Practical .NET
Leverage Joins in Entity Framework To Get Just the Data You Want
Every once in a while you'll need to use the LINQ Join to get the data you want.
By and large, I tell developers they won't use the LINQ Join clause because you always can just follow the navigation properties that link one entity to another. But that's not always true.
Let's say, for example, that you want to retrieve all of the SalesOrderDetails for SalesOrders that have a delivery date in the future. You could write this query to follow the SalesOrderDetail navigation property to get to the SalesOrderDetails:
Dim sods = From so In db.SalesOrders.Include(Function(so) so.SalesOrderDetails)
Where so.DueDate > DateTime.Now
Select so.SalesOrderDetails
Under the hood, Entity Framework will use a SQL Join to tie together the SalesOrder and SalesOrderDetail tables, but you don't have to use a LINQ Join in your query.
This will work fine ... but let's make the query more complicated: You don't want all of the SalesOrderDetails -- you only want SalesOrderDetails that have their IsDiscounted property is set to True. Unfortunately, you can't do a "conditional include" to retrieve just those SalesOrderDetails. If you stick with the query I just showed then you could, potentially, retrieve a lot of SalesOrderDetails that you don't want.
In this case, a LINQ Join is probably the best way to get just the data you want:
Dim sods = From so In db.SalesOrders
Join sod In db.SalesOrderDetails
On so.SalesOrderId = sod.SalesOrderID
Where so.DueDate > DateTime.Now And
sod.IsDiscounted = True
Select sod
If you're familiar with SQL this syntax will look very familiar, the one "addition" being the In keyword in the Join clause that establishes the variable name for referring to the SalesOrderDetails.
About the Author
Peter Vogel is a system architect and principal in PH&V Information Services. PH&V provides full-stack consulting from UX design through object modeling to database design. Peter tweets about his VSM columns with the hashtag #vogelarticles. His blog posts on user experience design can be found at http://blog.learningtree.com/tag/ui/.