Why is a custom resolver needed here (AutoMapper)?

Question

Given the following entity model:

public class Location
{
    public int Id { get; set; }
    public Coordinates Center { get; set; }
}
public class Coordinates
{
    public double? Latitude { get; set; }
    public double? Longitude { get; set; }
}

... and the following view model:

public class LocationModel
{
    public int Id { get; set; }
    public double? CenterLatitude { get; set; }
    public double? CenterLongitude { get; set; }
}

The LocationModel properties are named such that mapping from the entity to the model does not require a custom resolver.

However when mapping from a model to an entity, the following custom resolver is needed:

CreateMap<LocationModel, Location>()
    .ForMember(target => target.Center, opt => opt
        .ResolveUsing(source => new Coordinates 
            { 
                Latitude = source.CenterLatitude, 
                Longitude = source.CenterLongitude 
            }))

Why is this? Is there a simpler way to make AutoMapper to construct a new Coordinates value object based on the naming conventions in the viewmodel?

Update

To answer the first comment, there is nothing special about the entity to viewmodel mapping:

CreateMap<Location, LocationModel>();

Can you include the mapping from the entity to the model? – Davin Tryon Feb 03 '12 at 17:31 — Davin Tryon, Feb 03 '12 at 17:31
I have included the mapping from the entity to the model. – danludwig Feb 03 '12 at 19:01 — danludwig, Feb 03 '12 at 19:01

devuxer · Accepted Answer · 2012-02-03T19:38:37.753

1

Edit

Please see comment thread below. This answer is actually for the opposite mapping.

You're doing something else wrong. You are following the convention correctly, so the mapping should work without any need for a resolver.

I just tried this test, and it passed:

public class Location
{
    public int Id { get; set; }
    public Coordinates Center { get; set; }
}

public class Coordinates
{
    public double? Latitude { get; set; }
    public double? Longitude { get; set; }
}

public class LocationModel
{
    public int Id { get; set; }
    public double? CenterLatitude { get; set; }
    public double? CenterLongitude { get; set; }
}

[Test]
public void LocationMapsToLocationModel()
{
    Mapper.CreateMap<Location, LocationModel>();

    var location = new Location
    {
        Id = 1,
        Center = new Coordinates { Latitude = 1.11, Longitude = 2.22 }
    };

    var locationModel = Mapper.Map<LocationModel>(location);

    Assert.AreEqual(2.22, locationModel.CenterLongitude);
}

edited Feb 03 '12 at 19:38

answered Feb 03 '12 at 18:08

devuxer

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Yes. Mapping from the entity to the viewmodel works just fine. It's going the other way that I'm finding need for the resolver: `Mapper.CreateMap()`. – danludwig Feb 03 '12 at 18:59
@olivehour, oops sorry about that. In that case, yes, I think you do need a custom resolver. As far as I know, AutoMapper doesn't do "unflattening", just flattening. You might want to look into ValueInjecter (note the spelling). – devuxer Feb 03 '12 at 19:10
Thanks, never heard of that lib before. – danludwig Feb 03 '12 at 19:34

Why is a custom resolver needed here (AutoMapper)?

1 Answers1