的LINQ取不同

Question

我有查詢：

ID ItemID RecommendItemID Message 
------------------------------------------ 
1  25  3     Msg here 
2  26  3     Something else 
3  27  8     Another message 

我需要查詢，以篩選出具有相同RecommendItemID，這應該結果：

var q = (
    from c in db.tblStoreRecommendations 
    where 
    itemIDsInCart.Contains(c.ItemID) 
    && !itemIDsInCart.Contains(c.RecommendItemID) 
    select c 
); 

它將沿着線返回的東西不會多次出現在返回的結果中。

如果存在兩個，它可以使用（隨機選擇將是最好的）。所以返回的結果應該省略記錄ID 1或2.

任何人都可以告訴我如何做到這一點嗎？謝謝！

Answer 1

一種方法是使用的GroupBy，然後從每組中選擇第一項：

var q = (
    from c in db.tblStoreRecommendations 
    where 
     itemIDsInCart.Contains(c.ItemID) 
    && !itemIDsInCart.Contains(c.RecommendItemID) 
    select c 
).GroupBy(c => c.RecommendItemID) 
.Select(g => g.First()); 

---

如果您使用此顯示隨機審查，我會建議強加於到這使用代碼而不是LINQ查詢，通過省略First像這樣：

var q = (
    from c in db.tblStoreRecommendations 
    where 
     itemIDsInCart.Contains(c.ItemID) 
    && !itemIDsInCart.Contains(c.RecommendItemID) 
    select c 
).GroupBy(c => c.RecommendItemID) 
.Select(g => g.ToArray()); 

var random = new Random(); 
foreach (var r in q) 
{ 
    var rec = r[random.Next(r.Length)]; 
    // use your recommendation 
} 

Answer 2

我相信你可以添加一個明顯的對比您的LINQ表達式的末尾，就像：

var q = (
    from c in db.tblStoreRecommendations 
    where 
    itemIDsInCart.Contains(c.ItemID) 
    && !itemIDsInCart.Contains(c.RecommendItemID) 
    select c 
) 
.Distinct((x, y) => x.RecommendItemID == y.RecommendItemID); 

---

編輯

我才意識到我該使用擴展的方法，我寫我自己，通常保持方便的做...這裏是它的代碼：

/// <summary> 
    /// Provides a way to get a distinct list of items based on a lambda comparison operator. 
    /// </summary> 
    /// <typeparam name="T"></typeparam> 
    /// <param name="source"></param> 
    /// <param name="equalityComparer">A comparison function that returns true of the two items are considered equal.</param> 
    /// <returns>The list of distinct items.</returns> 
    public static IEnumerable<T> Distinct<T>(this IEnumerable<T> source, Func<T, T, bool> equalityComparer) 
    { 
     var distincts = new List<T>(); 
     foreach (var item in source) 
     { 
      var found = false; 
      foreach (var d in distincts) 
      { 
       found = equalityComparer(item, d); 
       if (found) 
        break; 
      } 

      if (!found) 
      { 
       distincts.Add(item); 
       yield return item; 
      } 
     } 
    } 

Answer 3

讓我們儘可能使用查詢表達式來堅持這一點：

var q = from c in db.tblStoreRecommendations 
     where itemIDsInCart.Contains(c.ItemID) 
      && !itemIDsInCart.Contains(c.RecommendItemID) 
     group c by c.RecommendItemID into matches 
     select matches.First(); 

上下文關鍵字into當然在這裏派上用場。

這裏是如何randomize它：

private static Random random = new Random(); 

---

var q = from c in db.tblStoreRecommendations 
     where itemIDsInCart.Contains(c.ItemID) 
      && !itemIDsInCart.Contains(c.RecommendItemID) 
     group c by c.RecommendItemID 
     into matches 
     select matches.AsEnumerable().ElementAt(random.Next(matches.Count())); 

由於LINQ到SQL無法處理ElementAt電話，我選擇前編輯答案執行查詢（通過調用AsEnumerable）分組中的Random元素。