Fun with NoRM 4

NoRM, c#, mongodb, nosql No Responses »
Jul 262010

This time, a short post on how to model inheritance, which (at least in a class-oriented programming language) is one of the foundations of object-oriented programming.

Let’s take an example with a person, who has a home address that can be either domestic or foreign. Consider this:

public class Person
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
 
    public Address HomeAddress { get; set; }
}
 
public abstract class Address
{
    public abstract string FormatAddress(string separator);
}
 
public class DomesticAddress : Address
{
    public string Street { get; set; }
    public string Number { get; set; }
    public string PostalCode { get; set; }
    public string City { get; set; }
    // and 17 other fields here, according to whatever is standard in your country
 
    public override string FormatAddress(string separator)
    {
        return string.Join(separator, new[] { Street + " " + Number, PostalCode + " " + City });
    }
}
 
public class ForeignAddress : Adress
{
    public string[] AddressLines { get; set; }
 
    public override string FormatAddress(string separator)
    {
        return string.Join(separator, AddressLines);
    }
}

Now, when I create a Person with a DomesticAddress and save it to my local Mongo, it looks like this:

var people = mongo.GetCollection<Person>();
people.Insert(new Person
{ 
    FirstName = "Mogens Heller", 
    LastName = "Grabe",
    HomeAddress = new DomesticAddress 
    { 
        Street = "Torsmark", 
        Number = "4", 
        // etc
    }
});

which is all fine and dandy – and in the db:

> db.Person.findOne();
{
    "FirstName": "Mogens Heller",
    "LastName": "Grabe",
    "HomeAddress": {
        "Street": "Torsmark",
        "Number": "4",
        // etc...
    }
}

which looks pretty good as well. BUT when I try to load the person again by doing this:

var people = mongo.GetCollection<Person>();
var me = people.FindOne();

I get BOOM!!: Norm.MongoException: Could not find the type to instantiate in the document, and Address is an interface or abstract type. Add a MongoDiscriminatedAttribute to the type or base type, or try to work with a concrete type next time.

Why of course! JSON (hence BSON) only specifies objects – even though we consider them to be logical instances of some class, they’re actually not! – they’re just objects!

So, we need to help NoRM a little bit. Actually the exception message says it all: Add a MongoDiscriminated attribute to the abstract base class, like so:

[MongoDiscriminated]
public abstract class Address
{
    public abstract string FormatAddress(string separator);
}

That was easy. Now, if I do a db.People.drop(), followed by my people.Insert(...)-code from before, I get this in the db:

> db.Person.findOne();
{
    "FirstName": "Mogens Heller",
    "LastName": "Grabe",
    "HomeAddress": {
        "__type": "MongoTest.DomesticAddress, MongoTest",
        "Street": "Torsmark",
        "Number": "4",
        // etc...
    }
}

See the __type field that NoRM added to the object? As you can see, it contains the assembly-qualified name of the concrete type that resulted in that particular object, allowing NoRM to deserialize properly when loading from the db.

Now, this actually makes working with inheritance hierarchies and specialization pretty easy – just add [MongoDiscriminated] to a base class, resulting in concrete type information being saved along with objects of any derived type.

Only thing that would be better is if NoRM would issue a warning or an exception when saving something that could not be properly deserialized – this way, one would not easily get away with saving stuff that could not (easily) be retrieved again.

Fun with NoRM 3

NoRM, c#, mongodb, nifty, nosql No Responses »
Jul 222010

Third post in “Fun With NoRM” will be about how “the dynamism” of JavaScript and JSON is bridged into the rigid and statically typed world of C#. The thing is, in principle there’s no way to be certain that a JSON object returned from MongoDB will actually fit into our static object model.

Consider a situation where, for some reason, some of our orders have a field, PlacedBy, containing the name of the person who placed the order. Let’s see how things will go when adding the field and then querying all orders:

> use dbname
> var order = db.Order.findOne();
> order.PlacedBy = "El Duderino";
> db.Order.save(order);
var orders = mongo.GetCollection<Order>();
 
foreach(var order in orders.Find())
{
    Console.WriteLine("Order #{0}", order.Number);
}

- and BOOM ! – Cannot deserialize!: Norm.MongoException: Deserialization failed: type MongoTest.Order does not have a property named PlacedBy

This is actually pretty good, because this way we will never accidentally load a document with un-deserializable properties and save it back, thus truncating the document. But how can we handle this?

