Projecting from an IDbAsyncQueryProvider - entity-framework

Question

I have a project in which we use Automapper to Project() entities from our data access layer into our domain classes which are consumed by external callers (i.e. WebAPI, windows services, etc). The idea is that we want to abstract our domain model from the actual database implementation, but often the domain model is comprised of at least the members from the database, and so Automapper makes constructing those domain models a lot easier using it's projection features.
This results in code that looks something like:
public class DbTask
{
public int Id { get; set; }
public string Name { get; set; }
}
public class Task
{
public int Id { get; set; }
public string Name { get; set; }
public User Assignee { get; set; }
public Priority CurrentPriority { get; set; }
}
And then a business service implementation that looks like:
public IQueryable<Task> QueryTasksByUser(int userId)
{
return dbContext
.Where(x => x.Assignee.Id == userId)
.Project()
.To<Task>();
}
So then at the API layer we want to leverage async/await as much as possible, and so I'm wondering if Automapper has any support for this. If I write my method like:
public async Task<IHttpActionResult> GetTopTaskForCurrentUser()
{
var task = await _taskService
.GetTasksByUser(Thread.CurrentPrincipal.AsUser().UserId)
.Where(x => x.CurrentPriority == Priority.Top)
.SingleOrDefaultAsync();
return Ok(task);
}
I get the following exception:
System.InvalidOperationException : The provider for the source IQueryable doesn't implement IDbAsyncQueryProvider. Only providers that implement IDbAsyncQueryProvider can be used for Entity Framework asynchronous operations. For more details see http://go.microsoft.com/fwlink/?LinkId=287068.
Is this something that Automapper just doesn't have support for [yet], or is there another method or extension that I'm missing?

Answer 1

You just need to make sure you call these methods in the correct order:
return await dbContext.Users
.Where(user => user.Id == id)
.Project().To<UserDto>()
.ToListAsync();
Put Project.To in the same place you'd put "Select" and it works fine.
I typically create an extension method to wrap the projection and async'ing:
public static async Task<List<TDestination>>
ToListAsync<TDestination>(
this IProjectionExpression projectionExpression)
{
return await projectionExpression.To<TDestination>().ToListAsync();
}
return await dbContext.Users
.Where(u => u.Id == id)
.Project().ToListAsync<UserDto>();