Entity Framework left 加入两个数据库答案

【问题标题】：Entity Framework left Join two databasesEntity Framework left 加入两个数据库
【发布时间】：2013-08-01 22:29:27
【问题描述】：

我需要使用实体框架将列表加入数据库查询。基本上，我有两个数据库，我需要将一个数据库中的表外部连接到另一个数据库中的表。为了做到这一点，我想从第一个数据库中获取一个列表，然后将它加入到第二个查询的查询中。问题是，我必须将结果作为 IQueryable 返回，因此该方法的使用者可以在查询执行之前对查询进行额外的过滤。

我尝试了两种不同的方法：

选项 1：

 Public Shared Function List() As IQueryable(Of ContractEquipment)
' Get Subcontractors from Database 1
Dim subcontractors As List(Of SubContractor) = GetSubContractorList().ToList()
' Query Database 2
Return (From c In DB.ContractEquipments
        Select New With {
            .ContractID = c.ContractID,
            .EquipmentID = c.EquipmentID,
            .Inactive = c.Inactive,
            .SubContractorName = (From x In subcontractors Where c.SubContractorID = x.SubContractorID Select x.Name).FirstOrDefault()
        })

结束函数

选项 2：

Public Shared Function List() As IQueryable(Of ContractEquipment)
' Get Subcontractors from Database 1
Dim subcontractors As List(Of SubContractor) = GetSubContractorList().ToList()
' Query Database 2
Return (From c In DB.ContractEquipments
        Group Join s in subcontractors on c.SubContractorID equals s.SubContractorID into tblSubContractors = Group
        Select New With {
            .ContractID = c.ContractID,
            .EquipmentID = c.EquipmentID,
            .Inactive = c.Inactive,
            .SubContractorName = tblSubContractors.Select(Function(z) z.Name).FirstOrDefault()
        })

结束函数

我不断收到同样的错误： “无法创建 type 的常量值。此上下文仅支持原始类型（'例如 Int32、String 和 Guid'）。”

有人有什么建议吗？

【问题讨论】：

标签： vb.net entity-framework

【解决方案1】：

好的，我找到了解决方法。在数据库 2 中创建了一个视图：

CREATE VIEW dbo.ViewContractEquipment
AS SELECT e.ContractEquipmentID, s.Name as SubContractorName
FROM dbo.ContractEquipments AS e 
LEFT OUTER JOIN DB2.dbo.SubContractors AS s ON e.SubContractorID = s.SubContractorID

然后我将视图添加到我的查询中并将其与主表连接：

    Return (From d In DB.ViewContractEquipments
        Join c in DB.ContractEquipments ON c.ContractEquipmentID equals d.ContractEquipmentID
        Select New With {
            .ContractID = c.ContractID,
            .EquipmentID = c.EquipmentID,
            .Inactive = c.Inactive,
            .SubContractorName = d.SubContractorName
        })

不优雅，但它有效！

【讨论】：