An Enthusiastic Programmer

EF Core Explicit Loading


Explicit Loading means that related entities are explicitly loaded from the database at a later time.

This project can be found in Github

using (var context = new SchoolContext())
    var lake_team =  await context.teams.Where(t => t.Name == "lake").FirstOrDefaultAsync();

    //The lake.Players is a collection navigation property, which should use the Collection method.
    await context.Entry(lake_team).Collection(lake => lake.Players).LoadAsync();

    //load collection navigation property Address for each player 
    foreach (Player player in lake_team.Players) {
        await context.Entry(player).Collection(p => p.Addresses).LoadAsync();
    return lake_team;

In the above example, the await context.Entry(lake_team).Collection(lake=>lake.Players).LoadAsync() loads the Player entities, the Collection method provides access to tracking and loading information for a collection navigation property that associates this entity to a collection of another entites. The LoadAsync method executes the SQL query and fill up the specified reference property or collection property in the memory, all of these complete asynchronously.

As we have known. The Player and Addresses are all collection property. one Team has many players, and one player has many homes. their structures defined as below:

class Team{
public List<Player> Players { set; get; }
class Player{
public List<HomeAddress> Addresses { set; get; }

The opposite of the collection property is the reference property. The collection property is the to-many relationship, but the reference property is the to-one relationship.

for example:

//class School{
//   public string SchoolName{set;get;}
//   public string loc{set;get;}
class Team{
public School shool{set;get;}

The Team only belongs to one school, so the school is a reference property.



The Collection() or Referece() methods return a CollectionEntry object or a ReferenceEntry object, both objects expose change tracking information. and they are not IQueryable instances, So we can’t continue to write further LINQ queries to filter out related data after these methods.

How we can do it? luckily, we have Query() method which can offer us to write further queries.

context.Entry(lake_team).Collection(lake => lake.Players).
        Where(p=>p.Name== "Kobe Bean Bryant").