Eagerly Loading
Eager loading is the process whereby a query for one type of entity also loads related entities as part of the query. Eager loading is achieved by use of the Include method. For example, the queries below will load blogs and all the posts related to each blog.
using (var context = new BloggingContext())
{
// Load all blogs and related posts
var blogs1 = context.Blogs
.Include(b => b.Posts)
.ToList();
// Load one blogs and its related posts
var blog1 = context.Blogs
.Where(b => b.Name == “ADO.NET Blog”)
.Include(b => b.Posts)
.FirstOrDefault();
// Load all blogs and related posts
// using a string to specify the relationship
var blogs2 = context.Blogs
.Include(“Posts”)
.ToList();
// Load one blog and its related posts
// using a string to specify the relationship
var blog2 = context.Blogs
.Where(b => b.Name == “ADO.NET Blog”)
.Include(“Posts”)
.FirstOrDefault();
}Eagerly loading multiple levels
It is also possible to eagerly load multiple levels of related entities. The queries below show examples of how to do this for both collection and reference navigation properties.
using (var context = new BloggingContext())
{
// Load all blogs, all related posts, and all related comments
var blogs1 = context.Blogs
.Include(b => b.Posts.Select(p => p.Comments))
.ToList();
// Load all users their related profiles, and related avatar
var users1 = context.Users
.Include(u => u.Profile.Avatar)
.ToList();
// Load all blogs, all related posts, and all related comments
// using a string to specify the relationships
var blogs2 = context.Blogs
.Include(“Posts.Comments”)
.ToList();
// Load all users their related profiles, and related avatar
// using a string to specify the relationships
var users2 = context.Users
.Include(“Profile.Avatar”)
.ToList();
}Lazy Loading
Lazy loading is the process whereby an entity or collection of entities is automatically loaded from the database the first time that a property referring to the entity/entities is accessed. When using POCO entity types, lazy loading is achieved by creating instances of derived proxy types and then overriding virtual properties to add the loading hook. For example, when using the Blog entity class defined below, the related Posts will be loaded the first time the Posts navigation property is accessed:
public class Blog
{
public int BlogId { get; set; }
public string Name { get; set; }
public string Url { get; set; }
public string Tags { get; set; }
public virtual ICollection Posts { get; set; }
} Turning off lazy loading for specific navigation properties
Lazy loading of the Posts collection can be turned off by making the Posts property non-virtual:
public class Blog
{
public int BlogId { get; set; }
public string Name { get; set; }
public string Url { get; set; }
public string Tags { get; set; }
public ICollection Posts { get; set; }
} Turn off lazy loading for all entities
Lazy loading can be turned off for all entities in the context by setting a flag on the Configuration property. For example:
public class BloggingContext : DbContext
{
public BloggingContext()
{
this.Configuration.LazyLoadingEnabled = false;
}
}Explicitly Loading
Even with lazy loading disabled it is still possible to lazily load related entities, but it must be done with an explicit call. To do so you use the Load method on the related entity’s entry. For example:
using (var context = new BloggingContext())
{
var post = context.Posts.Find(2);
// Load the blog related to a given post
context.Entry(post).Reference(p => p.Blog).Load();
// Load the blog related to a given post using a string
context.Entry(post).Reference(“Blog”).Load();
var blog = context.Blogs.Find(1);
// Load the posts related to a given blog
context.Entry(blog).Collection(p => p.Posts).Load();
// Load the posts related to a given blog
// using a string to specify the relationship
context.Entry(blog).Collection(“Posts”).Load();
}Applying filters when explicitly loading related entities
The Query method provides access to the underlying query that the Entity Framework will use when loading related entities. You can then use LINQ to apply filters to the query before executing it with a call to a LINQ extension method such as ToList, Load, etc. The Query method can be used with both reference and collection navigation properties but is most useful for collections where it can be used to load only part of the collection. For example:
using (var context = new BloggingContext())
{
var blog = context.Blogs.Find(1);
// Load the posts with the ‘entity-framework’ tag related to a given blog
context.Entry(blog)
.Collection(b => b.Posts)
.Query()
.Where(p => p.Tags.Contains(“entity-framework”)
.Load();
// Load the posts with the ‘entity-framework’ tag related to a given blog
// using a string to specify the relationship
context.Entry(blog)
.Collection(“Posts”)
.Query()
.Where(p => p.Tags.Contains(“entity-framework”)
.Load();
}Note that while the relationship can be specified as a string instead of a lambda expression, the returned IQueryable is not generic when a string is used and so the Cast method is usually needed before anything useful can be done with it.
Using Query to count related entities without loading them
Sometimes it is useful to know how many entities are related to another entity in the database without actually incurring the cost of loading all those entities. The Query method with the LINQ Count method can be used to do this. For example:
using (var context = new BloggingContext())
{
var blog = context.Blogs.Find(1);
// Count how many posts the blog has
var postCount = context.Entry(blog)
.Collection(b => b.Posts)
.Query()
.Count();
}
