Giving database context to object Factory

问题

There is a question I always ask myself when I'm using a Factory pattern inside my code (C#, but it applies to any language I suppose).

I have a "Service" that takes care of interacting with my database, do stuff with objects and interacts with my object model.

This Service uses a Factory sometimes to delegate the instanciation of an object. But this factory obviously needs to interact by itself with the database to instanciate my object properly. Is it a good/bad practice to pass the Database context to the Create method for example?

Like this :

var myNewObject = MyFactory.Create(myDatabaseContext);

the other way would be to let the Service always be the only one to talk with the database.

var myNewObject = MyFactory.Create();
var extraProperty = myDatabaseContext.Get(something);
myNewObject.extraProp = extraProperty;

Any advices?

回答1:

The idea of passing the database context into the factory create method is called method injection. This is a form of dependency injection, so you are on the right track.

You can use dependency injection to manage your database context inside of your factory via the constructor. The factory could look something like this:

public class MyFactory 
{
    private readonly IMyDbContext dbContext;

    public MyFactory(IMyDbContext dbContext)
    {
        this.dbContext = dbContext;
    }

    public object Create()
    {
        // Use the dbContext, etc
    }
}

Constructor injection is usually favored because it leaves method signatures less cluttered. We will also most likely have one type of database context so there will be no need to take advantage of polymorphism based on some other runtime information.

You can choose to use a Dependency Injection Container like Ninject or, my favorite, SimpleInjector to manage the dependencies for you.

It is OK to have the DbContext only used by the factory. One thing you may want to watch out for is that a user of your factory may not realize that the factory is calling to the database. This could be a bad thing and have negative performance implications. Typically, construction information is passed into the factory method, not initialized into the factory method from the DB. You could even take it a step further and use the Repository Pattern to abstract away some more of the data access logic if you think it is necessary and you don't have that already.

To learn more about Dependency Injection, in case you are unfamiliar, you can start here.

My ideal structure may look like this:

public class MyFactory : IFactory
{  
    public object Create(object someProperty)
    {
        // build object
    }
}

public class MyService
{
    private readonly IMyDbContext dbContext;
    private readonly IFactory factory;

    public MyService(IMyDbContext dbContext, IFactory factory)
    {
        this.dbContext = dbContext;
        this.factory = factory;
    }

    public void DoWork()
    {
        var property = dbContext.Get(something);
        var newObj = factory.Create(property);
        // Use stuff
    }
}

回答2:

In the project I am working on, we try to keep all database access inside the Service. If the Factory needs objects that must be loaded from the DB, the Service should load them and pass them to the Factory. If the object returned by the Factory shall be persisted, the Service should add it to the DbContext.

This corresponds to the second way you have shown. The advantage is that the Factory can be unit tested without any need to mock the DbContext.

If you want to keep the DB access inside the Factory anyways, I would inject the DbContext into the constructor of the Factory, instead of passing it to the Create() method.

The Service gets an instance of the Factory injected in turn (instead of accessing static methods of the Factory). Again, this will make mocking much easier.

public class Service {

    private readonly IMyDbContext _myDatabaseContext;
    private readonly IMyFactory _myfactory;

    public Service (IMyDbContext myDbContext, IMyFactory myfactory) {
        _myDatabaseContext = myDbContext;
        _myfactory = myfactory
    }

    public void Create() {
        var extraProperty = myDatabaseContext.Get(something);
        var myNewObject = _myFactory.Create(extraProperty);
        _myDatabaseContext.Add(myNewObject);
        _myDatabaseContext.SaveChanges();
    }
}

来源：https://stackoverflow.com/questions/43120132/giving-database-context-to-object-factory