我正在尝试在客户端和ASP.NET MVC服务器之间进行android同步。逻辑很简单，我的下一个方法接收一个数据字典，其中key = idGroup和value = LastMessageIdKnown，最后我应该得到每个组的下一条消息，Id高于LastMessageIdKnown（我的dicctionary的值）。 / p>
现在我正在迭代地图，对于每个键，我对SQL数据库进行查询，但这是低效的，如果我有N个键，你可以想象暗示着什么。
这是我目前的方法
public Dictionary<int, List<Messages>> SynchronizedChatMessages(Dictionary<int, int> data)
{
Dictionary<int, List<Messages>> result = new Dictionary<int, List<Messages>>();
foreach(int item in data.Keys){
var idMessage= data[item];
var listMessages= _context.Messages.Where(x => x.Grupo_ID == item && x.ID > idMessage).ToList();
result.Add(item,listMessages);
}
return result;
}
如何以唯一且最佳的方式改进此查询以获得我需要的所有内容？
谢谢。
答案 0 :(得分：1)
如果这样可行会很好，但我怀疑它可以一次性转换为SQL语句：
var toInsert =
from msg in _context.Messages
group msg by msg.Grupo_ID into g
where data.Keys.Contains(g.Key)
select new {
Item = g.Key,
Messages = g.Where(x => x.ID > data[g.Key])
};
我认为第二个Where条款x => x.ID > data[g.Key]无法翻译。
所以你可能需要两次通过，如下所示：
// This is a single SQL query.
var groups =
from msg in _context.Messages
group msg by msg.Grupo_ID into g
where data.Keys.Contains(g.Key)
select new {
Item = g.Key,
// ordering helps us when we do the in-memory part.
Messages = g.OrderByDescending(x => x.ID).ToList()
};
// This iterates the result set in memory
foreach (var g in groups)
result.Add(
g.Item,
// input is ordered, we stop when an item is <= data[g.Item].
g.Messages.TakeWhile(m => m.ID > data[g.Item]).ToList())
答案 1 :(得分：1)
这是一次尝试，使用Predicate来制作它，以便对整个邮件集合只有一个Where。
请注意，我在没有数据库的情况下对此进行了模拟，因此我将List传递给SynchronizedChatMessages函数，而您可以使用上下文。
还有待证明的是，这种做事方式只会对数据库生成一个查询（因为我只在对象中执行）。整个程序在下面进一步说明，但首先，只显示使用谓词来实现仅Where触发一次的函数。
public static Dictionary<int, List<Message>> SynchronizedChatMessages(List<Message> messages, Dictionary<int, int> data)
{
List<Predicate<Message>> predList = new List<Predicate<Message>>();
//Built of list of indivIdual predicates
foreach (var x in data)
{
var IdMessage = x.Key;
var lastMessageId = x.Value;
Predicate<Message> pred = m => m.IdGroup.Id == IdMessage && m.Id > lastMessageId;
predList.Add(pred);
}
//compose the predicates
Predicate<Message> compositePredicate = m =>
{
bool ret = false;
foreach (var pred in predList)
{
//If any of the predicates is true, the composite predicate is true (OR)
if (pred.Invoke(m) == true) { ret = true; break; }
}
return ret;
};
//do the query
var messagesFound = messages.Where(m => compositePredicate.Invoke(m)).ToList();
//get the individual distinct IdGroupIds
var IdGroupIds = messagesFound.Select(x => x.IdGroup.Id).ToList().Distinct().ToList();
//Create dictionary to return
Dictionary<int, List<Message>> result = new Dictionary<int, List<Message>>();
foreach (int i in IdGroupIds)
{
result.Add(i, messagesFound.Where(m => m.IdGroup.Id == i).ToList());
}
return result;
}
以下是整个事情：
using System;
using System.Collections.Generic;
using System.Linq;
namespace ConsoleApplication20
{
public class Program
{
public class Message
{
public int Id { get; set; }
public IdGroup IdGroup { get; set; }
}
public class IdGroup
{
public int Id { get; set; }
public List<Message> Messages { get; set; }
}
public static Dictionary<int, List<Message>> SynchronizedChatMessages(List<Message> messages, Dictionary<int, int> data)
{
List<Predicate<Message>> predList = new List<Predicate<Message>>();
//Built of list of indivIdual predicates
foreach (var x in data)
{
var IdMessage = x.Key;
var lastMessageId = x.Value;
Predicate<Message> pred = m => m.IdGroup.Id == IdMessage && m.Id > lastMessageId;
predList.Add(pred);
}
//compose the predicates
Predicate<Message> compositePredicate = m =>
{
bool ret = false;
foreach (var pred in predList)
{
//If any of the predicates is true, the composite predicate is true (OR)
if (pred.Invoke(m) == true) { ret = true; break; }
}
return ret;
};
//do the query
var messagesFound = messages.Where(m => compositePredicate.Invoke(m)).ToList();
//get the individual distinct IdGroupIds
var IdGroupIds = messagesFound.Select(x => x.IdGroup.Id).ToList().Distinct().ToList();
//Create dictionary to return
Dictionary<int, List<Message>> result = new Dictionary<int, List<Message>>();
foreach (int i in IdGroupIds)
{
result.Add(i, messagesFound.Where(m => m.IdGroup.Id == i).ToList());
}
return result;
}
public static void Main(string[] args)
{
var item1 = new IdGroup { Id = 2, Messages = new List<Message>() };
var item2 = new IdGroup { Id = 45, Messages = new List<Message>() };
var item3 = new IdGroup { Id = 36, Messages = new List<Message>() };
var item4 = new IdGroup { Id = 8, Messages = new List<Message>() };
var message1 = new Message { Id = 3, IdGroup = item1 };
var message2 = new Message { Id = 7, IdGroup = item1 };
var message3 = new Message { Id = 9, IdGroup = item1 };
item1.Messages.Add(message1);
item1.Messages.Add(message2);
item1.Messages.Add(message3);
var message4 = new Message { Id = 4, IdGroup = item2 };
var message5 = new Message { Id = 10, IdGroup = item2 };
var message6 = new Message { Id = 76, IdGroup = item2 };
item2.Messages.Add(message4);
item2.Messages.Add(message5);
item2.Messages.Add(message6);
var message7 = new Message { Id = 6, IdGroup = item3 };
var message8 = new Message { Id = 32, IdGroup = item3 };
item3.Messages.Add(message7);
item3.Messages.Add(message8);
var message9 = new Message { Id = 11, IdGroup = item4 };
var message10 = new Message { Id = 16, IdGroup = item4 };
var message11 = new Message { Id = 19, IdGroup = item4 };
var message12 = new Message { Id = 77, IdGroup = item4 };
item4.Messages.Add(message9);
item4.Messages.Add(message10);
item4.Messages.Add(message11);
item4.Messages.Add(message12);
List<IdGroup> items = new List<IdGroup> { item1, item2, item3, item4 };
List<Message> messages = new List<Message> { message1, message2, message3, message4, message5, message6,message7, message8, message9, message10, message11, message12};
Dictionary<int, int> lastMessagesPerItem = new Dictionary<int, int> { { 2, 3 }, { 45, 10 }, { 36, 6 }, { 8, 11 } };
var result = SynchronizedChatMessages(messages, lastMessagesPerItem);
var discard = Console.ReadKey();
}
}
}