public class Document
{
    public virtual int Id { get; set; }
    public virtual IList<File> Files { get; set; }
}
public class File
{
    public virtual int Id { get; protected set; }
    public virtual Document Document { get; set; }
}

public class HasManyConvention : IHasManyConvention
{
    public bool Accept(IOneToManyPart target)
    {
        return true;
    }
    public void Apply(IOneToManyPart target)
    {
        target.Cascade.All();
    }
}

public class DocumentMappingOverride : IAutoMappingOverride<Document>
{
    public void Override(AutoMap<Document> mapping)
    {
        mapping.HasMany(x => x.Files)
            .Inverse()
            // this line has no effect
            .Cascade.AllDeleteOrphan();
    }
}

public class FileMappingOverride : IAutoMappingOverride<File>
{
    public void Override(AutoMap<File> mapping)
    {
        mapping.References(x => x.Document).Not.Nullable();
    }
}

public class DocumentConvention : IClassConvention
{
    public bool Accept(IClassMap target)
    {
        return target.EntityType == typeof(Document);
    }
    public void Apply(IClassMap target)
    {
        target.SetAttribute("cascade", "all-delete-orphan");
    }
}

public class DocumentConvention : IClassConvention
{
    public bool Accept(IClassMap target)
    {
        return target.EntityType == typeof(Document);
    }
    public void Apply(IClassMap target)
    {
        target.HasMany<Document, File>(x => x.Files)
            .Inverse()
            .Cascade.AllDeleteOrphan();
    }
}

"Duplicate collection role mapping Document.Files"

mapping.IgnoreProperty(x => x.Files);

return target.EntityType == typeof(Document);