如何在.NET中以ORM友好的方式存储可扩展元数据？

Question

我似乎无法找到"如何使用ORM工具的EAV方法"问题的答案,所以我会在这里试试运气.

假设我有一张Entities表:

ID -> int
Name -> nvarchar(50)

一个Images表:

EntityID -> int
Width -> int
Height -> int

一张Songs桌子:

EntityID -> int
Duration -> decimal(12,3)

我需要向实体添加可扩展的元数据(具有类型信息的未知键值对),以便我能够发出如下查询:

找到所有Duration超过3分钟的歌曲,Name以"The"开头,元数据符合以下条件:

• HasGuitarSolo 设置为true
• GuitarSoloDuration 大于30秒

并按GuitarSoloDuration降序对结果进行排序.

我不想创建HasGuitarSolo,GuitarSoloDuration数据库等栏目,我非常希望将它们存储在一个EAV样的模式,或者说不需要钥匙了前面的知识的替代模式.

Answer 1

Add a column to the tables called 'metadata' and put XML in it. SQL server allows you to look at a blob full of XML as if it were additional columns (with limitations).

For ORM, it depends on how your object is structured.

• Infinitely Customizable metadata items: you put the name-value pairs from the XML in a collection. If your ORM won't allow this, put it straight into a string property, the setter could parse it into an XML doc (or faster object if you need speed). Getter would return the string. Then a separate property 'MetaDataItem(ItemName->string)' that is not ORM'd would read values from the metadata list and update/add them with its setter.
• Metadeta is hardcoded properties - map them using a query that pulls them from the XML.
• Hybrid of the two - hardcoded properties for some items - have their setters/getters call MetaDataItem.
• Reverse hybrid if certain properties need to be directly stored (esp if you are sorting large lists on them): you have to hardcode properties for that metadata with their own private members, but don't ORM those properties. Hardcoded the saving/loading of those values into the string property that is ORM'd, and if you want to be able to update those hardcoded metadata items from the MetaDataItem property as well, hardcode them it that spot, too!

如果您有一大堆硬编码的元数据属性，除了无限数量之外，您还可以使用列表和反射来轻松处理 XML 属性和 MetaDataItem 属性中的数据。如果所有内容都经过硬编码，您仍然可以使用 XML 文本属性来加载/保存它们，映射该属性，而不是其他属性。

使用对象上的 LINQ 查询对它们进行排序。

我取得了巨大的成功，并且对每个项目符号进行了编码，事情变得越来越好！2005/.Net 1.1，所以没有 ORM、LINQ、我的第一个 VB.net 程序等。但其他开发人员确实使用 SQL Server 的 XML 查询来读取我的 XML。当然，我忘记了这一点，改变了它，并绊倒了他们:-(

这是片段。这一切的关键是：ORM 友好 = ORM 某些属性，而不是其他属性；允许消费者使用其他属性，但不允许使用某些属性。如果您的 ORM 不允许这种按菜单点选的属性选择，您也许可以使用继承或组合来欺骗它。抱歉，我没有时间为您的目的发布完整的示例。

嗯，我家里没有代码示例。我明天将编辑并粘贴它。

按照承诺进行编辑，这是代码片段：

   Public Property ItemType(ByVal stTagName As String) As String
        Get
            Dim obj As Object
            obj = Me.lstMemberList.Item(stTagName)
            If Not obj Is Nothing Then
                Return CType(obj, foDataItem).Type
            End If
        End Get
        Set(ByVal Value As String)
            Dim obj As Object
            obj = Me.lstMemberList.Item(stTagName)
            If Not obj Is Nothing Then
                CType(obj, foDataItem).Type = Value
            End If
        End Set
    End Property

    Public Function ItemExists(ByVal stTagName As String) As Boolean
        Return Me.lstMemberList.ContainsKey(stTagName)
    End Function

