4

I’m developing an application with a model similar to Stack Overflow (question / answer etc...) Modelling a NoSQL Forum Application with C# / ASP.net MVC

The model looks something like this (simplified)

class Question
{
    public string Title { get; set; }
    public string Body { get; set; }
    public DateTime DateCreated { get; set; }

    public string UserName { get; set; }

    public List<Answer> Replies { get; set; }
}

class Answer
{
    public string Body { get; set; }
    public DateTime DateCreated { get; set; }

    public string UserName { get; set; }
}

So my documents are just one document, with the "answers" embedded in them

I’m trying to design my repositories for this approach.

Should I have 2 separate repositories? For example:

interface IQuestionRepository
{
    void PutQuestion(Question question);
    Question GetQuestion(string questionID);
}  


interface IAnswerRepository
{
    void PutAnswer(string questionID, Answer Answer);
    Answer GetAnswer(string answerID);
}

Or something like this:

interface IPostRepository
{
    void PutQuestion(Question question);
    Question GetQuestion(string questionID);
    void PutAnswer(string questionID, Answer Answer);
    Answer GetAnswer(string answerID);
}
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3 Answers 3

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6

Your model is inherently flawed.

Question should be a root document.

Answer should be a root document.

While written in regards to RavenDB the document modeling information is mostly directly usable by you: http://codeofrob.com/archive/2010/12/21/ravendb-document-design-with-collections.aspx

Edit: FWIW the reason why your model is flawed is with document databases you want your documents to model transaction boundaries. Think of the editing scenario with stack overflow and how much of a nightmare it would be to maintain consistency with multiple people adding and updating answers which all alter the root document, and the poster is updating the question. The amount of contention on the single object will very problematic.

RavenDB provides what they call "patching" that lets you manipulate part of a document structure instead of the entire document exactly to solve problems like this but this design is best avoided up front instead of trying to make it work by greatly increasing the complexity of your persistence model having to do partial updates and handle elaborate concurrency situations.

And to answer the specific question after this, then you would have an AnswersRepository and a QuestsionsRepository

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4 Comments

I hadn't noticed this post by Rob.. I've read it now, and made some changes to my model.
@alexjamesbrown I know you accepted the answer to this question, do you feel that this was the right approach now that time has progressed since then?
At the risk of getting flamed, I don't agree that the model is inherently flawed. The idea that the transaction boundary is the correct design model is incomplete in my opinion because it does not take into account the most common use case, which it the stackoverflow example is reading the data, not writing the data. Following the logic here, we impose an over head of an additional query to "simplify" the least common use case. Further, MongoDB has an update in place feature specifically for the "lots of people writing simultaneously" scenario
@eggheaddesign we will have to agree to disagree, I absolutely stand by my words on where the actual transaction boundary exists. NoSQL systems are also generally read orientated, so the "cost" of having to query separately is not something I would choose over the cost of dealing with extremely concurrent data access. Further, that is a concern for optimization reducing the number of queries and there are ways to overcome this dependent on the provider. I can only speak of RavenDB specifically but it is possible to store pseudo-subdocuments that you could retrieve this in a single query.
0

I think that it will be better to create repository for each aggregate rute(only for Question Document)

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Comments

-1

You don't need an Answer's repository. From a domain point of view, you should just add the answer to your Question object. The question repository should do the job, as Question looks like an aggregate root and you should have a repository per aggregate root (not per entity).

You should be careful not to create a Anemic Domain Model.

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1 Comment

-1 this design would cause an immense amount of work. See my answer why this is bad design. Domain Modeling & Persistence Modeling are 2 very different topics, with the latter being extremely geared to the persistence mechanism. Many times the persistence model can be made to accept domain entities but there are times that this would be bad, such as this case.

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