指定的 LINQ 表达式包含对与不同上下文关联的查询的引用
Posted
技术标签:
【中文标题】指定的 LINQ 表达式包含对与不同上下文关联的查询的引用【英文标题】:The specified LINQ expression contains references to queries that are associated with different contexts 【发布时间】:2011-07-30 07:07:47 【问题描述】:我知道在 *** 上有类似的问题 - 我查看了它们并认为我的问题有些相似,但无法通过查看任何其他问题/答案来找到解决方案。 尝试执行以下代码时出现错误:
Private Sub btnReserve_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnReserve.Click
' Check that the room is still available.
Dim dbCheckOccupants As New pbu_housingEntities
Dim hall As String = CStr(Session("hall"))
Dim room As String = CStr(Session("room"))
Dim checkOccupants = From p In dbCheckOccupants.Rooms _
Let building_id = p.Building1.id _
Where p.building_name = hall _
Where p.room1 = room _
Select p.current_occupancy, p.max_occupancy, p.id, building_id
If checkOccupants.First.current_occupancy >= checkOccupants.First.max_occupancy Then
' If it isn't available, let student know.
lblResult.Text = "Sorry, this room is now fully occupied. Please choose another room."
Else
' If it is available, add the student to the room.
Dim AddOccupant As New pbu_housingEntities
Dim Occupant As New Resident
Dim gender As String = CStr(Session("gender"))
Dim person_name As String = CStr(Session("person_name"))
Dim class_level As String = CStr(Session("class_level"))
Dim semester As String = CStr(Session("term"))
Dim people_code_id As String = CStr(Session("people_code_id"))
Dim first_name As String = CStr(Session("first_name"))
Dim last_name As String = CStr(Session("last_name"))
Dim building_id As String = checkOccupants.First.building_id
Dim room_id As String = checkOccupants.First.id
Occupant.building = building_id
Occupant.room = room_id
Occupant.gender = gender
Occupant.person_name = person_name
Occupant.class_level = class_level
Occupant.semester = semester
Occupant.people_code_id = people_code_id
Occupant.create_date = Date.Now
Occupant.first_name = first_name
Occupant.last_name = last_name
AddOccupant.Residents.AddObject(Occupant)
AddOccupant.SaveChanges()
' Increment the number of occupants in the room.
Dim UpdateRoomOccupancy As New pbu_housingEntities
Dim UpdateOccupancy = (From p In UpdateRoomOccupancy.Rooms _
Where p.building_name = hall _
Where p.room1 = room _
Select p).First
UpdateOccupancy.current_occupancy = UpdateOccupancy.current_occupancy + 1
UpdateRoomOccupancy.SaveChanges()
' Add the student to a bed.
Dim AddBed As New pbu_housingEntities
Dim UpdateBed = From p In AddBed.Beds _
Where p.building = building_id _
Where p.room = room_id _
Where p.occupant = "" _
Select p
' Get the student's ID from the residency table.
Dim GetID = From p In AddBed.Residents _
Where p.people_code_id = people_code_id _
Order By p.id Descending _
Select p
Dim myID As String = GetID.First.id.ToString
UpdateBed.First.occupant = myID
AddBed.SaveChanges()
lblResult.Text = "Success! You have successfully requested residency in this room!"
End If
End Sub
在这一行发现一个错误:
Dim myID As String = GetID.First.id.ToString
据我所知,我没有使用多个上下文?
【问题讨论】:
【参考方案1】:我不明白为什么你会在你指出的那一行得到一个异常,但我认为代码有两个缺陷:你在这个方法中实例化了 4 个不同的上下文(虽然我没有看到你混合了那些上下文)并且您不处置任何一个。 (数据库上下文引用一个外部资源(数据库连接),显式地处理它是一个很好的做法。)
尝试重写它,使您只有一个上下文dbContext
,SaveChanges
只被调用一次,并且这个单个上下文被放置在方法的末尾(通过Using
块):
Private Sub btnReserve_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnReserve.Click
' Check that the room is still available.
Using (dbContext As pbu_housingEntities = New pbu_housingEntities)
' Check that the room is still available.
