根据带有 LINQ 的 Id 列表按顺序选择多行

Question

我有一个 id 列表。如何选择 id 在列表中的所有行并保留顺序？

这就是我所拥有的：

var ids = new int[]{5, 1, 8, 2, 3};
var items = Db.Items.Where(x => ids.Contains(x.Id));

但它不会保留顺序。

我知道我可以根据 id 对项目进行排序，但我想要 O(n) 复杂度。

这个问题类似于：Select multiple records based on list of Id's with linq 但我需要保持行的顺序

Answer 1

我不太喜欢查询和数据库，但你可以通过使用 C# 技巧很快地做到这一点

var ids = new int[]{5, 1, 8, 2, 3};
var dict = ids.Select((id, i) => new { Index = i, Id = id })
    .ToDictionary(x => x.Id, x => x.Index);
var items = Db.Items
    .Where(x => ids.Contains(x.Id))
    .OrderBy(x => dict[x.Id]);

我不知道它将如何被翻译成数据库查询

---

我还没有测试过，但这里有一个没有OrderBy 的版本，但空间效率较低（实际上可能更慢）：

var ids = new int[]{5, 1, 8, 2, 3};
var temp = Db.Items
.Where(x => ids.Contains(x.Id))
.ToLookup(x => x.Id);

var tempList = new List<IGrouping<int, Item>>();
for(int i = 0; i < ids.Length; i++)
{
    tempList.Add(temp[ids[i]]);
}

var items = tempList.SelectMany(x => x);

---

还有另一种方法 - 只需进行反向连接：

var ids = new int[]{5, 1, 8, 2, 3};
var items = from id in ids
            join item in Db.Items
            on id equals item.Id
            select item;

这将导致按 id 排序的查询

Answer 2

投影到中间类型以保留原始索引、构建联合并对索引进行排序怎么样？

class ItemWithIndex
{
    public int Index { get; set; }
    public Item Item { get; set; }
}

class Item
{
    public int Id { get; set; }
}

int[] ids = { 5, 1, 8, 2, 3 };

IQueryable<ItemWithIndex> query = null;

for(int index = 0; index < ids.Length; index++)
{
    int currentIndex = index;
    int currentId = ids[index];

    IQueryable<ItemWithIndex> next = db.Items
        .Where(i => i.Id == currentId)
        .Select(i => new ItemWithIndex { Index = currentIndex, Item = i });

    query = query == null ? next : query.Concat(next);
}

ItemWithIndex[] items = query
    .OrderBy(i => i.Index)
    .ToArray();

这是生成的查询：

SELECT 
    [UnionAll4].[Id] AS [C1], 
    [UnionAll4].[C1] AS [C2], 
    [UnionAll4].[Id1] AS [C3]
    FROM  (SELECT 
        [Extent1].[Id] AS [Id], 
        @p__linq__1 AS [C1], 
        [Extent1].[Id] AS [Id1]
        FROM [dbo].[Items] AS [Extent1]
        WHERE [Extent1].[Id] = @p__linq__0
    UNION ALL
        SELECT 
        [Extent2].[Id] AS [Id], 
        @p__linq__3 AS [C1], 
        [Extent2].[Id] AS [Id1]
        FROM [dbo].[Items] AS [Extent2]
        WHERE [Extent2].[Id] = @p__linq__2
    UNION ALL
        SELECT 
        [Extent3].[Id] AS [Id], 
        @p__linq__5 AS [C1], 
        [Extent3].[Id] AS [Id1]
        FROM [dbo].[Items] AS [Extent3]
        WHERE [Extent3].[Id] = @p__linq__4
    UNION ALL
        SELECT 
        [Extent4].[Id] AS [Id], 
        @p__linq__7 AS [C1], 
        [Extent4].[Id] AS [Id1]
        FROM [dbo].[Items] AS [Extent4]
        WHERE [Extent4].[Id] = @p__linq__6
    UNION ALL
        SELECT 
        [Extent5].[Id] AS [Id], 
        @p__linq__9 AS [C1], 
        [Extent5].[Id] AS [Id1]
        FROM [dbo].[Items] AS [Extent5]
        WHERE [Extent5].[Id] = @p__linq__8) AS [UnionAll4]

Answer 3

我会这么说，

1st，你可以像以前一样得到物品：

var ids = new int[]{5, 1, 8, 2, 3};
var items = Db.Items.Where(x => ids.Contains(x.Id));

然后你可以这样做：

var orderedItems = new int[ids.Length()] // sorry, I'm codign in SO edit, not sure on the syntax
foreach(id in items)
{
var position = Array.IndexOf(items, id)
orderedITems[position] = id;
}

这应该符合您的要求（也可以简化为一行）。

希望对你有帮助

胡安

Answer 4

这是一个可能的解决方案：

var ids = new int[] {5, 1, 8, 2, 3};
var items = new List<Item>();
for (int i = 0; i < ids.Length; i++)
{
    items.Add(Db.Items.Find(ids[i]));
}

但是，它执行 N 次查询，所以应该有更好的方法。