    Public Property ItemValue(ByVal stTagName As String, Optional ByVal Type4NewItem As String = "") As String
        Get
            Dim obj As Object
            obj = Me.lstMemberList.Item(stTagName)
            If obj Is Nothing Then
                Dim stInternalKey As String = ""
                Try
                    stInternalKey = Me.InternalKey.ToString
                Catch
                End Try
                If stTagName <> "InternalKey" Then '' // avoid deadlock if internalkey errs!
                    Throw New ApplicationException("Tag '" & stTagName & _
                      "' does not exist in FO w/ internal key of " & stInternalKey)
                End If
            Else
                Return CType(obj, foDataItem).Value
            End If
        End Get
        Set(ByVal Value As String)
            '' // if child variation form...
            If bLocked4ChildVariation Then
                '' // protect properties not in the list of allowed updatable items 
                If Not Me.GetChildVariationDifferentFields.Contains(stTagName) Then
                    Exit Property
                End If
            End If
            '' // WARNING - DON'T FORGET TO UPDATE THIS LIST OR YOU WILL NEVER FIND THE BUG!
            Select Case stTagName
                Case "PageNum"
                    _PageNum = CInt(Value)
                Case "Left"
                    _Left = CInt(Value)
                Case "Top"
                    _Top = CInt(Value)
                Case "Width"
                    _Width = CInt(Value)
                Case "Height"
                    _Height = CInt(Value)
                Case "Type"
                    _Type = String2Type(Value)
                Case "InternalKey"
                    _InternalKey = CInt(Value)
                Case "UniqueID"
                    _UniqueID = Value
            End Select
            Static MyError As frmErrorMessage
            Dim obj As Object
            If Me.lstMemberList.ContainsKey(stTagName) Then
                Dim foi As foDataItem = CType(Me.lstMemberList.Item(stTagName), foDataItem)
                If foi.Type = "Number" Then
                    Value = CStr(Val(Value))
                End If
                If foi.Value <> Value Then
                    If bMonitorRefreshChanges Then
                        LogObject.LoggIt("Gonna Send Update for Change " & stTagName & " from " & _
                          foi.Value & " to " & Value)
                        If Not Me.FormObjectChanged_Address Is Nothing Then
                            FormObjectChanged_Address(Me, stTagName)
                        End If
                    End If
                End If
                foi.Value = Value
            Else
                Me.lstMemberList.Add(stTagName, New foDataItem(Value, Type4NewItem))
                Me.alOrderAdded.Add(stTagName)
            End If
        End Set
    End Property


  Public Function StringVal(ByVal st As String, Optional ByVal stDefault As String = "") As String
        Try
            StringVal = stDefault
            Return CType(Me.ItemValue(st), String)
        Catch ex As Exception
            Dim bThrowError As Boolean = True
            RaiseEvent ConversionError(ex, "String=" & Me.ItemValue(st), Me, st, bThrowError)
            If bThrowError Then
                LogObject.LoggIt("Error setting tag value in fo.StringVal: " & st)
                Throw New Exception("Rethrown Exception getting value of " & Me.ID & "." & st, ex)
            End If
        End Try
    End Function
    Public Function IntVal(ByVal st As String, Optional ByVal iDefault As Integer = 0) As Integer

    ...

 '' // 'native' values - are normal properties instead of XML properties, which 
    '' // actually makes it harder to deal with b/c of extra updates to sync them, BUT,
    '' // worth it - as they are read much more than written (sorts, wizard builds,
    '' // screen redraws, etc) I can afford to be slow when writing to them, PLUS
    '' // retain the benefits of referencing everything else via ItemValue, PLUS
    '' // these are just the more 'popular' items. 
    Private _Top As Integer
    Private _Left As Integer
    Private _Width As Integer
    Private _Height As Integer
    Private _PageNum As Integer
    Private _Type As pfoType
    Private _InternalKey As Integer
    Private _UniqueID As String

    Public Sub SetNativeValuesFromMyXML()
        _Top = CInt(CType(Me.lstMemberList("Top"), foDataItem).Value)
        _Left = CInt(CType(Me.lstMemberList("Left"), foDataItem).Value)
        _Width = CInt(CType(Me.lstMemberList("Width"), foDataItem).Value)
        _Height = CInt(CType(Me.lstMemberList("Height"), foDataItem).Value)
        _PageNum = CInt(CType(Me.lstMemberList("PageNum"), foDataItem).Value)
        _Type = String2Type(CType(Me.lstMemberList("Type"), foDataItem).Value)
        _InternalKey = CInt(CType(Me.lstMemberList("InternalKey"), foDataItem).Value)
        _UniqueID = CType(Me.lstMemberList("UniqueID"), foDataItem).Value
    End Sub

    Public Property Top() As Integer
        Get
            Return _Top '' // CInt(ItemValue("Top"))
        End Get
        Set(ByVal Value As Integer)
            ItemValue("Top") = Value.ToString
        End Set
    End Property

    Public Property Left() As Integer
        Get
            Return _Left '' //CInt(ItemValue("Left"))
        End Get

    ...