LINQ中的左联接和按列区分

Question

I am having two relations: Users and UserProjects .

I want to check which user is already in project. So far it`s like this:

 var usrs = (from users in context.Users
                        join userProj in context.UserProjects
                        on users.UserId equals userProj.UserId into gj
                        from subuserProject in gj.DefaultIfEmpty()
                        select new ProjectUsersDTO
                        {
                            UserName = users.Name,
                            Rate = users.RatePerHour,
                            UserId = users.UserId,
                            alreadyInProject = subuserProject == null || subuserProject.ProjectId != projectId ? false : true
                        })
                      .ToList();

The problem is I want to get them distinct. Right now I am getting one user with alreadyInProject == true and alreadyInProject == false .

Answer 1

If you are checking for some particular project, then you don't need to join all projects - simply filter user projects before joining:

from user in context.Users
join userProj in context.UserProjects.Where(up => up.ProjectId == projectId)
    on user.UserId equals userProj.UserId into g
from subuserProject in g.DefaultIfEmpty()
select new ProjectUsersDTO {
    UserName = user.Name,
    Rate = user.RatePerHour,
    UserId = user.UserId,
    alreadyInProject = (subuserProject != null)
}

Generated query will look like

SELECT
  [Extent1].[UserId] AS [UserId],
  [Extent1].[Name] AS [Name],
  [Extent1].[RatePerHour] AS [RatePerHour],
  CASE WHEN ([Extent2].[UserProjectId] IS NOT NULL) 
       THEN cast(1 as bit)
       ELSE cast(0 as bit)
  END AS [C1]
  FROM  [dbo].[Users] AS [Extent1]
  LEFT OUTER JOIN [dbo].[UserProjects] AS [Extent2] 
     ON ([Extent2].[ProjectId] = @p__linq__0) AND ([Extent1].[UserId] = [Extent2].[UserId])