Open DataReader Error when using AsEnumerable - linq

Question

I am using the following function to Query an EF6 model using a bunch of query expressions that I have built programmatically:
/// <summary>
/// Common application of generated queries to select the required resources
/// </summary>
/// <typeparam name="T">The EF model type</typeparam>
/// <param name="source">The source of models (a DbSet)</param>
/// <param name="queries">A collection of query pairs (database, local) to be applied as Where clauses</param>
/// <returns></returns>
protected static IEnumerable<string> ApplyCriteriaBase<T>(IQueryable<T> source, IEnumerable<(Expression, Expression)> queries) where T : DbResource
{
// Apply the database queries sequentially as Where clauses
var resources = queries
.Select(p => p.Item1)
.Aggregate(source, (ds, qry) => ds.Where(qry as Expression<Func<T, bool>>))
.Distinct();
// If there are no local queries then don't bother fetching the whole object, just select the id in the generated SQL
if(queries.All(q => q.Item2 == null))
return resources
.Select(a => a.Id.ToString());
// Apply local queries
// AsEnumerable() escapes from the IQueryable monad so the subsequent queries are executed locally on IEnumerable
// This requires fetching the whole object because we dont know which bit is to be tested by the local query
// Note that IEnumerable doesn't understand query expressions so we have to compile them to code and then invoke them
return queries
.Select(p => p.Item2)
.Where(q => q != null)
.Aggregate(
resources.ToList() as IEnumerable<T>, // .AsEnumerable() doesn't work
(ds, qry) => ds.Where((qry as Expression<Func<T, bool>>).Compile().Invoke))
.Select(a => a.Id.ToString());
}
The idea is that queries contains 2-tuples of query expressions where the first of each is to be run against the database (MySql) and the second, if present, is to be run against the EF models returned.
It is my understanding that AsEnumerable() should permit the second query to be executed locally and lazily without pulling all of the results into memory. Unfortunately this fails with the error:
"There is already an open DataReader associated with this Connection
which must be closed first."
The fix shown in the example, converting the result stream to a List, works correctly but is strict.
Why doesn't AsEnumerable() work and is there another way to process the results lazily? Some instances of DbResource are very large and I only want to accumulate the Id of each resource.
Thanks,
Andy