Updating Entities from DTOs with AutoMapper
About a year ago, Patrick Steele did a great job of describing AutoMapper: an absolutely necessary tool if you're using Data Transfer Objects to move data between parts of your application. AutoMapper will automatically move data from your business entities into the corresponding properties on your DTO, saving you many, many lines of repetitious assignment statements. It also means that if you add a new property to your DTO, AutoMapper will automatically pick up the corresponding property from the entity; no further changes to your code required.
You do need to be careful, however, when moving your data from a DTO and into an Entity Framework (EF) entity object. By default, AutoMapper creates a new instance of the object that the data is being moved into. If you write code like the following, for instance, the Customer object in custExisting that's returned from AutoMapper is a different object from the Customer object retrieved from the DbContext (even though the custExisting variable points to both of those objects):
Mapper.CreateMap(Of CustomerDTO, Customer)()
custExisting = dbs.Customers.Single(Function(c) c.Id = custDTO.Id)
custExisting = Mapper.Map(Of CustomerDTO, Customer)(custDTO)
That's bad, because EF won't track the changes made to that new custExisting object: When you call SaveChanges, nothing will be saved back to the database. This is a common-enough problem that there's even some misinformation about it floating around. I've seen a couple of forums, for instance, where people with this problem have been told that AutoMapper can't be used to update entities.
You can update entities with AutoMapper. Here's how: pass both the DTO and the entity object to AutoMapper's Map method. That's what this code does:
custExisting = Mapper.Map(Of CustomerDTO, Customer)(custDTO, custExisting)
Rather than creating a new entity object to return from the Map method, AutoMapper now updates the entity object passed in the second parameter and returns it to my custExisting variable. As a result, EF will track the changes made to the entity object; and, when you call SaveChanges, your updates will be sent to the database.
Posted by Peter Vogel on 11/06/2013