Linq-to-SQL：如何使用group by来整形数据？

Question

I have an example database, it contains tables for Movies, People and Credits. The Movie table contains a Title and an Id. The People table contains a Name and an Id. The Credits table relates Movies to the People that worked on those Movies, in a particular role. The table looks like this:

CREATE TABLE [dbo].[Credits] (
    [Id] [int] IDENTITY (1, 1) NOT NULL PRIMARY KEY,
    [PersonId]  [int] NOT NULL FOREIGN KEY REFERENCES People(Id),
    [MovieId]  [int] NOT NULL  FOREIGN KEY REFERENCES Movies(Id),
    [Role]  [char] (1) NULL

In this simple example, the [Role] column is a single character, by my convention either 'A' to indicate the person was an actor on that particular movie, or 'D' for director .

I'd like to perform a query on a particular person that returns the person's name, plus a list of all the movies the person has worked on, and the roles in those movies.

If I were to serialize it to json, it might look like this:

{
  "name" : "Clint Eastwood",
  "movies" : [
     { "title": "Unforgiven",        "roles": ["actor", "director"] },
     { "title": "Sands of Iwo Jima", "roles": ["director"] },
     { "title": "Dirty Harry",       "roles": ["actor"] },
     ...
  ]
}

How can I write a LINQ-to-SQL query that shapes the output like that?

I'm having trouble doing it efficiently.

---

Try #1

if I use this query:

  int personId = 10007;
  var persons =
      from p in db.People
      where p.Id == personId
      select new
      {
          name   = p.Name,
          movies =
                (from m in db.Movies
                 join c in db.Credits on m.Id equals c.MovieId
                 where (c.PersonId == personId)
                 select new {
                         title = m.Title,
                         role = (c.Role=="D"?"director":"actor")
                 })
      };

I get something like this:

{
  "name" : "Clint Eastwood",
  "movies" : [
     { "title": "Unforgiven",        "role": "actor" },
     { "title": "Unforgiven",        "role": "director" },
     { "title": "Sands of Iwo Jima", "role": "director" },
     { "title": "Dirty Harry",       "role": "actor" },
     ...
  ]
}

That's not quite right. As you can see there's a duplicate of each movie for which Eastwood played multiple roles. I would expect that because there are multiple rows in the credits table for that movie+person combination, one for each role.

---

Try #2

I thought I'd use a group by , like this:

  var persons =
      from p in db.People
      where p.Id == personId
      select new
      {
          name   = p.Name,
          movies =
                (from m in db.Movies
                 join c in db.Credits  on m.Id equals c.MovieId
                 where (c.PersonId == personId)
                 orderby m.Year
                 group ((c.Role == "A")? "actor":"director")
                 by m.Id
                 into g
                 select new {roles = g })
      };

The output is pretty close to what I want. It looks like this:

{
  "name" : "Clint Eastwood",
  "movies" : [
     { "roles": ["actor", "director"]}, 
     { "roles": ["director"]}, 
     { "roles": ["actor"]},
     ...
  ]
}

That's close, but of course I don't have the movie titles.

---

Try #3

If I use a group by and include the movie title, like this:

  var persons =
      from p in db.People
      where p.Id == personId
      select new
      {
          name   = p.Name,
          movies =
                (from m in db.Movies
                 join c in db.Credits  on m.Id equals c.MovieId
                 where (c.PersonId == personId)
                 orderby m.Year
                 group ((c.Role == "A")? "actor":"director")
                 by m.Id
                 into g
                 select new { title = m.Title, roles = g })
      };

...then it won't compile, due to

error CS0103: The name 'm' does not exist in the current context

---

How can I shape the output the way I want?

Answer 1

It's a lot easier to reason about if you start from the relation table (credits):

var query =
    from c in context.Credits
    where c.PersonId == 1
    group c by c.Person into g
    select new
    {
        PersonName = g.Key.Name,
        Credits = from cr in g
                  group cr by cr.Movie into g2
                  select new
                  {
                      MovieTitle = g2.Key.Name,
                      Roles = g2.Select(ci =>
                          (ci.Role == 'A') ? "Actor" : "Director")
                  }
    };

Here's code that will display the results:

foreach (var result in query)
{
    Console.WriteLine(result.PersonName);
    foreach (var credit in result.Credits)
    {
        string roles = string.Join(",", credit.Roles.ToArray());
        Console.WriteLine("  " + credit.MovieTitle + ": " + roles);
    }
}

Answer 2

I believe that you'll need to materialize the query, then group by name and title and use string.Join to collate the roles.

  int personId = 10007;
  var persons = db.People.Where( p => p.Id == personId );
  var movies = db.Movies
                 .Join( db.Credits.Where( c => c.PersonId == personId),
                        m => m.Id,
                        c => c.MovieId,
                       (m,c) => new {
                   personid = c.PersonId,
                   title = m.title,
                   role = c.Role == "D" : "director", "actor"
                  })
                 .GroupBy( g => new { g.personid, g.title } )
                 .ToList()
                 .Select( g => new {
                     personid = g.Key.personid,
                     title = g.Key.title
                     roles = string.Join( ",", g.Select( g => g.role ).ToArray() )
                  });

  var personsWithMovies = people.Join( movies, p => p.PersonId, m => m.personid, (p,m) => new {
                            name = p.Name,
                            movies = m 
                          });

Answer 3

Thanks to the hint from tvanfosson , I was able to come up with this, which works for me!

var persons =
     from p in db.People
     where p.Id == personId
     select new
     {
         name   = p.Name,
         movies =
               (from m in db.Movies
                join c in db.Credits on m.Id equals c.MovieId
                where (c.PersonId == personId)
                group ((c.Role =="A")?"actor":"director") by m into sg
                orderby sg.Key.year
                select new { title = sg.Key.Title, roles = sg } )
     };

---

I also took some hints from Aaronaught and tried starting with the Credits table, and using the generated associations. That made things simpler. This code also works:

var persons =
    from c in db.Credits
    where c.PersonId == arg
    group c by c.People into g
    select new
    {
        name = g.Key.Name,
        credits = from cr in g
            group ((cr.Role == "A") ? "actor" : "director")
            by cr.Movies into g2
            orderby g2.Key.Year
            select new { title = g2.Key.Title, roles = g2 }
    };

...and it produces the same (or equivalent) output when serialized the the JavaScriptSerializer.

---

The key realization for me, that allowed me to get this done, was that I could use a compound key for the group, and that I could select on the fields within the key. The second key realization was that I should use the generated associations.