One to many relation in pharo - pharo

Question

how can we implement associations between classes in pharo. I tried this code to create a new classe. but, it doesn't work...
Object subclass: #SlotExampleMovie
slots: {
#name.
#year.
#director => ToOneRelationSlot opposite: #directedMovies class: #SlotExamplePerson.
#actors => ToManyRelationSlot opposite: #actedInMovies class: #SlotExamplePerson. }
classVariables: { }
category: 'SlotAssociations-Tests-Example'

Answer 1

I don't know where you got the selector opposite:class: from... The selector you're looking for is named inverse:inClass:. Your code would then look like this:
Object subclass: #SlotExampleMovie
slots: {
#name.
#year.
#director => ToOneRelationSlot inverse: #directedMovies inClass: #SlotExamplePerson.
#actors => ToManyRelationSlot inverse: #actedInMovies inClass: #SlotExamplePerson. }
classVariables: { }
category: 'SlotAssociations-Tests-Example'

Answer 2

I believe you do not need slots for that. You can just use instance variables.
Suppose you want to model the relationship between a book (an instance of class Book) and its authors (instances of class Person).
Then, you declare an "instance variable" called authors in class Book. That variable will hold a collection (a instance of some collection class), that in turn will hold the authors (instances of class Person)
Object subclass: #Book
instanceVariableNames: 'authors'
classVariableNames: ''
category: 'examples'
Then, you need to create (first of all) the collection. You do that in the initialize method of your Book class (this is the first message an object receives when it is created).
>> initialize
authors := Set new.
I used a set, but there are other collection objects.
Then you need a method in that class to tell a book when it has a new author:
>> addAuthor: someObject
authors add: someObject
Then, you can just use it, like this:
a := Book new.
p := Person new.
a addAuthor: p.
Because Pharo is dynamically typed (you do not need to explicitly type variables), the parameter to author: could be any object. With documentation and good variable names you can tell it should be a Person (in fact, the parameter should be better called aPerson).
Summary: The Book has a collection in its instance variable "authors". When you tell the book to add a new author (with the addAuthor: message ), it tells the collection to hold the object (with the add: message).