LINQ Inner-Join vs Left-Join
Using extension syntax I'm trying to create a left-join using LINQ on two lists that I have. The following is from the Microsoft help but I've modified it to show that the pets list has no elements. What I'm ending up with is a list of 0 elements. I assume that this is because an inner-join is taking place. What I want to end up with is a list of 3 elements (the 3 Person objects) with null data filled in for the missing elements. i.e. a Left-Join. Is this possible?
Person magnus = new Person { Name = "Hedlund, Magnus" };
Person terry = new Person { Name = "Adams, Terry" };
Person charlotte = new Person { Name = "Weiss, Charlotte" };
//Pet barley = new Pet { Name = "Barley", Owner = terry };
//Pet boots = new Pet { Name = "Boots", Owner = terry };
//Pet whiskers = new Pet { Name = "Whiskers", Owner = charlotte };
//Pet daisy = new Pet { Name = "Daisy", Owner = magnus };
List<Person> people = new List<Person> { magnus, terry, charlotte };
//List<Pet> pets = new List<Pet> { barley, boots, whiskers, daisy };
List<Pet> pets = new List<Pet>();
// Create a list of Person-Pet pairs where
// each element is an anonymous type that contains a
// Pet's name and the name of the Person that owns the Pet.
var query =
people.Join(pets,
person => person,
pet => pet.Owner,
(person, pet) =>
new { OwnerName = person.Name, Pet = pet.Name }).ToList();
I think if you want to use extension methods you need to use the GroupJoin
var query =
people.GroupJoin(pets,
person => person,
pet => pet.Owner,
(person, petCollection) =>
new { OwnerName = person.Name,
Pet = PetCollection.Select( p => p.Name )
.DefaultIfEmpty() }
).ToList();
You may have to play around with the selection expression. I'm not sure it would give you want you want in the case where you have a 1-to-many relationship.
I think it's a little easier with the LINQ Query syntax
var query = (from person in context.People
join pet in context.Pets on person equals pet.Owner
into tempPets
from pets in tempPets.DefaultIfEmpty()
select new { OwnerName = person.Name, Pet = pets.Name })
.ToList();
You need to get the joined objects into a set and then apply DefaultIfEmpty as JPunyon said:
Person magnus = new Person { Name = "Hedlund, Magnus" };
Person terry = new Person { Name = "Adams, Terry" };
Person charlotte = new Person { Name = "Weiss, Charlotte" };
Pet barley = new Pet { Name = "Barley", Owner = terry };
List<Person> people = new List<Person> { magnus, terry, charlotte };
List<Pet> pets = new List<Pet>{barley};
var results =
from person in people
join pet in pets on person.Name equals pet.Owner.Name into ownedPets
from ownedPet in ownedPets.DefaultIfEmpty(new Pet())
orderby person.Name
select new { OwnerName = person.Name, ownedPet.Name };
foreach (var item in results)
{
Console.WriteLine(
String.Format("{0,-25} has {1}", item.OwnerName, item.Name ) );
}
Outputs:
Adams, Terry has Barley
Hedlund, Magnus has
Weiss, Charlotte has
I the following error message when faced this same problem:
The type of one of the expressions in the join clause is incorrect. Type inference failed in the call to 'GroupJoin'.
Solved when I used the same property name, it worked.
(...)
join enderecoST in db.PessoaEnderecos on
new
{
CD_PESSOA = nf.CD_PESSOA_ST,
CD_ENDERECO_PESSOA = nf.CD_ENDERECO_PESSOA_ST
} equals
new
{
enderecoST.CD_PESSOA,
enderecoST.CD_ENDERECO_PESSOA
} into eST
(...)
Here's a good blog post that's just been posted by Fabrice (author of LINQ in Action) which covers the material in the question that I asked. I'm putting it here for reference as readers of the question will find this useful.
Converting LINQ queries from query syntax to method/operator syntax
Left joins in LINQ are possible with the DefaultIfEmpty() method. I don't have the exact syntax for your case though...
Actually I think if you just change pets to pets.DefaultIfEmpty() in the query it might work...
EDIT: I really shouldn't answer things when its late...