how write unit test for this?

public partial class Loan
    {
        public virtual PledgePriceChange InitPledgePriceChangeDocument(PledgePriceChange document)
        {
            if (document == null)
                document = new PledgePriceChange();

            if (document.TranDate == default(DateTime))
            {
                document.TranDate = _core.TransactionDate();
            }
            document.CurrencyId = _config.DefaultCurrencyId;
            document.DocTypeId = DocumentType.PledgePriceChange;

            var plegde = _db.PledgeSales.Include(x => x.PledgeSaleJewelry)
                                        .Include(x => x.Items.Select(i => i.PledgeSaleJewelry))
                            .SingleOrDefault(x => x.Num == document.PledgeNum);

            if (plegde == null)
                throw BankCore.CreateException(String.Format("Pledge {0} does not exisis",
                                               document.PledgeNum));

            plegde.Price = _core.SubAccBalByConnection(plegde.SubAccountId,
                                 AccConnection.PledgePrice, document.TranDate);
            foreach (var item in plegde.Items)
            {
                item.Price = _core.SubAccBalByConnection(item.SubAccountId,
                                 AccConnection.PledgePrice, document.TranDate);
            }

            document.Jewelry = (JewelrySale)plegde;

            return document;
        }