Well, NoRM makes it pretty easy: Make your model class inherit Expando, thus effectively becoming a dictionary. E.g. like so:

public class Order : Expando
{
   // ...
}

Now we can do this:

var orders = mongo.GetCollection<Order>();
 
foreach(var order in orders.Find())
{
    Console.WriteLine("Order #{0}", order.Number);
 
    if (order.AllProperties().Any())
    {
        var props = order.AllProperties().Select(p => string.Format("{0}: {1}", p.PropertyName, p.Value));
        Console.WriteLine("\t{0}", string.Join(", ", props.ToArray()));
    }
}

- which yields:

Order #1
    PlacedBy: El Duderino
Order #2
Order #3

when run with a small DB containing three orders. Nifty, huh?

If you’re sad that you’ve given up your single opportunity to inherit something by deriving from Expando, just go ahead and implement IExpando instead. Then you need to suply a few members, but you can just redirect to an internal Expando in your class.

Next up, a post on how to model inheritance hierarchies… one of my favorites! :)

Fun with NoRM 2

NoRM, c#, mongodb, nosql No Responses »
Jul 192010

This second post in “Fun With NoRM” will be about querying…

How to get everything

Querying collections can be done easily with the anonymous types of C# 3 – e.g. the Order collection from my previous post can be queried for all orders like so:

var orders = mongo.GetCollection<Order>();
 
var allOrders = orders.Find();

How to use criteria

If we’re looking for some particular order, we can query by field values like so:

var orderNumber2 = orders.Find(new { Number = 2 });

or by using the query operators residing in the static class Q:

var ordersWithNumberGreaterThan2 = orders.Find(new { Number = Q.GreaterThan(2) });

More advanced criteria

The query operators can even be combined by combining criteria like so:

var ordersWithNumberBetween5And10 = orders.Find(new { Number = Q.GreaterThan(5).And.LessThan(10) });

Now, what about the nifty dot notation? This an example where C#’s capabilities don’t cut it anymore, as everything on the left side in an anonymous type need to be valid identifiers – so no dots in property names!

This is solved in NoRM by introducing Expando! (not to be confused with ExpandoObject of .NET 4, even though they have similarities…)

Expando is just a dictionary, so to query by the field of an embedded object, do it like so:

var q = new Expando();
q["Items.Name"] = "beer";
var ordersWithBeer = orders.Find(q);

As you can see, querying with NoRM is pretty easy – and I think the NoRM guys have found a pretty decent workaround in the case of dot notation, where C#’s syntax could not be bent further.

Stay tuned for more posts…

Fun with NoRM 1

NoRM, c#, mongodb, nifty, nosql No Responses »
Jul 162010

My previous posts on MongoDB have been pretty un-.NETty, in that I have focused almost entirely on how to work the DB through its JavaScript API. To remedy that, I shall write a few short posts on how to get rolling with MongoDB using NoRM, the coolest C# driver for MongoDB at the moment.

First post will be on how to connect and shove data into MongoDB.

Short introduction to NoRM

NoRM is “No Object-Relational Mapping”. It’s a .NET-driver, that allows you to map objects and their fields and aggregated objects into documents. I like NoRM because it’s successfully preserved that low-friction MongoDB-feeling, bridging C#’s capabilities nicely to those of JavaScript in the best possible way, providing some extra C#-goodies along the way. Please read on, you’ll see…

Connect to MongoDB

Easy – can be done like so:

using(var mongo = Mongo.Create("mongodb://hostname/dbname"))
{
   // go crazy in here!!1
}

Inserting a few documents

Inserting documents with NoRM is easy – just create a class with fields and aggregated objects, and make sure the class either has a property named something like “Id” or has a property decorated with [MongoIdentifier], e.g. like so:

public class Order
{
    public Order()
    {
        Id = ObjectId.NewObjectID();
        Items = new List<Item>();
    }
 
    public ObjectId Id { get; set; }
    public int Number { get; set; }
    public List<Item> Items { get; set; }
}
 
public class Item
{
    public string Name { get; set; }
    public int Amount { get; set; }
}

- and then go ahead and pull a strongly typed collection and insert documents into it:

var orders = mongo.GetCollection<Order>();
 
orders.Insert(new Order {
    Number = 1,
    Items = {
        new Item { Name = "beer" },
        new Item { Name = "nuts" },
    }
});

Now, to make this work I need to create five 200-line XML-files with mapping info etc. </kidding> no, seriously – that’s all it takes to persist an entire aggregate root!!

Pretty cool, eh? That’s what I meant when I said low friction. Stay tuned for more posts, e.g. on how to query a collection…

So you think you're domain-driven?

ddd 2 Responses »
Jun 242010

In a project I am currently involved with, a core part of the system involves a couple of fairly complicated (at least to me :) ) computations involving time, power, energy, fuel, volumes etc.

At first, we just implemented the computations “as specified”, i.e. we went ahead and did stuff like this:

public double GetRemainingCapacity()
{
    double remainingRuntimeHours = remainingRuntimeSeconds / 3600;
    double remainingCapacityMWh = currentProductionKW * remainingRuntimeHours / 1000;
    return remainingCapacityMWh;
}

to calculate how many MHhs a given device can deliver. This is just an example, we have a lot of this stuff going on, and this will be a major extensibility point in the system in the future.

I don’t know about you, but I got a tiny headache every time I looked at code like this, part from trying to understand what was going on, part because I knew errors could hide in there forever.

To remedy my headache (and the other team members’ headaches), we started migrating all that funky double stuff to some types, that do a better job at representing – i.e. Power for power, Energy for energy, and so on!

All of them, of course, as proper immutable value types (even though we use classes for that).

And then we utilized C#’s ability to supply operator overloading on all our domain types, allowing stuff like this to happen:

public Energy GetRemainingCapacity()
{
    return currentProduction * remainingRuntime;
}

where currentProduction is an instance of Power and remainingRuntime is a TimeSpan. And whoever gets the Energy that comes out of this, will never have to doubt whether its Whs, KWhs, or MWhs – it’s just pure energy!

Now, this may seem like a small change, but it has already proven to have huge ramifications for the way we implement our computations:

  1. There is no such thing as multiplying two powers by accident, or subtracting two values that cannot be subtracted and still make sense, etc.
  2. We have no errors due to wrong factors, e.g. like KWs mistakenly being treated as MWs
  3. We have gained a cleaner, more intuitive core API, which is just friggin’ sweet!

In retrospective, I have done so much clumsy work in the past that could have been avoided by introducing proper value types for core domain concepts, like e.g. money, percentages, probabilities, etc.

I usually call myself “pretty domain-driven”, but now I realize that there’s an entire aspect of being just that, that I have overseen.

Do you think you’re domain-driven?

I will be speaking about NoSQL and MongoDB

blablabla, c#, mongodb, nosql 1 Response »
Jun 132010

as seen from the eyes of a .NET developer at two events in June (in Danish).

The first event is a JAOO Geek Night at Dong Energy in Skærbæk on Tuesday June 29th at 4:30 pm. You can read more about the free JAOO Geek Nights here.

The other event is a meeting in Århus .NET User Group, which is the day after, on Wednesday June 30th at 6 pm – you can sign up via Facebook here.

I’m really looking forward to it, because I think we will have some interesting discussions. And perhaps we can widen a few people’s horizons :)

Hope to see a lot of engaged people at both events.

Big brownfield codebase, NDepend, and a Kaizen mind

code quality, ndepend No Responses »
Jun 022010

I have meant to write a post for quite a while now, on how my team and I got up and running with NDepend on a big legacy codebase. So, here goes:

Preface

I am currently employed as a developer on a mortgage bond administration system. The project was started almost 6 years ago when SOA was (apparently!!) incredibly hot, so it’s got a lot of web services which was some architect’s interpretation of service-orientation.

