Question

这就是我的消息数据的结构。如您所见，每个帖子都有时间戳，userId和主题的ID。

[
  {
    "_id" : ObjectId("5b0abb48b20c1b4b92365145"),
    "topicId" : "XN7iqmCFD4jpgJZ6f",
    "timestamp" : 1527429960,
    "user" : "5b0869636f4e363e300d105a",
    "content" : "lorem ipsum"
  }    
]

现在我需要检查是否有主题，最新帖子（=最高时间戳）与我自己的ID不匹配。有了这个，我确实知道哪个主题有一个新的答案（这不是我自己的帖子）。

所以我从这开始：

db.messages.find({
  $query: { 
    user: { $ne: "myUserId" }
  },
  $orderby: { 
   timestamp: -1 
  }
}).limit(1).toArray()

我的问题是，我不知道如何通过topicId对我的查询进行分组。不知怎的，我的尝试似乎有错误的语法。

Answer 1

您必须使用汇总按$sort分组，然后$limit按时间分类，然后db.messages.aggregate([ { $match: { user: { $ne: "myUserId" } }}, { $group: { _id: "$topicId", timestamp: { $first: "$timestamp"}, user: { $first: "$user" }, content: { $first: "$content" } }}, { $sort: { timestamp: -1 } }, { $limit: 1 } ])来限制查询

Artifacts >> Development (trigger on build success) >> Production (manual trigger after Development)

Answer 2

使用聚合管道来代替查找。

1）$ match $ ne：＆＃34; myUserId＆＃34;

2）$ sort timestamp：-1

3）$ group topicId $ first

我过去写的一个示例查询..

Public Class CheckForDuplicateFiles
    Private Sub btnDirectory_Click(sender As Object, e As EventArgs) Handles btnDirectory.Click
        Directory.ShowDialog()
        Dim SelectedDirectory As String = Directory.SelectedPath
        Dim SourceDirectory As DirectoryInfo = New DirectoryInfo(SelectedDirectory)
        Dim lstFiles As New List(Of FileData)
        For Each childFile As FileInfo In SourceDirectory.GetFiles("*", SearchOption.AllDirectories).Where(Function(file) file.Extension.ToLower = cboExtensions.SelectedItem.ToString)
            Dim hash As MD5
            hash = MD5.Create
            Dim hashvalue() As Byte
            Dim filestream = childFile.OpenRead
            hashvalue = hash.ComputeHash(filestream)
            Dim has_hex1 As String = PrintByteArray(hashvalue)
            Dim f As New FileData()
            f.Name = Path.GetFileNameWithoutExtension(childFile.Name)
            f.Size = childFile.Length
            f.Extension = childFile.Extension
            f.Hash = has_hex1
            f.Location = childFile.FullName
            f.AddedToListView = False
            For i As Integer = 0 To lstFiles.Count - 1
                If lstFiles.Item(i).Hash = f.Hash Then
                    AddToListView(f)
                    f.AddedToListView = True
                    If lstFiles.Item(i).AddedToListView = False Then
                        AddToListView(lstFiles.Item(i))
                        lstFiles.Item(i).AddedToListView = True
                    End If
                    Exit For
                End If
            Next
            lstFiles.Add(f)
        Next
    End Sub
    Private Sub AddToListView(file As FileData)
        'Adding in ListView
        Dim NewItem As ListViewItem = ListView1.Items.Add(file.Name)
        NewItem.SubItems.Add(file.Size.ToString)
        NewItem.SubItems.Add(file.Extension)
        NewItem.SubItems.Add(file.Hash)
        NewItem.SubItems.Add(file.Location)
    End Sub
    Private Function PrintByteArray(bytes As Byte()) As String
        Dim sb As New StringBuilder
        For Each b As Byte In bytes
            sb.Append(b)
        Next
        Return sb.ToString
    End Function
    Public Class FileData
        Public Property Name As String
        Public Property Size As Long
        Public Property Extension As String
        Public Property Hash As String
        Public Property Location As String
        Public Property AddedToListView As Boolean
    End Class
End Class

获取$ group by之后的最新文档

2 个答案: