LINQ嵌套选择新

Question

I've 2 db tables like as Family and Cars and have a class named Families. They connected eachother by a foreign key.

Family

ID | Surname | Address

Cars

ID | FamilyID | Name

A family can have multiple cars.

I want to bind this to a grid and I wrote this query:

var data = from d in Master.Connection.Familys.Where(x => some conditions).OrderByDescending(d => d.ID)
                       select new Families
                       {
                           Id = d.ID,
                           Surname = d.Surname,
                           Address = d.Address,
                           Cars = ""
                       };

foreach (var item in data)
            {
                var cr = Master.Connection.Cars.Where(x => x.FamilyID == item.ID);
                if (!cr.Any()) continue;
                foreach (var crItem in cr)
                {
                    item.Cars += crItem.Name + ", ";
                }
            }

public class Families
{
    public int Id { get; set; }
    public string Surname { get; set; }
    public string Address { get; set; }
    public string Cars { get; set; }
}

Grid filled by data. However Cars column is empty. How can I do this in the first query? Or, is there a simple way to do this?

Answer 1

Try this:

var data = from d in Master.Connection.Familys.Where(x => some conditions).OrderByDescending(d => d.ID)
                   select new Families
                   {
                       Id = d.ID,
                       Surname = d.Surname,
                       Address = d.Address,
                       Cars = ""
                   };


data.ForEach(f => f.Cars = String.Join(",", Master.Connection.Cars.Where(x => x.FamilyID == item.ID).Select(cr => cr.Name).ToArray()));

Answer 2

Your linq is returning IEnumerable<T> which is Lazy by design. In other words, when your first linq is executed, it doesn't fetch the data immediately. The data is actually fetched when it is used for the first time. As a side-effect, item.Cars += crItem.Name + ", "; is getting overwritten. A possible workaround is to add a .ToList() to fetch the data immediately.

var data = Master.Connection.Familys
    .Where(x => some conditions)
    .OrderByDescending(d => d.ID)
    .Select(d => new Families
                   {
                       Id = d.ID,
                       Surname = d.Surname,
                       Address = d.Address,
                       Cars = ""
                   })
    .ToList();

Update Your code is making multiple calls to the database in var cr = Master.Conn ... item.ID); which is totally unnecessary. Instead you can use GroupJoin as follows:

var data = Connection.Families   //main table
                .Where(x => some conditions)
                .GroupJoin(Connection.Cars   //left outer join
                    , f => f.ID   //join condition
                    , c => c.FamilyID // join condition
                    , (f, c) => new {Family = f, CarNames = c.Select(o => o.Name)})
                    .OrderByDescending(d => d.Family.ID)
                    .ToList()
                    .Select(o => new Families
                    {
                        Id = o.Family.ID,
                        Surname = o.Family.SurName,
                        Address = o.Family.Address,
                        // Check possible null from left join
                        Cars = o.CarNames.Count() > 0 ? o.CarNames.Aggregate((a, b) => a + "," + b) : string.Empty
                    })                    
                .ToList();

PS: Don't use .Join() as suggested by tbean because .Join() will produce INNER Join not a LEFT OUTER Join .

Answer 3

Another way:

   private IQueryable<Family> GetFamilys()
    {
        return Master.Connection.Familys;
    }

    private IQueryable<Car> GetCars()
    {
         return Master.Connection.Cars;
    }

    public List<Families> FillFamilies()
    {
      var list = (from family in GetFamilys()
                   join car in GetCars() on family.ID equals car.familyID 
                   into groupJoin from subcar in groupJoin.DefaultIfEmpty()
                   select new Families
                  {
                   Id = family.ID,
                   Surname = family.Surname,
                   Address = family.Address,
                   Cars = subcar?.Name ?? String.Empty
                  }).ToList();
       return list;
     }