Lazy Loading | Explicit Loading | Eager Loading in EntityFramework and EntityFramework.Core

EntityFramework

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();
}

Note that Include is an extension method in the System.Data.Entity namespace so make sure you are using that namespace.

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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();
}

Note that it is not currently possible to filter which related entities are loaded. Include will always bring in all related entities.

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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<Post> Posts { get; set; }
}

Turn lazy loading off for serialization

Lazy loading and serialization don’t mix well, and if you aren’t careful you can end up querying for your entire database just because lazy loading is enabled. Most serializers work by accessing each property on an instance of a type. Property access triggers lazy loading, so more entities get serialized. On those entities properties are accessed, and even more entities are loaded. It’s a good practice to turn lazy loading off before you serialize an entity. The following sections show how to do this.

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<Post> Posts { get; set; }
}

Loading of the Posts collection can still be achieved using eager loading (see Eagerly Loading above) or the Load method (see Explicitly Loading below).

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;
    }
}

Loading of related entities can still be achieved using eager loading (see Eagerly Loading above) or the Load method (see Explicitly Loading below).

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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();
}

Note that the Reference method should be used when an entity has a navigation property to another single entity. On the other hand, the Collection method should be used when an entity has a navigation property to a collection of other entities.

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();
}

When using the Query method it is usually best to turn off lazy loading for the navigation property. This is because otherwise the entire collection may get loaded automatically by the lazy loading mechanism either before or after the filtered query has been executed.

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();
}

---

EntityFramework.Core

Entity Framework Core allows you to use the navigation properties in your model to load related entities. There are three common O/RM patterns used to load related data.

• Eager loading means that the related data is loaded from the database as part of the initial query.
• Explicit loading means that the related data is explicitly loaded from the database at a later time.
• Lazy loading means that the related data is transparently loaded from the database when the navigation property is accessed. Lazy loading is not yet possible with EF Core.

Eager Loading

You can use the Include method to specify related data to be included in query results. In the following example, the blogs that are returned in the results will have their Posts property populated with the related posts.

using (var context = new BloggingContext())
{
    var blogs = context.Blogs
        .Include(blog => blog.Posts)
        .ToList();
}

Entity Framework Core will automatically fix-up navigation properties to any other entities that were previously loaded into the context instance. So even if you don't explicitly include the data for a navigation property, the property may still be populated if some or all of the related entities were previously loaded.

You can include related data from multiple relationships in a single query.

using (var context = new BloggingContext())
{
    var blogs = context.Blogs
        .Include(blog => blog.Posts)
        .Include(blog => blog.Owner)
        .ToList();
}

Including multiple levels

You can drill down thru relationships to include multiple levels of related data using the ThenInclude method. The following example loads all blogs, their related posts, and the author of each post.

using (var context = new BloggingContext())
{
    var blogs = context.Blogs
        .Include(blog => blog.Posts)
            .ThenInclude(post => post.Author)
        .ToList();
}

You can chain multiple calls to ThenInclude to continue including further levels of related data.

using (var context = new BloggingContext())
{
    var blogs = context.Blogs
        .Include(blog => blog.Posts)
            .ThenInclude(post => post.Author)
                .ThenInclude(author => author.Photo)
        .ToList();
}

You can combine all of this to include related data from multiple levels and multiple roots in the same query.

using (var context = new BloggingContext())
{
    var blogs = context.Blogs
        .Include(blog => blog.Posts)
            .ThenInclude(post => post.Author)
            .ThenInclude(author => author.Photo)
        .Include(blog => blog.Owner)
            .ThenInclude(owner => owner.Photo)
        .ToList();
}

Ignored includes

If you change the query so that it no longer returns instances of the entity type that the query began with, then the include operators are ignored.

In the following example, the include operators are based on the Blog, but then the Select operator is used to change the query to return an anonymous type. In this case, the include operators have no effect.

using (var context = new BloggingContext())
{
    var blogs = context.Blogs
        .Include(blog => blog.Posts)
        .Select(blog => new
        {
            Id = blog.BlogId,
            Url = blog.Url
        })
        .ToList();
}

By default, EF Core will log a warning when include operators are ignored. See Logging for more information on viewing logging output. You can change the behavior when an include operator is ignored to either throw or do nothing. This is done when setting up the options for your context - typically in DbContext.OnConfiguring, or in Startup.cs if you are using ASP.NET Core.

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
    optionsBuilder
        .UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=EFQuerying;Trusted_Connection=True;")
        .ConfigureWarnings(warnings => warnings.Throw(CoreEventId.IncludeIgnoredWarning));
}

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Explicit loading

You can explicitly load a navigation property via the DbContext.Entry(...) API.

using (var context = new BloggingContext())
{
    var blog = context.Blogs
        .Single(b => b.BlogId == 1);
    context.Entry(blog)
        .Collection(b => b.Posts)
        .Load();
    context.Entry(blog)
        .Reference(b => b.Owner)
        .Load();
}

Querying related entities

You can also get a LINQ query that represents the contents of a navigation property.

This allows you to do things such as running an aggregate operator over the related entities without loading them into memory.

using (var context = new BloggingContext())
{
    var blog = context.Blogs
        .Single(b => b.BlogId == 1);
    var postCount = context.Entry(blog)
        .Collection(b => b.Posts)
        .Query()
        .Count();
}

You can also filter which related entities are loaded into memory.

using (var context = new BloggingContext())
{
    var blog = context.Blogs
        .Single(b => b.BlogId == 1);
    var goodPosts = context.Entry(blog)
        .Collection(b => b.Posts)
        .Query()
        .Where(p => p.Rating > 3)
        .ToList();
}

Lazy Loading

Lazy loading is not yet supported by EF Core. You can view the lazy loading item on our backlog to track this feature.