A simple guide to understanding object-oriented ruby relationships (finally!)

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As a beginner, I have had the hardest time wrapping my head around the concept of object-oriented relationships in code. After weeks (or try months) of reading explainers and watching tutorial videos, I’m beginning to see the light.

Here, I’ll distill examples of belongs to, has many, and has many through relationships down to bare bones, which has helped me build comprehension. And hopefully, it’s useful to you too. Let me know?

Belongs to relationship

The belongs to relationship means that ONE object (a song, a book, a dog, etc.) has an owner (an artist, an author, a shelter, etc).

We’re able to show that in code by creating an attr_accessor for the owner.

class Dog
 attr_accessor :shelter
end
 
class Shelter
end

The above code allows us to create a new dog and assign ownership of the dog to a shelter.

Step 1: Create new dog and assign it equal to Bandit variable.

Step 2: Create new shelter

Step 3: Assign bandit’s shelter attribute to your instance of Shelter that you just created

Voila, Bandit is now a instance of the Dog class with a shelter attribute.

bandit = Dog.new 
nycshelter = Shelter.new 
bandit.shelter = nycshelter #assigning the dog
bandit.shelter
bandit
=> #<Dog:0x000055b18e671ca0 @shelter=#<Shelter:0x000055b18e671c78>>

But there’s trouble in paradise. At this point, the instance of Shelter does not recognize or see the dog it owns.

Here’s how we know that:

nycshelter.dogs
=> undefined method `dogs’ for #<Shelter:0x00005557b6fca1f8>
(repl):15:in `<main>’

When we ask nycshelter, an instance of Shelter, what dogs it has many of, it has no clue. Looking at the error, we have a clue, which points to what we need to do next. We need to define a method for dogs for the shelter. But every instance of shelter in general should probably know what dogs it has many of.

Has Many Relationships

Here’s the flip side of the belongs to relationship. A shelter has many dogs or you could say the shelter has a collection of dogs.

To establish a has many relationship, our owner instance needs an empty array to store its collection of items (in this case, dogs), where every instance of the dog that’s instantiated can be added.

class Shelter
 attr_accessor :dogs, :name
  def initialize(name)
    @name = name
    @dogs = []
  end
end

Here, we’re just setting up a new shelter named brooklynshelter:

brooklynshelter = Shelter.new(brooklynshelter)
=> #<Shelter:0x000055a164d8b1d8 @name=nil, @dogs=[]>

We’re going to reset our dog (belongs to) and shelter (has many) relationship.

brooklynshelter = Shelter.new(brooklynshelter)
snacks = Dog.new
snacks.shelter = brooklynshelter
snacks
=> #<Dog:0x00005614828bf708 @shelter=#<Shelter:0x00005614828bf758 @name=nil, @dogs=[]>>

So, we can that snacks now belongs to brooklynshelter, an instance of Shelter, which has a collection (empty array) of dogs.

brooklynshelter.dogs
=> []

When we ask brooklynshelters for its dogs attribute, it’s still showing up as being empty. Because there’s another essential step we’re forgetting: ADDING THE INSTANTIATED DOG INTO THE EMPTY ARRAY. We’re going to do that with an #add_dog method

brooklynshelter = Shelter.new(brooklynshelter)
snacks = Dog.new
snacks.shelter = brooklynshelter
brooklynshelter.add_to_shelter(snacks)
brooklynshelter
=> #<Shelter:0x0000563fc0797240 @name=nil, @dogs=[#<Dog:0x0000563fc07971f0 @shelter=#<Shelter:0x0000563fc0797240 …>>]>

Now, the flip side of the relationship — the has many relationship — is complete. The instance of Shelter — brooklynshelter — knows it has a dog instance whose shelter attribute is equal to the shelter.

Has Many Through Relationship

One way I get my brain to comprehend complicated Ruby concepts is by breaking them down into simple rules or its smallest components. I’m going to try that, here, with the has many relationship.

The has many through relationship describes the connection between 3 classes, wherein 1 of 3 classes provides an indirect link (or relationship) between the other 2.

• One item will be the middle man and provides an indirect relationship between or through two objects
• The shared item will belong to the other two items
• The other two items will have many of the shared item

For example:
→shelter → dogs→ owner

In this relationship:
* shelter has many dogs
* dog belongs to shelter and owner
* owners has many dogs
* through dogs, shelter has many owners and owners has many shelters

class Shelter #has many dogs
 attr_accessor :name, :city
 @@all = []
def initialize (name, city)
 @name = name
 @city = city
 @dogs = [] 
 @@all << self
 end
def add_dog(dog) #always should have an argument so we can pass in an instance of dog
 dog.shelter = self
 @dogs << dog
 end
def dogs
 @dogs
 end
def self.all
 @all
 end
end

class Dog #belong to a shelter & an owner
 attr_accessor :name, :age, :breed, :shelter, :owner
 @@all = []
def initialize (name, age, breed)
 @name = name
 @age = age
 @breed = breed
 @@all << self
 end
def self.all
 @all
 end
end

class Owner #has many dogs 
 attr_accessor :name, :age, :dogs
 @@all = []
def initialize (name, age)
 @name = name
 @age = age
 @dogs = []
 @shelters = []
 @@all << self
 end
def add_dog(dog)
 @dogs << dog
 dog.owner = self
 end
def shelters
 self.dogs.each do |dog| 
 @shelters << dog.shelter if dog.shelter
 end
 @shelters
 end
 
 def self.all
 @@all
 end
end

With all this, we can invoke:

ruffles = Dog.new(“ruffles”, 1, “corgi”)
queensshelter = Shelter.new(“Queens Humane Society”, “NYC”)
ruffles.shelter = queensshelter
ruffles.shelter
queensshelter.add_dog(ruffles)
joe = Owner.new(“joe”, 30)
joe.add_dog(ruffles)
joe.shelters
joe.shelters.each {|shelter| puts shelter.name}
=> Queens Humane Society
=> [#<Shelter:0x00005612a3437d38 @name=”Queens Humane Society”, @city=”NYC”, @dogs=[#<Dog:0x00005612a3437db0 @name=”ruffles”, @age=1, @breed=”corgi”, @shelter=#<Shelter:0x00005612a3437d38 …>, @owner=#<Owner:0x00005612a3437cc0 @name=”joe”, @age=30, @dogs=[#<Dog:0x00005612a3437db0 …>], @shelters=[…]>>]>, #<Shelter:0x00005612a3437d38 @name=”Queens Humane Society”, @city=”NYC”, @dogs=[#<Dog:0x00005612a3437db0 @name=”ruffles”, @age=1, @breed=”corgi”, @shelter=#<Shelter:0x00005612a3437d38 …>, @owner=#<Owner:0x00005612a3437cc0 @name=”joe”, @age=30, @dogs=[#<Dog:0x00005612a3437db0 …>], @shelters=[…]>>]>]