Reference to an ITVF raises a “second operation started on this context before a previous operation completed” exception

Question

I am attempting to reference an Inline Table-Valued Function (ITVF) in a Linq query:

var results = await (
    from v in _context.Vehicles
    from r in _con         


        

Answer 1

Sorry, my fault. The technique from the answer to your previous question is applicable for calling ITVF with constant/variable parameters, but not with correlated subqueries like in your case (and my wrong example).

The solution is to remove the ITVF parameter and extend the result to include that column as well (effectively turning it into parameterless view):

CREATE FUNCTION dbo.UnitRepairStatus()
RETURNS TABLE
AS
RETURN
  SELECT u.UnitNumber, h.InRepair
  FROM    Schema2.Unit u
  INNER JOIN Schema2.History h on u.ID = h.UnitID

Also remove the parameter from the context method

EF Core 2.x:

public IQueryable<UnitRepairStatus> UnitRepairStatus() =>
    Query<UnitRepairStatus>().FromSql("SELECT * FROM UnitRepairStatus()");

EF Core 3.x:

public IQueryable<UnitRepairStatus> UnitRepairStatus() =>
    Set<UnitRepairStatus>().FromSqlRaw("SELECT * FROM UnitRepairStatus()");

and change the LINQ query to use join:

var results = await (
    from v in _context.Vehicles
    join r in _context.UnitRepairStatus() on v.VehicleNumber equals r.UnitNumber // <---
    orderby v.VehicleNumber
    select new FooViewModel { 
        ID = v.ID, 
        VehicleNumber = v.VehicleNumber,
        InRepair = Convert.ToBoolean(r.InRepair)
    }
).ToListAsync();

Now it should translate and execute server side, and successfully get materialized at the client.

The problem with the original approach was that EF Core silently switched the query execution to client evaluation (hate that), and then hit its protection for executing multiple async operations on one and the same context.