Technique for carrying metadata to View Models with AutoMapper

Question

I use AutoMapper to map my domain objects to my view models. I have metadata in my domain layer, that I would like to carry over to the view layer and into ModelMetadata. (This metadata is not UI logic, but provides necessary information to my views).

Right now, my solution is to use a separate MetadataProvider (independently of ASP.NET MVC), and use conventions to apply the relevant metadata to the ModelMetadata object via an AssociatedMetadataProvider. The problem with this approach is that I have to test for the same conventions when binding the ModelMetadata from the domain as I do with my AutoMapping, and it seems like there should be a way to make this more orthogonal. Can anyone recommend a better way to accomplish this?

Answer 1

I use the approach below to automatically copy data annotations from my entities to my view model. This ensures that things like StringLength and Required values are always the same for entity/viewmodel.

It works using the Automapper configuration, so works if the properties are named differently on the viewmodel as long as AutoMapper is setup correctly.

You need to create a custom ModelValidatorProvider and custom ModelMetadataProvider to get this to work. My memory on why is a little foggy, but I believe it's so both server and client side validation work, as well as any other formatting you do based on the metadata (eg an asterix next to required fields).

Note: I have simplified my code slightly as I added it below, so there may be a few small issues.

Metadata Provider

public class MetadataProvider : DataAnnotationsModelMetadataProvider
{        
    private IConfigurationProvider _mapper;

    public MetadataProvider(IConfigurationProvider mapper)
    {           
        _mapper = mapper;
    }

    protected override System.Web.Mvc.ModelMetadata CreateMetadata(IEnumerable<Attribute> attributes, Type containerType, Func<object> modelAccessor, Type modelType, string propertyName)
    {           
        //Grab attributes from the entity columns and copy them to the view model
        var mappedAttributes = _mapper.GetMappedAttributes(containerType, propertyName, attributes);

        return base.CreateMetadata(mappedAttributes, containerType, modelAccessor, modelType, propertyName);

}
}

Validator Provivder

public class ValidatorProvider : DataAnnotationsModelValidatorProvider
{
    private IConfigurationProvider _mapper;

    public ValidatorProvider(IConfigurationProvider mapper) 
    {
        _mapper = mapper;
    }

    protected override System.Collections.Generic.IEnumerable<ModelValidator> GetValidators(System.Web.Mvc.ModelMetadata metadata, ControllerContext context, IEnumerable<Attribute> attributes)
    {   
        var mappedAttributes = _mapper.GetMappedAttributes(metadata.ContainerType, metadata.PropertyName, attributes);
        return base.GetValidators(metadata, context, mappedAttributes);
    }
}

Helper Method Referenced in above 2 classes

public static IEnumerable<Attribute> GetMappedAttributes(this IConfigurationProvider mapper, Type sourceType, string propertyName, IEnumerable<Attribute> existingAttributes)
{
    if (sourceType != null)
    {
        foreach (var typeMap in mapper.GetAllTypeMaps().Where(i => i.SourceType == sourceType))
        {
            foreach (var propertyMap in typeMap.GetPropertyMaps())
            {
                if (propertyMap.IsIgnored() || propertyMap.SourceMember == null)
                    continue;

                if (propertyMap.SourceMember.Name == propertyName)
                {
                    foreach (ValidationAttribute attribute in propertyMap.DestinationProperty.GetCustomAttributes(typeof(ValidationAttribute), true))
                    {
                        if (!existingAttributes.Any(i => i.GetType() == attribute.GetType()))
                            yield return attribute;
                    }
                }
            }
        }
    }

    if (existingAttributes != null)
    {
        foreach (var attribute in existingAttributes)
        {
            yield return attribute;
        }
    }

}

Other Notes

• If you're using dependency injection, make sure your container isn't already replacing the built in metadata provider or validator provider. In my case I was using the Ninject.MVC3 package which bound one of them after creating the kernel, I then had to rebind it afterwards so my class was actually used. I was getting exceptions about Required only being allowed to be added once, took most of a day to track it down.

Answer 2

if your metadata are provided with attributes define the attributes in MetaDataTypes, then apply the same MetaDataType to both your domain class and to your viewmodels. You can define all MetaDataTypes in a separate dll that is reference by both layers. There are some issues with this approach if your ViewModel classes have not some properties that is used in the MetaDataType, but this can be fixed with a custom Provider(I have the code if youlike this approach).