Linq to Entity with multiple left outer joins - join

Question

I am trying to understand left outer joins in LINQ to Entity. For example I have the following 3 tables:
Company, CompanyProduct, Product
The CompanyProduct is linked to its two parent tables, Company and Product.
I want to return all of the Company records and the associated CompanyProduct whether the CompanyProduct exists or not for a given product. In Transact SQL I would go from the Company table using left outer joins as follows:
SELECT * FROM Company AS C
LEFT OUTER JOIN CompanyProduct AS CP ON C.CompanyID=CP.CompanyID
LEFT OUTER JOIN Product AS P ON CP.ProductID=P.ProductID
WHERE P.ProductID = 14 OR P.ProductID IS NULL
My database has 3 companies, and 2 CompanyProduct records assocaited with the ProductID of 14. So the results from the SQL query are the expected 3 rows, 2 of which are connected to a CompanyProduct and Product and 1 which simply has the Company table and nulls in the CompanyProduct and Product tables.
So how do you write the same kind of join in LINQ to Entity to acheive a similiar result?
I have tried a few different things but can't get the syntax correct.
Thanks.

Answer 1

Solved it!
Final Output:
theCompany.id: 1
theProduct.id: 14
theCompany.id: 2
theProduct.id: 14
theCompany.id: 3
Here is the Scenario
1 - The Database
--Company Table
CREATE TABLE [theCompany](
[id] [int] IDENTITY(1,1) NOT NULL,
[value] [nvarchar](50) NULL,
CONSTRAINT [PK_theCompany] PRIMARY KEY CLUSTERED
( [id] ASC ) WITH (
PAD_INDEX = OFF,
STATISTICS_NORECOMPUTE = OFF,
IGNORE_DUP_KEY = OFF,
ALLOW_ROW_LOCKS = ON,
ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY];
GO
--Products Table
CREATE TABLE [theProduct](
[id] [int] IDENTITY(1,1) NOT NULL,
[value] [nvarchar](50) NULL,
CONSTRAINT [PK_theProduct] PRIMARY KEY CLUSTERED
( [id] ASC
) WITH (
PAD_INDEX = OFF,
STATISTICS_NORECOMPUTE = OFF,
IGNORE_DUP_KEY = OFF,
ALLOW_ROW_LOCKS = ON,
ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY];
GO
--CompanyProduct Table
CREATE TABLE [dbo].[CompanyProduct](
[fk_company] [int] NOT NULL,
[fk_product] [int] NOT NULL
) ON [PRIMARY];
GO
ALTER TABLE [CompanyProduct] WITH CHECK ADD CONSTRAINT
[FK_CompanyProduct_theCompany] FOREIGN KEY([fk_company])
REFERENCES [theCompany] ([id]);
GO
ALTER TABLE [dbo].[CompanyProduct] CHECK CONSTRAINT
[FK_CompanyProduct_theCompany];
GO
ALTER TABLE [CompanyProduct] WITH CHECK ADD CONSTRAINT
[FK_CompanyProduct_theProduct] FOREIGN KEY([fk_product])
REFERENCES [dbo].[theProduct] ([id]);
GO
ALTER TABLE [dbo].[CompanyProduct] CHECK CONSTRAINT
[FK_CompanyProduct_theProduct];
2 - The Data
SELECT [id] ,[value] FROM theCompany
id value
----------- --------------------------------------------------
1 company1
2 company2
3 company3
SELECT [id] ,[value] FROM theProduct
id value
----------- --------------------------------------------------
14 Product 1
SELECT [fk_company],[fk_product] FROM CompanyProduct;
fk_company fk_product
----------- -----------
1 14
2 14
3 - The Entity in VS.NET 2008
alt text http://i478.photobucket.com/albums/rr148/KyleLanser/companyproduct.png
The Entity Container Name is 'testEntities' (as seen in model Properties window)
4 - The Code (FINALLY!)
testEntities entity = new testEntities();
var theResultSet = from c in entity.theCompany
select new { company_id = c.id, product_id = c.theProduct.Select(e=>e) };
foreach(var oneCompany in theResultSet)
{
Debug.WriteLine("theCompany.id: " + oneCompany.company_id);
foreach(var allProducts in oneCompany.product_id)
{
Debug.WriteLine("theProduct.id: " + allProducts.id);
}
}
5 - The Final Output
theCompany.id: 1
theProduct.id: 14
theCompany.id: 2
theProduct.id: 14
theCompany.id: 3

Answer 2

IT should be something like this....
var query = from t1 in db.table1
join t2 in db.table2
on t1.Field1 equals t2.field1 into T1andT2
from t2Join in T1andT2.DefaultIfEmpty()
join t3 in db.table3
on t2Join.Field2 equals t3.Field3 into T2andT3
from t3Join in T2andT3.DefaultIfEmpty()
where t1.someField = "Some value"
select
{
t2Join.FieldXXX
t3Join.FieldYYY
};
This is how I did....

Answer 3

You'll want to use the Entity Framework to set up a many-to-many mapping from Company to Product. This will use the CompanyProduct table, but will make it unnecessary to have a CompanyProduct entity set in your entity model. Once you've done that, the query will be very simple, and it will depend on personal preference and how you want to represent the data. For example, if you just want all the companies who have a given product, you could say:
var query = from p in Database.ProductSet
where p.ProductId == 14
from c in p.Companies
select c;
or
var query = Database.CompanySet
.Where(c => c.Products.Any(p => p.ProductId == 14));
Your SQL query returns the product information along with the companies. If that's what you're going for, you might try:
var query = from p in Database.ProductSet
where p.ProductId == 14
select new
{
Product = p,
Companies = p.Companies
};
Please use the "Add Comment" button if you would like to provide more information, rather than creating another answer.

Answer 4

LEFT OUTER JOINs are done by using the GroupJoin in Entity Framework:
http://msdn.microsoft.com/en-us/library/bb896266.aspx

Answer 5

The normal group join represents a left outer join. Try this:
var list = from a in _datasource.table1
join b in _datasource.table2
on a.id equals b.table1.id
into ab
where ab.Count()==0
select new { table1 = a,
table2Count = ab.Count() };
That example gives you all records from table1 which don't have a reference to table2.
If you omit the where sentence, you get all records of table1.

Answer 6

Please try something like this:
from s in db.Employees
join e in db.Employees on s.ReportsTo equals e.EmployeeId
join er in EmployeeRoles on s.EmployeeId equals er.EmployeeId
join r in Roles on er.RoleId equals r.RoleId
where e.EmployeeId == employeeId &&
er.Status == (int)DocumentStatus.Draft
select s;
Cheers!

Answer 7

What about this one (you do have a many-to-many relationship between Company and Product in your Entity Designer, don't you?):
from s in db.Employees
where s.Product == null || s.Product.ProductID == 14
select s;
Entity Framework should be able to figure out the type of join to use.