private static void ExampleExplicitLoading()
{
// Here we get the code of Eager Loading, and modified the code to use Explicit Loading;
// You can run the application and see the queries in SQL Profiler;
var context = new PlutoContext();
// The Include was removed, and this is going to simplify my query, because EF is not
// going to join the Authors table with the Courses table;
// And then we need to explicity load the courses for this author; and this is what we call Explicit Loading;
var author = context.Authors.Single(a => a.Id == 1);
// There are two ways to do Explicit Loading:
// MSDN way (this only works for single entries - for example, here he have only one author object):
context.Entry(author).Collection(a => a.Courses).Load();
// Other way (Mosh way):
context.Courses.Where(c => c.AuthorId == author.Id).Load();
foreach (var course in author.Courses)
{
Console.WriteLine("{0}", course.Name);
}
// With the Explicit Loading we can simplify the complex query that is result of too many
// includes and too much Eager Loading; Of course, we're going to end up with multiple round
// trips to the database, but sometimes this can be more efficient than running a huge query
// on the database;
// But Explicit Loading also has another benefit: we can apply filters to the related objects:
// MSDN Way:
context.Entry(author).Collection(a => a.Courses).Query().Where(c => c.FullPrice == 0).Load();
// Other way (Mosh way):
context.Courses.Where(c => c.AuthorId == author.Id && c.FullPrice == 0).Load();
// ------------------------------------------------------------------------------
var authors = context.Authors.ToList();
// Prefer to use the second approach - the Mosh way - because it's simpler and it's more flexible;
// for example, to get the free courses for these authors, we cannot use the MSCN approach, because
// if I call context.Entry, I cannot pass the authors object, because the Entry method is used to
// reference only a single entry, a single object;
//context.Entry()...
// with the other approach, see:
var authorIds = authors.Select(a => a.Id);
context.Courses.Where(c => authorIds.Contains(c.AuthorId) && c.FullPrice == 0).Load();
}
static void Main(string[] args)
{
var context = new PlutoContext();
// Lazy Loading - Don't use in web application!!
//var courses = context.Courses.Single(c => c.Id == 2);
//foreach (var tag in courses.Tags)
// Console.WriteLine(tag.Name);
// Eager Loading
var courses = context.Courses.Include(c => c.Author).ToList();
foreach (var course in courses)
{
Console.WriteLine($"{course.Name} by {course.Author.Name}");
}
// Eager Loading - Multiple Levels
// For ingle properties
//context.Courses.Include(c => c.Author.Address);
// For collection properties
//context.Courses.Include(c => c.Tags.Select(t => t.Moderator));
// Explicit Loading - Singleton
var author = context.Authors.Single(a => a.Id == 1);
// MSDN way
context.Entry(author).Collection(a => a.Courses).Query().Where(c => c.FullPrice == 0).Load();
// Mosh way
context.Courses.Where(c => c.AuthorId == author.Id && c.FullPrice == 0).Load();
foreach (var course in author.Courses)
{
Console.WriteLine($"{course.Name}");
}
// Explicit Loading - Multiple
var authors = context.Authors.ToList();
var authorIds = authors.Select(a => a.Id);
context.Courses.Where(c => authorIds.Contains(c.AuthorId) && c.FullPrice == 0).Load();
}
public static void ExplicitLoading(PlutoContext context)
{
Console.WriteLine("Explicit loading 1");
Console.WriteLine();
//var author = context.Authors.Include(a => a.Courses).Single(a => a.Id == 1); // Eager loading
var author = context.Authors.Single(a => a.Id == 1);
//MSDN - Only works for a single author not multiples ie if the line above was ToList and not Single
context.Entry(author).Collection(a => a.Courses).Load();
//context.Entry(author).Collection(a => a.Courses).Query().Where(c => c.FullPrice == 0).Load();
//Better way
context.Courses.Where(c => c.AuthorId == author.Id).Load();
//context.Courses.Where(c => c.AuthorId == author.Id && c.FullPrice == 0).Load();
foreach (var course in author.Courses)
{
Console.WriteLine("{0}", course.Name);
}
}