Loading Related Entities in Entity Framework

Output

In the code above, initially, one query is fired to fetch Department Master data. When the Employee Masters property of the Department is accessed, the Entity Framework loads the related data automatically. In short, whenever the navigation property is accessed, the Entity Framework loads the related entity from the database. In the example above if one department has two employees then the for loop retrieves the related employee as well as the employee details table from the database.

Explicit Loading

If Lazy Loading is disabled then it is still possible to fetch the related entities without the use of Eager Loading. It is possible with Explicit Loading. To explicitly retrieve the related entities from the database we can use the "Load" method. In the following example, If Lazy Loading is disabled then when I check whether the employee has related detail data it returns null.

Output

using (var context = new Entities())
{
    context.Configuration.LazyLoadingEnabled = false; // If using DbContext

    var emp = context.EmployeeMasters.FirstOrDefault(p => p.EmployeeId == 1);

    if (emp.EmployeeDetail == null)
    {
        Console.WriteLine("No Employee Detail Data or Employee Detail is not loaded");
    }

    Console.ReadKey();
}

Now I want to load the related detail data explicitly. There is a one-to-one relationship between EmployeeMaster and the EmployeeDetail entity. The following will explicitly load one-to-one related entities

using (var context = new Entities())
{
    var emp = context.EmployeeMasters.FirstOrDefault(p => p.EmployeeId == 1);

    if (emp.EmployeeDetail == null)
    {
        Console.WriteLine("No Employee Detail Data or Employee Detail is not loaded");
        Console.WriteLine("Loading Employee Detail Data....");

        if (!context.Entry(emp).Reference(e => e.EmployeeDetail).IsLoaded)
        {
            context.Entry(emp).Reference(e => e.EmployeeDetail).Load();
            Console.WriteLine("Employee Detail Data load Complete....");
            Console.WriteLine("Employee Email: " + emp.EmployeeDetail.PersonalEmail);
        }
    }

    Console.ReadKey();
}

Output

The following will explicitly load one-to-many related entities

using (var context = new Entities())
{
    var dep = context.DepartmentMasters.FirstOrDefault(p => p.DepartmentId == 1);

    Console.WriteLine("Department Employees are not currently loaded");

    if (!context.Entry(dep).Collection(d => d.EmployeeMasters).IsLoaded)
    {
        context.Entry(dep).Collection(d => d.EmployeeMasters).Load();

        Console.WriteLine("Department Employees load complete");

        foreach (var emp in dep.EmployeeMasters)
        {
            Console.WriteLine(emp.Code);
        }
    }

    Console.ReadKey();
}

Output

Conclusion

Both the Lazy Loading and Explicit Loading make multiple round trips to the database to load related entities while Eager Loading retrieves all the related data in a single round trip to the database.