The aforementioned “architect” left the project pretty early though, and after that our team has consisted of 4 to 8 people in various configurations.

This, combined with a couple of hectic deadlines along the way, has led to a big, fragmented codebase, where some areas have been left almost untouched for years, other areas are big balls of mud that everyone currently on the team fears to touch, other areas are characterized by developers having been in a hurry when they wrote the code etc etc.

A few times along the way, the idiomatic way of implementing new stuff has been radically changed to make things better. One example is that instead of orchestrating a bunch of web services RPC style, all new functionality is now being implemented in a single web service, messaging style.

Another thing is that the system is not built with an IoC container in mind, but that did not prevent us from introducing Castle Windsor. That means that services must be pulled from Windsor, service location style – and even though we try to encourage people to reduce their calls to the service locator in only a few well-defined places, some developers did not understand why, and they went ahead and used the locator in all kinds of places.

To put it like we do in Danish when someone owes more than their mortgage is worth: We have technical debt rising above our chimney!

What to do?

When you have so many problems that you don’t know where to begin, how do you solve your problems then?

Well, the Japanese have a word, Kaizen, which is beautifully capturing the concept of constantly improving things.

It’s a philosophy I try to consider all day long, when I write code, modify code, and even when I look at code: I constantly make small changes, remove cruft, and refactor into better more idiomatic ways. How can I do that without breaking code as well? Simple: We have automated tests!

NDepend is a really cool tool that lets us achieve Kaizen on a higher level :)

Quick introduction if you don’t know anything at all about NDepend

In NDepend, you create a project and add all the assemblies and .PDBs from your build. Now NDepend can analyze your assemblies and tell you all kinds of interesting stuff about your code.

E.g. it can show you a dependency matrix, which visualizes dependencies between assemblies and namespaces. That way, you can see if your application adheres to a layered structure, or if dependencies are circular.

But the feature I want to focus on here, is CQL rules. CQL is Code Query Language, which resembles SQL a bit, but is used to query assemblies, types, methods, fields etc. Using CQL, I can also generate warnings if certain conditions are not met.

What makes this extremely interesting in our case, is that we can run our CQL rules from our automated build, via CruiseControl.NET, which integrates nicely with the automatedness (is that a word?) that is required for an agile team.

A few examples

Simple stuff – people not adhering to our naming conventions

I am a firm believer that code must be clean, streamlined, and uniform, in order to reduce the perceived noise when reading it. For some reason, a couple of team members re-installed their ReSharper and got their R# naming rules reset, which resulted in numerous cases where field names were prefixed with _.

To address this annoyance, I created the following CQL warning:

 WARN IF Count > 0 IN 
  SELECT FIELDS FROM ASSEMBLIES 
    "SomeDomainAssembly.Core", 
    "SomeDomainAssembly.CoreServices", 
    "AnotherAssembly.Common", 
    "FourthAssembly.Foundation" 
  WHERE 
    NameLike "^_"

- which from now on will yield a warning and the names and locations of violations in the build report in CruiseControl.NET. Nifty!

More annoying stuff – people using the service locator to instantiate stuff in tests!

Building a pretty complex system with a complex domain can lead to complicated set ups when “unit” testing. E.g. when a payment is recorded in the system, a payment distributor will generate money transactions for interest, principal reduction, and more, in accordance with some particular customer’s strategy. Then, if the SUT needs a couple of payments to have been made, some of our developers took a shortcut in their test set ups, and just pulled a IPaymentDistributor from our service locator, which – unfortunately – is fully configured and functional in our tests.

The real problem is of course that a great portion of our core modules are so strongly coupled that they cannot be tested in isolation.

