Creating a common generic and extensible NHiberate Repository version 2

This is a follow up to my original post on this topic Creating a common generic and extensible NHiberate Repository and further progression of it taking advantage of DetachedCriterias and Future queries.

I’m going to start with my interface as a jumping off point.

public interface IRepository<T> { 
/// <summary>
/// Adds the specified <see cref="T"/>. 
/// </summary> 
/// <param name="obj">The obj.</param> 
void Add(T obj);  
/// <summary>  
/// Deletes the specified <see cref="T"/>. 
/// </summary> 
/// <param name="obj">The obj.</param> 
void Delete(T obj);  
/// <summary> 
/// Defers the detached crteria execution which will return a result set that is an <see cref="IEnumerable{T}"/>. 
/// </summary> 
/// <param name="detachedCriteria">The detached criteria.</param> 
/// <returns></returns> 
IEnumerable<T> FutureList(DetachedCriteria detachedCriteria); 
/// <summary> 
/// Executes the detached crteria immediately returning a single item <see cref="T"/>
/// This will also cause the execution of any defered queries to be executed. 
/// </summary> /// <param name="detachedCriteria">The detached criteria.</param> 
/// <returns></returns> 
T FutureSingle(DetachedCriteria detachedCriteria); 
/// <summary> 
/// Gets the specified <see cref="T"/>. 
/// </summary> 
/// <typeparam name="U"></typeparam> 
/// <param name="id">The id.</param>
 /// <returns></returns> 
T Get<U>(U id); 
/// <summary> 
/// Executes the detached crteria immediately returning a result set that is an <see cref="IList{T}"/>. 
/// This will also cause the execution of any defered queries to be executed. 
/// </summary> 
/// <param name="detachedCriteria">The detached criteria.</param> 
/// <returns></returns> 
IList<T> QueryList(DetachedCriteria detachedCriteria); 
/// <summary> 
/// Executes the detached crteria immediately returning a single item <see cref="T"/> 
/// </summary> 
/// <param name="detachedCriteria">The detached criteria.</param> 
/// <returns></returns> 
T QuerySingle(DetachedCriteria detachedCriteria); 
/// <summary> 
/// Inserts or updates the specified <see cref="T"/>. 
/// </summary>
/// <param name="obj">The obj.</param> 
void SaveOrUpdate(T obj); 
/// <summary> 
/// Updates the specified <see cref="T"/>. 
/// </summary> 
/// <param name="obj">The obj.</param>
 void Update(T obj); } 

I think my interface is very well defined and with the XMLDoc comments easy to follow what will be implemented. The most notable changes from my original post is the removal of the GetAll and GetRange methods that returned an ICriteria, these always felt like a kludge to me as they clearly bleed concern by allowing the raw ICriteria to be exposed however back then I didn’t have the tools and knowledge available to create a better solution.

The new methods are related to the DetachedCriterias QuerySingle, QueryList and to the Future based queries FutureSingle and FutureList. Future is a newer construct in NHibernate that allows you to specify a query that you want NHibernate to understand and know at some point in the Future it will need this execution so you can batch more than 1 query together and reduce the number of calls to a database. This is a great bonus as the database is usually always the biggest slow down in an application.

On to the actual implementation of the interface

public class Repository<T> : IRepository<T> { 
private readonly IBusinessConversation _conversation;

public Repository(IBusinessConversation conversation) { _conversation = conversation; } 

IList<T> IRepository<T>.QueryList(DetachedCriteria detachedCriteria) {  
 return detachedCriteria.GetExecutableCriteria(_conversation.Session).List<T>(); 
} 

T IRepository<T>.QuerySingle(DetachedCriteria detachedCriteria) {  
 return detachedCriteria.GetExecutableCriteria(_conversation.Session).UniqueResult<T>(); 
} 

T IRepository<T>.FutureSingle(DetachedCriteria detachedCriteria) {  
 return detachedCriteria.GetExecutableCriteria(_conversation.Session).FutureValue<T>().Value; 
} 

IEnumerable<T> IRepository<T>.FutureList(DetachedCriteria detachedCriteria) {  
 return detachedCriteria.GetExecutableCriteria(_conversation.Session).Future<T>(); 
} 

T IRepository<T>.Get<U>(U id) { return _conversation.Session.Get<T>(id); } 

void IRepository<T>.SaveOrUpdate(T obj) { _conversation.Session.SaveOrUpdate(obj); } 

void IRepository<T>.Add(T obj) { _conversation.Session.Save(obj); } 

void IRepository<T>.Update(T obj) { _conversation.Session.Update(obj); } 

void IRepository<T>.Delete(T businessObj) { _conversation.Session.Delete(businessObj); } } 

The one that might be confusing is the _conversation dependency this relates to another post Implementing NHibernate Long Conversation with no HttpModules or AOP build time weaving. You can take a look at this if you haven’t and are curious however it’s not needed to understand my code here as you can really view _conversation to be the raw NHibernate ISession and understand what is going on here.

