
You can achieve this using an extension method from IQueryable<T> to IQueryable<T> or by using expressions.
e.g.
static class MyExtensions
{
public static void IQueryable<T> FilterResults(this IQueryable<T> query)
{
return query.Where(i => i....);
}
}
and then use like so:
query2 = query.FilterResults().Where(...);
For details on the alternative of using Expression Trees see this MSDN article.