But given that we have 1 MLOC, it’s not feasible to change the design of our core module at this time. But what we can do, is to make it visible to developers when they pull stuff from the service locator during testing. That can be acieved with the following CQL:

WARN IF Count > 0 IN 
  SELECT TYPES FROM ASSEMBLIES 
    "SomeDomainAssembly.Core.Tests", 
    "SomeDomainAssembly.CoreServices.Tests"
  WHERE 
    IsDirectlyUsing "CoreStuff.ServiceLocator"
Most annoying stuff – people not understanding IoC containers, using the service locator from within services already pulled from a container

This is something, that makes me angry and sad at the same time :) but some team members did not understand Castle Windsor and what IoC containers can do, so they just went on and did calls to ServiceLocator.Resolve<I...> from within services that were already pulled from the container.

There’s too many reasons that this is just icky, so I will just show the CQL that will make sure that this misconception will not live on:

WARN IF Count > 0 IN
  SELECT TYPES FROM ASSEMBLIES
    "SomeDomainAssembly.Core", 
    "SomeDomainAssembly.CoreServices", 
    "AnotherAssembly.Common", 
    "FourthAssembly.Foundation" 
  WHERE 
    HasAttribute "FourthAssembly.AutoRegistration.Attributes.ServiceAttribute"
    AND IsDirectlyUsing "CoreStuff.ServiceLocator"

As you can see, this rule builds on the fact that all our service registrations are made by registering types decorated with the ServiceAttribute.

Conclusion

I have shown a few examples on how we set up automated assertion of certain properties in our architecture. NDepend has already proven to be extremely useful for this kind of stuff, and it allows us to continually add rules whenever we identify problems, which is what we need.

Complexities

blablabla, code quality No Responses »
May 212010

Recently, I had the opportunity to teach an introductory course in programming in C#, including topics on dependency injection, IoC containers, and test-driven development.

When I introduced the concept of dependency injection, and I suggested that this seemingly pretty simple and trivial task be accomplished by putting an IoC container to use, some of the attendees reacted with scepticism and doubt – would this thing, configured with either endless fluent spells or – shudder! – tonnes of XML, not make everything even more complex?

At the time, I tried to convince them that an IoC container was cool, and that learning its ins and outs could be considered some kind of investment that – over time – would actually reduce complexity. This is not something new, and everybody who has ever used an IoC container will probably agree with me that this investment pays off. But still, I think I needed some words that would support my argument better.

So, here goes a few random thought about complexity…

Complexity?

Right now, I want to focus on the kind of complexity that is inherent in non-trivial programs. That is, the complexity can not be removed from the program. The complexity can be anything, really, and it is there in some form or another, and we need to deal with it.

Immediate vs. deferred complexity

Assuming we need to deal with it, how do we do it then? Which strategy do we follow? Well, acknowledging its existence is not the same as dealing with it, but it’s definitely a first step. Thereafter, we should probably consider either A) solving the complexity on a case-to-case ad hoc kind of basis, or B) building some kind of mechanism, that abstracts the identified complexity away, thus dealing with the complexity right now.

Localized vs. dispersed complexity

Definitely coupled to the above somehow, but not inseparable, is the localized vs. dispersed complexity question: When you identify some kind of complexity, do you A) solve the problem in all the little places where the complexity surfaces, or B) build some kind of framework that makes all the other code oblivious of the complexity?

My opinion, and how it relates to IoC containers

I don’t know about you, but I prefer B & B: the abstraction and the framework. It might leak, and it might take some time for other developers to grok if they have never seen that particular problem solved like that before, but if the abstraction holds, it will probably assume some kind of pattern status that will be used, re-used, and recognized from then on.

The force of IoC containers is that they take control over object creation, thus providing hooks around whenever objects are created, and object creation is an incredibly important event in a running program. Having a hook in that proves to be infinitely useful (almost).

The IoC container solves the complexity of deciding when and how to create (which) objects. This would otherwise be a severely dispersed (spatially as well as temporally) complexity, or cross-cutting concern if you will, in that it is a really common thing to instantiate stuff and/or use existing instances of stuff, and the alternative would be to new up objects, maybe pulling from a cache somewhere, all around the place – thus infiltrating our program with our solution to that particular problem.

Conclusion

Use an IoC container. Learn to use its features. Allow the container to take responsibility. This will make you happy.

