如何使用 linq 从多个表中获取记录答案

【问题标题】：How to get Records from multiple tables using linq如何使用 linq 从多个表中获取记录
【发布时间】：2018-07-24 14:59:22
【问题描述】：

我有 5 个表 Workflow 为“A”，WorkflowSteps 为“B”，AppUser 为“C”，AppRoles 为“D”，AppDepartment 为“E”我想以如下方式获取记录

1) 根据 workflowId 匹配来自 A 和 B 的记录

2) 匹配来自 B 和 C、B 和 D、B 和 E 的记录，因为表 B 具有外键 UserId、RoleId 和 DepartmentId

但问题在于 B UserId 中的某些列为 null 有些将 DepartmentId 设为 Null，有些将 RoleId 设为 Null，因此内部连接不起作用

这是我的查询，它必须获取三行，但我得到 0

var workflows = (from q in _context.WorkFlowSteps
                         join p in _context.WorkFlow
                         on q.WorkFlowId equals p.WorkFlowId
                         join r in _context.AppUsers
                         on q.ApprovalUserId equals r.UserId
                         from dep in _context.AppDepartment
                         from role in _context.AppRoles
                         where (q.DepartmentId == dep.DepartmentId) &&
                               (q.RoleId == role.RoleId)&& 
                               (q.WorkFlowId == id && 
                                q.IsAllowed == true && 
                                p.WebSiteId == WebsiteId)
                         select new WorkFlowViewModel
                         {
                             UserId =Convert.ToInt32(q.ApprovalUserId.HasValue?
                                      q.ApprovalUserId:
                                      (q.DepartmentId.HasValue? 
                                       q.DepartmentId:
                                       q.RoleId)),
                             ModeId = p.ModeId,
                             WebSiteId = p.WebSiteId,
                             Step = q.StepNo,
                             Name =q.ApprovalUserId.HasValue ? 
                                   p.Name :
                                   (q.DepartmentId.HasValue? 
                                    dep.Name :
                                    r.Name),
                             WorkFlowId = p.WorkFlowId
                         }).ToList();

【问题讨论】：

but the problem is for some column in B UserId is null some have DepartmentId as Null and some have RoleId is Null so inner join won't work 为此做left join
作为@HemidAbbasov 的建议。您可以为此使用左连接。访问此页面以参考更多信息：devcurry.com/2011/01/linq-left-join-example-in-c.html.
您应该非常很少加入 LINQ to EF。相反，只需遵循导航属性。您的 EF 模型知道如何为您连接表。

标签： asp.net-mvc entity-framework linq join

【解决方案1】：

假设 WorkFlowViewModel、WorkFlow、WorkFlowSteps、AppUsers、AppDepartment、AppRoles 的类（最少）（根据您的 Linq 查询暗示）：

public class WorkFlow
{
    public int WorkflowId { get; set; }
    public virtual IQueryable<WorkFlowSteps> WorkFlowSteps { get; set; }
    public int WebSiteId { get; set; }
    public int ModeId { get; set; }
}
public class WorkFlowSteps
{
    public int Id { get; set; }
    public int StepNo { get; set; }
    public bool IsAllowed { get; set; }
    public int WorkFlowId { get; set; }
    public WorkFlow WorkFlow { get; set; }
    public int? AppUserId { get; set; }
    public virtual AppUser AppUser { get; set; }
    public int? DepartmentId { get; set; }
    public virtual AppDepartment AppDepartment { get; set; }
    public int? RoleId { get; set; }
    public virtual AppRole AppRole { get; set; }
}
public class AppUser
{
    public int Id { get; set; }
    public List<WorkFlowSteps> WorkFlowSteps { get; set; }
    public string Name { get; set; }
}
public class AppDepartment
{
    public int Id { get; set; }
    public List<WorkFlowSteps> WorkFlowSteps { get; set; }
    public string Name { get; set; }
}
public class AppRole
{
    public int Id { get; set; }
    public List<WorkFlowSteps> WorkFlowSteps { get; set; }
    public string Name { get; set; }
}
public class WorkFlowViewModel
{
    public int WorkFlowId { get; set; }
    public int Step { get; set; }
    public int ModeId { get; set; }
    public int WebSiteId { get; set; }
    public int ApproverId { get; set; }
    public string ApproverName { get; set; }
}

然后此代码应选择并映射您的数据。第一步使用 EF 和导航路径来选择您的数据。第二步创建您想要的输出或“装饰对象”。

// Inputs consist of workflow id & website id & EF context
var selectedRecords = _context.WorkFlows
    .Include("WorkFlowSteps")
    .Include("WorkFlowSteps.AppUser")
    .Include("WorkFlowSteps.AppDepartment")
    .Include("WorkFlowSteps.AppRole")
    .Select(r => r.Id == id && r.WebSiteId == websiteId && r.WorkFlowSteps.IsAllowed);

var myModel = (from q in selectedRecords
               select new WorkFlowViewModel
               {
                   ApproverId = Convert.ToInt32(q.WorkFlowStep.AppUserId.HasValue 
                        ? q.WorkFlowStep.AppUserId 
                        : (q.WorkFlowStep.AppDepartmentId.HasValue 
                            ? q.WorkFlowStep.AppDepartmentId 
                            : (q.WorkFlowStep.AppRoleId.HasValue ? q.WorkFlowStep.AppRoleId, "0"))),
                   ModeId = q.ModeId,
                   WebSiteId = q.WebSiteId,
                   Step = q.WorkFlowStep.StepNo,
                   ApproverName = q.WorkFlowStep.AppUserId.HasValue 
                        ? q.WorkFlowStep.AppUser.Name 
                        : (q.WorkFlowStep.AppDepartmentId.HasValue
                            ? q.WorkFlowStep.AppDepartment.Name 
                            : (q.WorkFlowStep.AppRoleId.HasValue ? q.WorkFlowStep.AppRole.Name, "")),
                   WorkFlowId = q.WorkFlowId
               }).ToList();

【讨论】：