Dim hall As String = CStr(Session("hall"))
Dim room As String = CStr(Session("room"))
Dim checkOccupants = From p In dbContext.Rooms _
Let building_id = p.Building1.id _
Where p.building_name = hall _
Where p.room1 = room _
Select p.current_occupancy, p.max_occupancy, p.id, building_id
If checkOccupants.First.current_occupancy >= checkOccupants.First.max_occupancy Then
' If it isn't available, let student know.
lblResult.Text = "Sorry, this room is now fully occupied. Please choose another room."
Else
' If it is available, add the student to the room.
Dim Occupant As New Resident
Dim gender As String = CStr(Session("gender"))
Dim person_name As String = CStr(Session("person_name"))
Dim class_level As String = CStr(Session("class_level"))
Dim semester As String = CStr(Session("term"))
Dim people_code_id As String = CStr(Session("people_code_id"))
Dim first_name As String = CStr(Session("first_name"))
Dim last_name As String = CStr(Session("last_name"))
Dim building_id As String = checkOccupants.First.building_id
Dim room_id As String = checkOccupants.First.id
Occupant.building = building_id
Occupant.room = room_id
Occupant.gender = gender
Occupant.person_name = person_name
Occupant.class_level = class_level
Occupant.semester = semester
Occupant.people_code_id = people_code_id
Occupant.create_date = Date.Now
Occupant.first_name = first_name
Occupant.last_name = last_name
dbContext.Residents.AddObject(Occupant)
' Increment the number of occupants in the room.
Dim UpdateOccupancy = (From p In dbContext.Rooms _
Where p.building_name = hall _
Where p.room1 = room _
Select p).First
UpdateOccupancy.current_occupancy = UpdateOccupancy.current_occupancy + 1
' Add the student to a bed.
Dim UpdateBed = From p In dbContext.Beds _
Where p.building = building_id _
Where p.room = room_id _
Where p.occupant = "" _
Select p
' Get the student's ID from the residency table.
Dim GetID = From p In dbContext.Residents _
Where p.people_code_id = people_code_id _
Order By p.id Descending _
Select p
Dim myID As String = GetID.First.id.ToString
UpdateBed.First.occupant = myID
dbContext.SaveChanges()
lblResult.Text = "Success! You have successfully requested residency in this room!"
End If
End Using
End Sub
幸运的是,错误消失了,代码仍然和以前一样。
编辑
我只是快速添加一个一般说明:也许您有一个(错误的)想法,即您需要为每个查询、插入或删除操作提供一个新的上下文。情况并非如此,实际上是一种不好的做法,在不同的上下文中混合对象的风险很高。在大多数情况下,您可以在方法中遵循标准模式,如下所示:
' Create one single context
Using (dbContext As pbu_housingEntities = New pbu_housingEntities)
' Use here this dbContext for as many queries, Adds, Deletes and Changes
' as you need and like
' Save ALL Adds, Deletes and Changes you have done in this block
dbContext.SaveChanges()
End Using
了解 SaveChanges 将执行数据库事务也很重要,因此它是一个 All-Or-Nothing 操作。我可以想象这在你的方法中也很重要。例如,您使用Increment the number of occupants in the room.
和Add the student to a bed.
评论的两个操作,它们不应该同时发生或都不发生以避免数据库中的不一致状态吗?根据您的不同上下文,第一个操作可能成功但第二个操作失败,从而使您的数据在数据库中处于不一致的状态。
【讨论】:
哇!谢谢!这非常有帮助。我认为我对这个主题的理解扩大了 2 倍。我将尝试按照您的建议重写代码,并让您知道它是如何进行的。 :) 需要一点时间来适应这一切,但很快就会开始变得更有意义! :)【参考方案2】:你不能一通电话得到身份证吗?
Dim MyID = (From p In AddBed.Residents _
Where p.people_code_id = people_code_id _
Order By p.id Descending _
Select p.id).SingleOrDefault()
从外观上看,您正在调用上下文的多个实例。按照其他答案的建议尝试使用块
【讨论】:
以上是关于指定的 LINQ 表达式包含对与不同上下文关联的查询的引用的主要内容,如果未能解决你的问题,请参考以下文章
错误:“指定的 LINQ 表达式包含对与不同上下文关联的查询的引用”[重复]