Eager Loading with Repository Pattern and Entity Framework
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.
CodeProject