如何分配给列表 <string> 在LinqToEntities C＃中

Question

Hoping that I'm just missing something obvious but here's my query

        var data = (from project in _db.Projects 
                    where project.Id == id
                    let primaryCategory = (from c in _db.Categories
                                           where c.CategoryID == project.PrimaryCategory
                                           select c.Name)
                    let categories = (from c in _db.ProjectCategories
                                      join pc in _db.Projects_ProjectCategories on c.ProjectCategoryID equals pc.ProjectCategoryID
                                      where pc.ProjectID == project.ProjectID
                                      select c.Name)
                    let owner = (from o in _db.Owners
                                 join po in _db.App_Projects_Owners on o.OwnerID equals po.OwnerID
                                               where po.ProjectID == project.ProjectID
                                               select new OwnerModel
                                               {
                                                   Owner = o,
                                                   Project = project,
                                                   PrimaryCategory = primaryCategory.FirstOrDefault(),
                                                   Categories = categories.ToList()
                                               })

                    select new
                    {
                        owner,
                        project
                    }).FirstOrDefault();

In there OwnerModel.Categories is a List of strings. I can't use ToList() in the query because it gives a materialization error. I've added a custom setter that takes the IQueryable, but that still makes another round trip to the database for every owner that the query returns.

So how are you supposed to assign basic lists in a subquery?

EDIT AND ANSWER (since Robert McKee lead me to the answer in his comment).

The answer is to use the group by clause like so

var data = (from project in _db.Projects 
            where project.Id == id
            let primaryCategory = (from c in _db.Categories
                                           where c.CategoryID == project.PrimaryCategory
                                           select c.Name)
                    let categories = (from c in _db.ProjectCategories
                                      join pc in _db.Projects_ProjectCategories on c.ProjectCategoryID equals pc.ProjectCategoryID
                                      where pc.ProjectID == project.ProjectID
                                      group c.Name by pc.ProjectCategoryID into x
                                      select x.ToList())
                    let owner = (from o in _db.Owners
                                 join po in _db.App_Projects_Owners on o.OwnerID equals po.OwnerID
                                               where po.ProjectID == project.ProjectID
                                               select new OwnerModel
                                               {
                                                   Owner = o,
                                                   Project = project,
                                                   PrimaryCategory = primaryCategory.FirstOrDefault(),
                                                   Categories = categories
                                               })
                    select new
                    {
                        owner,
                        project
                    }).FirstOrDefault();

Specifically note the bits involving

group c.Name by pc.ProjectCategoryID into x 
select x.ToList()

It may seem counter-intuitive that it works like this until you dig into exactly what's going on. The method that I was calling above with categories.ToList() was trying to use the System.Collection.Generics ToList function, and that list didn't have any way to convert the expression to sql. However by using the group by clause I was creating a specialized Enumerable IGrouping and calling the ToList function on that. This function is able to be translated into a sql statement and thus doesn't throw the exception.

Learn something new every day.

Answer 1

Set up your navigation properties, and then your query becomes something like:

var data=db.Projects
  .Include(p=>p.PrimaryCategory)
  .Include(p=>p.Categories)
  .Include(p=>p.Owner) // or .Include(p=>p.Owners) if projects can have multiple owners
  .First(p=>p.Id == id);

As for child objects, you are looking for the group by clause or .GroupBy method.