Queryable Extensions

When using an ORM such as NHibernate or Entity Framework with Automapper's standard Mapper.Map functions, you may notice that the ORM will query all the fields of all the objects within a graph when Automapper is attempting to map the results to a destination type.

If your ORM exposes IQueryables, you can use Automapper's QueryableExtensions helper methods to address this key pain.

Using Entity Framework for an example, say that you have an entity OrderLine with a relationship with an entity Item. If you want to map this to an OrderLineDTO with the Item's Name property, the standard Mapper.Map call will result in Entity Framework querying the entire OrderLine and Item table.

Use this approach instead.

Given the following entities:

public class OrderLine
{
  public int Id { get; set; }
  public int OrderId { get; set; }
  public Item Item { get; set; }
  public decimal Quantity { get; set; }
}

public class Item
{
  public int Id { get; set; }
  public string Name { get; set; }
}

And the following DTO:

public class OrderLineDTO
{
  public int Id { get; set; }
  public int OrderId { get; set; }
  public string Item { get; set; }
  public decimal Quantity { get; set; }
}

You can use the Queryable Extensions like so:

public List<OrderLineDTO> GetLinesForOrder(int orderId)
{
  Mapper.CreateMap<OrderLine, OrderLineDTO>()
    .ForMember(dto => dto.Item, conf => conf.MapFrom(ol => ol.Item.Name);

  using (var context = new orderEntities())
  {
    return context.OrderLines.Where(ol => ol.OrderId == orderId)
             .Project().To<OrderLineDTO>().ToList();
  }
}

The .Project().To<OrderLineDTO>() will tell AutoMapper's mapping engine to emit a select clause to the IQueryable that will inform entity framework that it only needs to query the Name column of the Item table, same as if you manually projected your IQueryable to an OrderLineDTO with a Select clause.

Note that for this feature to work, all type conversions must be explicitly handled in your Mapping. For example, you can not rely on the ToString() override of the Item class to inform entity framework to only select from the Name column, and any data type changes, such as Double to Decimal must be explicitly handled as well.