This class houses the generic CRUD operations that are made generic to allow you to handle any custom object easily. The methods that are once again of interest are the ones I discussed as new additions to my Repository. The beauty of all them regardless is in their simplicity that they can all be written in 1 concise line yet offer immense flexibility.

With the QuerySingle and QueryList methods these allow you to take in a DetachedCriteria and get back either a single object or a IList collection which should be pretty self explanatory from both the code and method names.

The FutureList and FutureSingle are not so much as obvious. As I stated before these allow you to make these happen in the future. In my implementation only the FutureList will really allow you to relegate multiple events into the future but it also makes the most sense since individual lookups are frequently done by indexed columns that should be quick even done in multiple seperate calls. For each call you make to FutureList it will have NHibernate queue up the specified DetachedQuery and wait for one of the following either code causes any of IEnumerables returned to be enumerated which will fire off 1 mulit-query to get all of the result sets or you call FutureSingle.

FutureSingle is some what of a more unusual animal because a regular object doesn’t have the lazy access method that IEnumerables do either it exists, or it doesn’t. There is no transient stage as there is with IEnumerables. To overcome this fact the developers of NHibernate created this transient stage by wrapping object with a generic interface for IFutureValue<T> this gives a construct to have a 3rd stage that you need to call .Value to invoke the actual execution which will process all future queries. In my repository I actually call .Value immediately on a DetachedCriteria being passed into this, why? Because I’d need to return a IFutureValue<T> instead of just T as the result from the method. This in my opinion would be just as much of a kludge as returning ICriteria was from my previous implementation as it bleeds NHibernate understanding logic outside of the repository. This purely a subjective point and if you disagree with my sentiment you could easily return IFutureValue<T> and have a wider range of flexibility with the Future support.

Now on to actual usage of this in a real world scenario. I am going to be taking advantage of a framework I recently learned about called NHibernate Lambda Expressions this allows you to write Criteria and DetachedCriteria queries without the need to rely on magic strings. The point of this post won’t be to cover the usage of this as it should be mostly understandable for my example.

 public class DataProvider : IDataProvider {  
private readonly IRepository<Person> _personRepository;  

public DataProvider(IRepository<Person> personRepository) { _personRepository = personRepository; } 

public IList<Person> GetPersons(State state) { 
  var query = DetachedCriteria.For<Person>() 
 .CreateCriteria<Person>(x => x.PrimaryZip) 
 .CreateCriteria<Zip>(x => x.State) 
 .Add<State>(x => x.StateCode == state.StateCode);  
 return _personRepository.FutureList(query); 
} 

public Person GetPerson(Guid personID) {  return _personRepository.Get(personID); } } 

If you look at my GetPersons method you should see I have a somewhat deep object graph that is realistic, I have a collection of Persons in my database where I assign them a zip code object that contains among other things the zip code and the state that zip code lives in. I create a DetachedCriteria that lets me find all persons that exist in a state and I pass that down to my generic IRepository by delegation instead of inheritance. Many examples similar to my post use inheritance where you will inherit from Repository<Person> instead of having a delegate inside of a non generic class I feel this is the wrong way because it then limits you to having 1 repository per data object instead of being able to drop multiple IRepositorys inside of a single DataProvider.

Taking this a step further I can even remove the need to ever create a physical implementation of Repository<Person> through the use of an inversion of control / dependency injection framework in this case specifically StructureMap. In literally 1 line I can create an infinite amount of IRepositories for any application I need.

For(typeof (IRepository<>)).LifecycleIs(new HybridLifecycle()).Use(typeof (Repository<>)); 

This instructs StructureMap that anytime I need a IRepository<Person> or any other data object that it’s to give it a usage of Repository
completely absolving me of the responsibility of defining a Repository<Person> combining the functionality I’ve outlined in this post it offers a broad range of usage that allows you to interact with NHibernate without ever needing to write a single line of code again except the specific DetachedCriterias which will need written in one shape or form irregardless because they will have custom sql be the end result of them and to just wire up Methods to allow access to the underlying CRUD methods. You could actually expose the generic methods themselves however I feel that is exposing too much logic to be available to an API and prefer to shroud my generic repositories in the classes I call DataProviders as it also lets me group repositories in meaningful ways.

