How to Map Sitecore Rules Field with Glass and TDS

tl;dr Copy lines 246 and 247 from here. Regenerate your Glass classes. 

Today a colleague asked how to map the rules field from Sitecore with Glass Mapper using TDS and T4 templates. It just so happens that I'd recently worked on this problem, and I thought I would share it with others here.

The basic issue is that the glassv3item.tt template doesn't know how to deal with the Rules field. The GetGlassFieldByType method is responsible for assigning a type to mapped field. It does this with a switch statement. Our rules field is falling all the way through to the default case which maps the field to an object. We need to add a case for the field.Type value when it equals "rules".

What type will we map to though? On first pass I thought something like XDocument would make a lot of sense. Problem is that doesn't work. The value is always null. I took a look inside the Glass.Mapper.Sc.dll at what I believe is the code responsible for returning a value. It looks to me like the mapping code isn't fully implemented, and Glass relies on a generic method that simply returns a string value.

Not a big deal. We can work with this.

After you've modified the T4 template and regenerated your Glass classes, you should have a mapped property on your class of type string. I've found for my purposes this is perfect for my needs, but that didn't stop me from extending my partial class with an XDocument property...just in case.

public partial class GeneratedClass
{
    public XDocument RuleAsXDocument
    {
        get { return XDocument.Parse(this.Rule); }
    }
}