Question

In the controllers generated by Visual Studio, as well as the sample application (ContosoUniversity), the Index action always has something like

var departments = db.Departments.Include(d => d.Administrator);

What’s the difference between that and

var departments = db.Departments;

First I suspected that the first one (with Include) enables the view to retrieve department.Administrator. But the second one (without Include) seems to be able to do that as well.

Answer 1

The Include tells Entity Framework work to eagerly load the Administrator for each Department in the results. In this case, Entity Framework can use a SQL join to grab the data from both tables in a single request.

The code will still work without the Include, but the first time you access a Department’s Administrator, EF will need to hit the database to load it (since it wasn’t pre-loaded). Loading data on demand (lazily) is a nice feature but it can be a serious performance problem (known as an N+1 problem). Especially if you are accessing the Administrator for each Department (for example, in a loop) – instead of one database call, you will end up with many!