Eager Loading with Repository Pattern and Entity Framework

One question that I received yesterday after I published the Revisiting the Repository  and Unit of Work Patterns with Entity Framework
post was how to include the eager loading ability of Entity Framework. This post is offering a solution.

Revisiting Eager Loading and Lazy Loading

Lazy loading is a design pattern that is commonly used to defer initialization of an object up until it is needed by the program. The gains of using the pattern include efficiency (if it’s used right) and sometime performance. Eager loading is the opposite pattern of lazy loading. In this pattern we initialize the object before hand and don’t wait to the second we really need it. In Entity Framework we can use the Include method in order to eager load an entity graph.

Eager Loading with Repository Pattern

In the interface of the Repository I will add a new method which will be called QueryObjectGraph. That method will receive a string which indicate the children to load. Now the interface will look like:

public interface IRepository<T> where T : class
{        
    T GetById(int id);
    IEnumerable<T> GetAll();
    IEnumerable<T> Query(Expression<Func<T, bool>> filter);
    IEnumerable<T> QueryObjectGraph(Expression<Func<T, bool>> filter, string children);
    void Add(T entity);
    void Remove(T entity);   
}

and the implementation of the abstract Repository will change to

public abstract class Repository<T> : IRepository<T>
                                  where T : class
{
  #region Members
 
  protected ObjectSet<T> _objectSet;
 
  #endregion
 
  #region Ctor
 
  public Repository(ObjectContext context)
  {
    _objectSet = context.CreateObjectSet<T>();
  }
 
  #endregion
 
  #region IRepository<T> Members
 
  public IEnumerable<T> GetAll()
  {
    return _objectSet;
  }
 
  public abstract T GetById(int id);
 
  public IEnumerable<T> Query(Expression<Func<T, bool>> filter)
  {
    return _objectSet.Where(filter);
  }
 
  public IEnumerable<T> QueryObjectGraph(Expression<Func<T, bool>> filter, string children)
  {
    return _objectSet.Include(children).Where(filter);
  }
 
  public void Add(T entity)
  {
    _objectSet.AddObject(entity);
  }
 
  public void Remove(T entity)
  {
    _objectSet.DeleteObject(entity);
  }
 
  #endregion
}

Pay attention that I replaced the IObjectSet<T> into it’s Entity Framework implementation of ObjectSet<T>. The reason is that the IObjectSet<T> interface doesn’t include the Include method. Now I can continue using the same Unit of Work implementation and also use the eager loading ability like in the following example:

using (SchoolEntities context = new SchoolEntities())
{
  UnitOfWork uow = new UnitOfWork(context);
  foreach (var department in uow.Departments.GetAll())
  {
    Console.WriteLine(department.Name);
  }
 
  foreach (var department in uow.Departments.Query(d => d.Budget > 150000))
  {
    Console.WriteLine("department with above 150000 budget: {0}",
        department.Name);
  }
 
  foreach (var department in uow.Departments.QueryObjectGraph(d => d.Budget > 150000, 
      "Courses"))       
  {
    Console.WriteLine("department with above 150000 budget: {0}, {1}",
        department.Name, department.Courses.First().Title);
  }
}

Summary

In the previous post I only showed the way to implement your repositories. As you can see in this post I can take the offered solution and make it specific to my needs.