Linq GroupJoin 使用

备忘:

var data = BoshccEntities.Current.TB_MB_1
                .GroupJoin(BoshccEntities.Current.TB_MB_2, o => o.HospitalID, m => m.MemberID, (t, m) => new { t, m })
                .GroupJoin(BoshccEntities.Current.TB_CM_3, o => o.t.ServiceCode, s => s.DicNum, (o, s) => new { t = o.t, m = o.m, s })
                .GroupJoin(BoshccEntities.Current.TB_CM_3, o => o.t.PaymentMethodCode, p => p.DicNum, (o, p) => new { t = o.t, m = o.m, s = o.s, p })
                .Where(o =>
                    (isenable.HasValue ? o.t.IsEnable == isenable.Value : true)
                    && (hasHospital ? hospitalIDs.Contains(o.t.HospitalID) : true)
                    && (hasService ? o.t.ServiceCode == service : true)
                    && (hasPayment ? o.t.PaymentMethodCode == payment : true))
                .OrderBy(o => o.t.ID)
                .Select(o => new
                {
                    ID = o.t.ID,
                    HospitalID = o.t.HospitalID,
                    ServiceCode = o.t.ServiceCode,
                    PaymentMethodCode = o.t.PaymentMethodCode,
                    IsEnable = o.t.IsEnable,
                    Remark = o.t.Remark,

                    Hospital = o.m.FirstOrDefault(), //关键1
                    Service = o.s.FirstOrDefault(),
                    Payment = o.p.FirstOrDefault()
                })
                .ToList() //关键2
                .Select(o => new TB_MB_4
                {
                    ID = o.ID,
                    HospitalID = o.HospitalID,
                    ServiceCode = o.ServiceCode,
                    PaymentMethodCode = o.PaymentMethodCode,
                    IsEnable = o.IsEnable,
                    Remark = o.Remark,

                    HospitalName = o.Hospital == null ? "" : o.Hospital.HospitalName, //关键3
                    ServiceName = o.Service == null ? "" : o.Service.DicName,
                    PaymentName = o.Payment == null ? "" : o.Payment.DicName
                });