(seriously, do it! :) )

IoC component registration

castle windsor, ioc, nifty 4 Responses »
May 072010

An interesting topic, that I always love to discuss, is how to find a balance between building a pure domain model and being pragmatic and getting the job done. It just so happens, that getting the job done, and being able to add features to a system quickly, usually conflicts with my inner purist, who really wishes to keep my domain model and services oblivious of everything infrastructure-related.

Usually, I go for pragmatism. Not only because I’m lazy, but also because I think it’s funny to come up with solutions that accelerate development, and generally make the sun shine brighter.

Actually, this post should have been #2 in a series called “Polluting My Domain”, and this post should have been #1, because in #1 I showed how I usually use attributes to give hints to the automapper in Fluent NHibernate – e.g. [Cascade] on a relation to configure NHibernate to cascade operations across that relation, or [Indexed("ix__something")] to make that column be indexed in the database, or [Encrypted] to make that particular property be backed by an encrypting IUserType.

This post, however, will show a pragmatic, elegant and flexible way to make component registration easy in an IoC container.

Most IoC containers that I know of can be configured with XML and through some kind of more or less fluent API in code. I’ll spare you the XML, so I’ll just show a small example on Castle Windsor’s fluent API:

container.Register(Component.For<ISomeService>().ImplementedBy<SomeServiceImplementation>())

or you can perform multiple registrations like this:

container.Register(AllTypes.FromAssemblyContaining<SomeService>()
                                .BasedOn<IService>()
                                .WithService.FromInterface())

This is all fine and dandy, but I think the fluent API becomes pretty complicated when you throw customized registrations per customer and/or environment into the mix – mostly because it will become kind of obscure which services get registered where.

Therefore, one of the first things I have put in my recent projects, have been a registration routine based on attributes. Pretty simple, and yes, it does pollute my domain services with infrastructure-related stuff, but this is a great example where I prefer pragmatism and simplicity over purity.

My most recent project has two attributes, ServiceAttribute and RegisterInAttribute that look something like this:

[AttributeUsage(AttributeTargets.Class, AllowMultiple = true)]
public class ServiceAttribute
{
    readonly Type serviceType;
 
    public ServiceAttribute()
    {
    }
 
    public ServiceAttribute(Type serviceType)
    {
        if (serviceType == null) throw new ArgumentNullException("serviceType");
        this.serviceType = serviceType;
    }
 
    public Type ServiceType
    {
        get { return serviceType; }
    }
}
 
public enum Environment
{
    Development,
    Test,
    Staging,
    Production,
}
 
[AttributeUsage(AttributeTargets.Class, AllowMultiple = true)]
public class RegisterInAttribute
{
    readonly Environment environment;
 
    public RegisterInAttribute(Environment environment)
    {
        this.environment = environment;
    }
 
    public Environment Environment
    {
        get { return environment; }
    }
}

and then the registration code looks like this:

public class ComponentRegistrar
{
	public static void RegisterComponentsFromAssemblyOf<TSomeType>(IWindsorContainer container, 
                                                                       Environment environment)
	{
		var components = typeof(TSomeType).Assembly.GetTypes()
					.Where(t => ShouldBeRegistered(t, environment))
					.SelectMany(t => ToComponentRegistrations(t));
 
		container.Register(components);
	}
 
	static IEnumerable<TAttribute> GetAttributes(ICustomAttributeProvider provider)
	{
		return provider.GetCustomAttributes(typeof(TAttribute), false).Cast<TAttribute>();
	}
 
	static bool ShouldBeRegistered(ICustomAttributeProvider provider, Environment environment)
	{
		var attributes = GetAttributes<RegisterInAttribute>();
 
		return !attributes.Any() || attributes.Any(a => a.Environment == environment)
	}
 
	static IEnumerable<IRegistration> ToComponentRegistrations(Type type)
	{
                var serviceType = a.ServiceType ?? type;
 
		return GetAttributes<ServiceAttribute>(type)
				.Select(a => Component.For(serviceType)
							.ImplementedBy(type)
							.Lifestyle.Transient);
	}
}

So, having established the current value of environment, my component registration will look like this:

var environment = DetermineEnvironmentFromAppSettingsOrSomethingLikeThat();
 
var container = new WindsorContainer();
ComponentRegistrar.RegisterComponentsFromAssemblyOf<HomeController>(container, environment);
ComponentRegistrar.RegisterComponentsFromAssemblyOf<SomeDomainService>(container, environment);
ComponentRegistrar.RegisterComponentsFromAssemblyOf<SomeInfrastructureService>(container, environment);
 
// yay!

