Question

我需要使用实体框架将列表加入到数据库查询中。基本上，我有两个数据库，我需要将表从一个数据库外连接到另一个数据库中的表。为了做到这一点，我想到从数据库1获取一个列表，然后将其连接到第二个查询的查询。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()
        })

结束功能

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()
        })

结束功能

我一直得到同样的错误： “无法创建类型的常量值。在此上下文中仅支持基本类型（例如Int32，String和Guid'）。”

有人有什么建议吗？

Answer 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，但有效！

实体框架离开加入两个数据库

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