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Query Objects vs Methods on a Repository

Wednesday, March 28th, 2007.

Given that we have some interface/class that we use to communicate with the persistence mechanism, in NHibernate terms this would be ISession, although that could be wrapper by some more technology-agnostic interface like IDbConversation – holding the generic methods: Insert, Update, Delete, GetById<T>, GetAll<T>, etc.

What design issues are better handled by option one (query objects) or option two (methods on a repository)?

Option 1:

No custom repository per persistence type. All calls are done on IDbConversation. Fetching objects with custom queries is done like so:

IList<T> IDbConversation.PerformQuery(IQuery<T> query);

and then specific classes implement IQuery<T>, for instance, GetCustomersWithOutstandingDebtForLastQuarter which implements IQuery<ICustomer>. This implementation is responsible for translating the query into the appropriate representation for the persistence framework – say a chain of ICriteria in terms of NHibernate.

This results in loose coupling between queries, such that changing the interface of one would not touch code used by another.

By putting all queries in the same parent namespace “Queries”, we get the nice intellisense support of typing “IList<ICustomer> customers = this.IDbConversation.PerformQuery(Queries.” and getting the list of available options.

Option 2: (the more common)

One custom repository per persistence type. The repository talks with IDbConversation. The repository exposes methods for custom queries.

IList<Customer> CustomerRepository.GetCustomersWithOutstandingDebtForLastQuarter();

The repository translates the query to the specific persistence framework.

This results in less moving parts. In order to find a specific query you just type “this.CustomerRepository.” and the list of available methods shows up with intellisense.

Since all code is in the same class, changing the implementation/interface of one query/method may cause unintended side effects.

As you can probably tell, I’m for option one. It also fits nicely with my fetching strategy design, which I’m going to post the detailed design for shortly.

What are your thoughts?

[Also posted on the Domain Driven Design group here.]

  
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6 Comments

  1. » Performant and Explicit Domain Models Says:

    [...] Query Objects vs Methods on a Repository – discussing the scalability (in terms of number of developers and queries) of Query Objects. 43ce [...]


  2. Score keeping hockey and DDD « Justin Rudd’s Drivel Says:

    [...] older posts on the subjects.


  3. Hendry Luk Says:

    I think the problem with first option is from unit-test perspective. Yes each query is easy to be unit-tested easily (just like it’s easy to test specific repository’s method in option 2), but it’s extremely difficult to mock the query out (e.g. to test the layer above it, like UI controller or app-service).
    In the option 2, we can just mock the whole repository and assert the interaction with their query methods.
    But in option 1, there’s no easy way to mock the query (without TypeMock).
    I’m quite a fan of the first option myself (particularly I don’t want my repository method to depend on filtering criteria on UI, because UI changes often and this ideally should not affect data layer).
    But I’m wondering how you can write unit-test for UI controller that uses querying approach 1? Yes, I could test the UI agains fake database, but that’s essentially testing the query (that has been tested in isolation), and I only want to care about it interacts with correct query.
    And another question is, in the case of query that reflects search-screen, which layer do you prefer the query to live in? Yes ideally it should be in data layer, but that feels wrong to create a class in data-layer for something very specific to particular UI design. What’s your opinion about query objects that live in UI or application layer?

    Thanks


  4. Part 8: DAOs, Repositories, or Query Objects - NHibernate blog - NHibernate Forge Says:

    [...] In fact, these days we’re not even clear which is which. There are differing opinions all over the [...]


  5. Kevin Says:

    I did something similar to your “Option 1″:

    public class SubjectDAO : AbstractDAO, ISubjectDAO
    {
    public SubjectDAO(IRepository queries) : base(queries) { }

    public Subject[] GetSubjectsBy(Status status)
    {
    ISubject_SelectByStatus selectSubjectsByStatus = Queries.Find();

    return selectSubjectsByStatus.Execute(status == Status.Active ? true : false);
    }
    }

    The SubjectDAO (in order to retrieve subjects by status) executes a ISubject_SelectByStatus (which in this particular example is a wrapper to a stored procedure) that receives the enumeration State by parameter, and behind the scenes opens a connection, gets an active transaction (or starts a new one), creates the paremeter(s) with the parameter received from the DAO and executes the query, converting the IDataReader to a Model “Subject” entity with a Converter class injected with IoC to the ISubject_SelectByStatus.

    The nice thing about this example is that the query wrapper (the ISubject_SelectByStatus) is autogenerated with a home-made code generator, that “reads” the stored procedures and generate typed interfaces to be accessed from the DAOs.


  6. udidahan Says:

    Kevin,

    Have you seen my more recent CQRS posts where I discuss simpler query options?


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