- and that’s it! But the best of it is that adding services to the system now becomes a breeze – check this out – registering a concrete type, offering itself as a service:

[Service]
public class HomeController : Controller
{
    // (....)
}

- and here’s registering different stuff depending on the environment:

[Service]
[RegisterIn(Environment.Development)]
public class DebugController : Controller
{
    // (....)
}
 
[Service(typeof(IMailSender))]
[RegisterIn(Environment.Production)]
public class SmtpMailSender : IMailSender
{
    // (...)
}
 
[Service(typeof(IMailSender))]
[RegisterIn(Environment.Staging)]
public class FakeSmtpMailSender : IMailSender
{
    // (...)
}
 
[Service(typeof(IMailSender))]
[RegisterIn(Environment.Development)]
[RegisterIn(Environment.Test)]
public class LoggingMailSender : IMailSender
{
    // (...)
}

This way of registering component has proven to me several times to be a simple and nifty way of managing the differences between environments, and even differences between customers (which would require a few extensions to the example above though), still being able to add services to the system quickly.

Another benefit is that it’s pretty clear what happens, even to developers who might not be that experienced in using IoC containers. If I were the only developer on a project, I would probably prefer component registration based on conventions, but when you have a team, you sometimes need to make some things more explicit.

Even more checking out MongoDB: The coolness continues

mongodb, nifty, nosql No Responses »
Mar 312010

One thing I started to think about after having looked at MongoDB was how to model things that are somehow connected – without the use of foreign keys and the ability to join stuff.

When working with documents, you generally just embed the data where it belongs. But what if I have the following documents:

"songs" collection:
{
    "name": "Factory",
    "artist": "Martha Wainwright"
}
 
{
    "name": "Winter At The Hamptons",
    "artist": "Josh Rouse"
}
 
"users" collection:
{
    "username": "mookid",
    "name": "Mogens Heller Grabe"
}
 
{
    "username": "duderino",
    "name": "The Dude"
}

- and I want to constrain access to the songs, allowing me to see both songs, and The Dude to see only Factory?

My first take was to simply add username in an array inside each song, like so:

"songs" collection:
{
    "name": "Factory",
    "artist": "Martha Wainwright",
    "allowed": ["mookid", "duderino"]
}
 
{
    "name": "Winter At The Hamptons",
    "artist": "Josh Rouse",
    "allowed": ["mookid"]
}

- and this will work well with indexing, which can be done like this:

db.songs.ensureIndex({"allowed": 1})
db.songs.find({"allowed": "mookid"})  // will use the index :)

But here comes the problem: What if each song should be displayed along with the name of who can access that particular song? I need to embed more stuff in the array, e.g. like so:

"songs" collection:
{
    "name": "Factory",
    "artist": "Martha Wainwright",
    "allowed": [{
        "username": "mookid", 
        "name": "Mogens Heller Grabe"
    }, {
        "username": "duderino", 
        "name": "The Dude"
    }]
}
 
{
    "name": "Winter At The Hamptons",
    "artist": "Josh Rouse",
    "allowed": [{
        "username": "mookid", 
        "name": "Mogens Heller Grabe"
    }]
}

There’s a challenge now in keeping this extra “denormalized” piece of information up-to-date in case a user changes his name, etc. – but let’s just assume we’ve handled that.

Now here comes the cool thing: It’s cool that MongoDB can index “into” an array, but it can actually index “into” anything! Just tell it where to go, using the Dot Notation.

That means I can perform the same search as I did above like so:

db.songs.ensureIndex({"allowed.username": 1})
db.songs.find({"allowed.username": "mookid"})  // will use the index :)

How cool is that?? (pretty cool, actually!)

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