public class SearchModelMapping : IAutoMappingOverride<SearchItemModel>
{
    public void Override(AutoMapping<SearchItemModel> mapping)
    {
        mapping.Table("MainTable");
        mapping.Id(model => model.Vendor, "ITEM_VENDOR");

        mapping.Map(model => model.Upc, "ITEM_UPC");
        mapping.Map(model => model.Vendor, "ITEM_VENDOR");
        mapping.Map(model => model.Description, "ITEM_DESCRIPTION");
        mapping.Map(model => model.ProductVendorNumber, "PRODUCT_VENDOR");
        mapping.Map(model => model.AddDate, "ADD_DATE").CustomType("Date");

        mapping.Join("TABLE1", part =>
        {
            part.KeyColumn("ITEM_VENDOR");
            part.Map(model => model.CompanyNumber);
            part.Table("TABLE1");
        });

        mapping.Join("TABLE2", part2 =>
        {
            part2.Table("TABLE2");
            part2.KeyColumn("COMPANY_NUMBER");
            part2.Map(model => model.CompanyName, "COMPANY_NAME");
        });
    }
}

[AS400Entity]
public class SearchItemModel
{
    public virtual decimal Upc { get; set; }
    public virtual string VendorItemCode { get; set; }
    public virtual int ProductVendorNumber { get; set; }
    public virtual string Description { get; set; }
    public virtual DateTime AddDate { get; set; }
    public virtual int Vendor { get; set; }
    public virtual DateTime VendorDiscontinueDate { get; set; }

    public virtual int CompanyNumber { get; set; }
    public virtual string CompanyName { get; set; }

    public override bool Equals(object obj)
    {
        var asSearchModel = obj as SearchItemModel;
        return asSearchModel != null && (asSearchModel.Upc == Upc && asSearchModel.VendorItemCode == VendorItemCode && asSearchModel.Vendor == Vendor);
    }

    public override int GetHashCode()
    {
        unchecked
        {
            var hash = (int)2166136261;
            hash = hash * 16777619 ^ Upc.GetHashCode();
            hash = hash * 16777619 ^ Vendor.GetHashCode();
            hash = hash * 16777619 ^ (VendorItemCode ?? string.Empty).GetHashCode();
            return hash;
        }
    }
}