How to model a Many to many-relationship in code?

Question

Suppose I have 2 tables in a database. eg: Dog & Boss This is a many to many relationship, cause a boss can have more than 1 dog, and a dog can have more than 1 owner. I am the owner of Bobby, but so is my wife.

But many to many is not allowed, so there is a helpertable: DogsPerBoss

How to model this in code?

Class Boss can have a collection of Dogs. Class Dog can have a collection of Bosses. --> at least, that is what I think. Perhaps there are better solutions?

How about extra data that is in the helper-table? Should that be in de Boss-class or in the Dog-class? eg: Nickname (I call the dog "good boy" and my wife calls him "doggie")

I hope my question is kinda clear? Are there any best-practices on what is the best way to achieve this? Can you give me some references?

An ORM (like NHibernate) is not an option.

Answer 1

Why are you talking about tables? Are you creating an object model or a database model?

For an object model, there's no reason a Dog can't have a List<Owner> and an owner have a List<Dog>. Only if you have attributes on the relationship do you need an intermediate class (what UML calls an Association Class). That's when you'd have a DogOwnership class with extra properties, and each Owner would have a List<DogOwnership>, and so would each Dog. The DogOwner would have a Dog, an Owner, and the extra properties.

Answer 2

public class Boss {    private string name;    private List<Hashtable> dogs;    private int limit;     public Boss(string name, int dogLimit)    {       this.name = name;       this.dogs = new List<Hashtable>();       this.limit = dogLimit;     }     public string Name { get { return this.name; } }     public void AddDog(string nickname, Dog dog)    {       if (!this.dogs.Contains(nickname) && !this.dogs.Count == limit)       {          this.dogs.Add(nickname, dog);          dog.AddBoss(this);       }     }     public void RemoveDog(string nickname)    {        this.dogs.Remove(nickname);        dog.RemoveBoss(this);    }     public void Hashtable Dogs { get { return this.dogs; } } }  public class Dog {    private string name;    private List<Boss> bosses;     public Dog(string name)    {       this.name = name;       this.bosses = new List<Boss>();    }     public string Name { get { return this.name; } }     public void AddBoss(Boss boss)    {       if (!this.bosses.Contains(boss))       {           this.bosses.Add(boss);       }    }     public void RemoveBoss(Boss boss)    {       this.bosses.Remove(boss);    }       public ReadOnlyCollection<Boss> Bosses { get { return new ReadOnlyCollection<Boss>(this.bosses); } } } 

The above maintains the relationship of Bosses can have multiple dogs (with a limit applied) and dogs having multiple bosses. It also means that when a boss is adding a dog, they can specify a nickname for the dog which is unique to that boss only. Which means other bosses can add the same dog, but with different nicknames.

As for the limit, I would probably have this as an App.Config value which you just read in before instantiating the boss object(s). So a small example would be:

var james = new Boss("James", ConfigurationManager.AppSettings["DogsPerBoss"]); var joe = new Boss("Joe", ConfigurationManager.AppSettings["DogsPerBoss"]);  var benji = new Dog("Benji"); var pooch = new Dog("Pooch");  james.AddDog("Good boy", benji); joe.AddDog("Doggy", benji);  james.AddDog("Rover", pooch); joe.AddDog("Buddy", pooch);  // won't add as the preset limit has been reached. 

You can obviously tweak this as you see fit, however, I think the fundamentals of what you are looking for are there.

• Boss can have multiple dogs with limit
• Dogs can have multiple bosses
• Bosses can have different nicknames for same dog.