如何只添加新的城市？

Question

我有CreateDiscountViewByUser discountViewByUser它包含用戶選擇的城市列表，但他們可能已經是那些已添加的城市。

List<DiscountCity> discountCities = (from city in db.DiscountCities 
           where city.DiscountId == discountViewByUser.Id 
           select city).ToList(); 
      for (int y = 0; y < discountCities.Count(); y++) 
      { 
       var dc = discountCities[y]; 
       bool flag = false; 
       for (int i = 0; i < discountViewByUser.DiscountCitys.Length; i++) 
       {  
        if (dc.CityId == discountViewByUser.DiscountCitys[i]) 
        { 
         flag = true; 
         discountCities.Remove(dc); 
         y--; 
        }      
       } 
       if (!flag) 
       { 
        db.DiscountCities.DeleteObject(dc);  
       } 
      } 
      foreach (var dc in discountCities) 
      { 
       DiscountCity discountCity = new DiscountCity 
               {Id = Guid.NewGuid(), 
               CityId = dc.CityId, 
               DiscountId = main.Id}; 
       db.DiscountCities.AddObject(discountCity); 
      } 

如何只添加新的城市？ 我的代碼不能正常工作=（

UPDATE：

discountViewByUser.DiscountCitys type int[]. 

db.DiscountCities table: Id DiscountId CityId. 

例如： 在數據庫：敖德薩，基輔

用戶組：敖德薩，莫斯科

我需要刪除基輔和添加莫斯科如何做到這一點？

Answer 1

我建議你分兩步做。

1）找到城市要刪除

var deleteCities = db.DiscountCities.Where(c => c.DiscountId == discountViewByUser.Id 
     && !discountViewByUser.DiscountCitys.Contains(c.CityId)); 
foreach(deleteCity in deleteCities) 
{ 
    db.DiscountCities.DeleteObject(deleteCity); 
} 

2）找到cityId的要插入

var insertCities = discountViewByUser.DiscountCitys.Except(
      db.DiscountCities.Where(c => c.DiscountId == discountViewByUser.Id) 
          .Select(c => c.CityId)); 
foreach(var insertCity in insertCities) 
{ 
    DiscountCity discountCity = new DiscountCity 
      {Id = Guid.NewGuid(), CityId = insertCity, DiscountId = discountViewByUser.Id}; 
    db.DiscountCities.AddObject(discountCity);  
} 

Answer 2

我建議添加所有th e項目，然後刪除重複。

// Where uniqueList is a List<T> of unique items: 
uniqueList.AddRange(valuesToAdd); 
uniqueList = uniqueList.Distinct(new CityEqualityComparer()).ToList(); 
// Sorry, I don't know how this would fit into your code 

既然你被他們的CityId的比較城市，你可能需要使用自定義的IEqualityComparer以確定哪些城市是重複的。

這裏是這樣的類的例子：

class CityEqualityComparer : IEqualityComparer<City> 
{ 
    public bool Equals(City arg1, City arg2) 
    { 
     return arg1.CityId == arg2.CityId; 
    } 

    public int GetHashCode(City arg) 
    { 
     return arg.CityId; 
    } 
} 

This question也可以是一定的幫助。