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Creating a common generic and extensible NHiberate Repository

In my previous post Conversation Per Business Transaction using PostSharp and IoC I dealt with the session management aspect of NHiberate however by the end of my development of the business conversation aspect I realized that my current data provider scheme created a lot of duplicate code. As we all know any code that is duplicated becomes a nightmare to maintain so I went through and refactored first a truly generic repository.

public interface IRepository<T>
{
    ICriteria GetAll();
    T Get<U>(U id);
    void Save(T obj);
    ICriteria GetRange(int resultSet, int rangeSize);
}

This is a rather basic interface that will handle the most common operations to my data providers, currently it can handle getting all of the items in my database for a type, a page collection into the list of items in my database, get a specific item and save a new item (or update an existing item). The only minorly unsightly part is the Get<U> method this just allows passing any type of ID instead of declaring it as int statically.

On to implementing the interface:

public class Repository<T> : IRepository<T>
{
    private readonly IConversation _conversation;

    public Repository(IConversation conversation)
    {
        _conversation = conversation;
    }

    #region IRepository<T> Members

    ICriteria IRepository<T>.GetAll()
    {
        return _conversation.Session.CreateCriteria(typeof (T))
            //.SetCacheable(true)
            //.SetCacheMode(CacheMode.Normal)
            ;
    }

    T IRepository<T>.Get<U>(U id)
    {
        return _conversation.Session.Get<T>(id);
    }

    void IRepository<T>.Save(T obj)
    {
        _conversation.Session.SaveOrUpdate(obj);
    }

    ICriteria IRepository<T>.GetRange(int resultSet, int rangeSize)
    {
        return _conversation.Session.CreateCriteria(typeof (T))
            //.SetCacheable(true)
            //.SetCacheMode(CacheMode.Normal)
            .SetFirstResult(resultSet*rangeSize)
            .SetMaxResults(rangeSize - 1)
            ;
    }

    #endregion
}

This class contains rather basic interactions with NHiberate, if you’re not familiar with the CreateCriteria querying method in NHibernate take a look at Chapter 12. Criteria Queries. Other than that I can at an application level enable or disable output caching for my generic repository, currently I’m still working on getting my second level caching implementation working so right now I have it disabled as you can see it’s commented out.

By returning the ICriteria from the repository it allows us to pass on the full power of NHibernate and allow us to create further data provider specific queries.

Next I created an abstract base class for my actual data providers:

public abstract class DataProviderBase<T>
{
    protected readonly IConversation _conversation;
    protected readonly IRepository<T> _repoistory;

    protected DataProviderBase(IConversation conversation)
    {
        _conversation = conversation;
        _repository = new Repository<T>(_conversation);
    }
}

This base class just holds the fields for the business conversation (or ISession) and my newly created generic repository. Now on to my true concrete implementation of my data provider:

public class EmployeeDataProvider : DataProviderBase<Employee>, IEmployeeDataProvider
{
    [InjectionConstructor]
    public EmployeeDataProvider(IConversation conversation) : base(conversation)
    {
    }

    #region IEmployeeDataProvider Members

    IList<Employee> IEmployeeDataProvider.GetAll()
    {
        return _repository.GetAll().List<Employee>();
    }

    IList<Employee> IEmployeeDataProvider.GetRange(int resultSet, int rangeSize)
    {
        return _repository.GetRange(resultSet, rangeSize).List<Employee>();
    }

    Employee IEmployeeDataProvider.Get(int employeeId)
    {
        return _repository.Get(employeeId);
    }

    void IEmployeeDataProvider.Save(Employee employee)
    {
        _repository.Save(employee);
    }

    IList<Territory> IEmployeeDataProvider.GetTerritories()
    {
        return _conversation.Session.CreateCriteria(typeof (Territory)).List<Territory>();
    }

    #endregion
}

In this class we have the delegation of the IRepository (sitting in the abstract base) and we also have direct access to the NHibernate Session so we can add further methods of any kind. This also allows us the flexibility if you’d rather the EmployeeDataProvider had GetEmpoyees() instead of GetAll() it would allow that easily.

Since the repository itself is not exposed at the provider level this allows us to return the actual results in anyway we want. It allows us to further modify the basic queries by adding NHibernate where clauses to the Criteria if we so choose. Since the actual ICriteria won’t be exposed outside of the data provider we don’t need to worry about client interactions causing database usage. This would also allow us if we added NHibernate LINQ to not be concerned with returning the Queryable results for the same reason.

This pattern would also work directly with LINQ2SQL / LINQ2Entities / DLINQ by changing where the NHibernate session is dealt with and using your entity provider. I feel this is a perfect to the data access patern as it protects the code from becoming too unweilding by basing all communication directly through the ICriteria and giving our data providers concrete result lists.

Let me know what you think!

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