Ruby Object Relations: Has-Many-Through

Objectives

Understand Has-Many-Through relationships
Construct indirect relationships between models (Customers, Waiters, and Meals)
Explore the concept of a 'joining' model
Continue to write code using a Single Source of Truth

Introduction

We've seen how objects can be related to one another directly when one object contains a reference to another. This is the "has-many"/"belongs-to" association, and is a direct relationship. For example, an artist may have many songs or a book might have many reviews.

In addition to these one-to-one and one-to-many relationships, some relationships also need something to join them together. For example, you don't need to have a direct relationship with the pilot of a flight you're on. You have a relationship with that flight (you're taking the flight after all), and the pilot has a relationship with the flight (they're flying it). So you have a relationship to that pilot through the flight.

If you take more than one flight, you'll have these kinds of relationships with more than one pilot, all still using your ticket as the middle man. The way we refer to this is that each customer has many pilots through tickets.

Check out some more examples:

A company that offers a network of doctors to their employees through the company's insurance program
A user on a popular media sharing site can have many "likes", that occur through the pictures they post
A Lyft driver that you are connected to through the rides you've taken with them

In this lesson, we'll build out just such a relationship using waiters, customers, and meals. A customer has many meals, and a customer has many waiters through those meals. Similarly, a waiter has many meals and has many customers through meals.

Building Out Our Classes

Let's start by building out the Customer class and Waiter class. We want to make sure when building out classes, that there's something to store each instance. That is to say: the Customer class should know about every customer instance that gets created.

# ./lib/customer.rb
class Customer
  attr_accessor :name, :age

  @@all = []

  def initialize(name, age)
    @name = name
    @age = age
    @@all << self
  end

  def self.all
    @@all
  end

end

As you can see, each customer instance has a name and an age. Their name and age are set upon initialization, and because we created an attribute accessor for both, the customer can change their name or age. If we wanted to limit this ability to read-only, we would create an attribute reader instead. The Customer class also has a class variable that tracks every instance of customer upon creation.

# ./lib/waiter.rb
class Waiter

  attr_accessor :name, :yrs_experience

  @@all = []

  def initialize(name, yrs_experience)
    @name = name
    @yrs_experience = yrs_experience
    @@all << self
  end

  def self.all
    @@all
  end

end

Each instance of the Waiter class has a name and an attribute describing their years of experience. Just like the Customer class, the Waiter class has a class variable that stores every waiter instance upon initialization.

The "Has-Many-Through" Relationship

In real life, as a customer, each time you go out to eat, you have a different meal. Even if you order the same exact thing in the exact same restaurant, it's a different instance of that meal. So it goes without saying that a customer can have many meals.

It's a safe bet to assume that unless you only eat at one very small restaurant, you'll have many different waiters as well. Not all at once of course, because you only have one waiter per meal. So it could be said that your relationship with the waiter is through your meal. The same could be said of the waiter's relationship with each customer.

That's the essence of the has-many-through relationship.

How Does That Work in Code?

Great question! The way we're going to structure this relationship is by setting up our Meal class as a 'joining' model between our Waiter and our Customer classes. And because we're obeying the single source of truth, we're going to tell the Meal class to know all the details of each meal instance. That includes not only the total cost and the tip (which defaults to 0) but also who the customer and waiter were for each meal.

# ./lib/meal.rb
class Meal

  attr_accessor :waiter, :customer, :total, :tip

  @@all = []

  def initialize(waiter, customer, total, tip=0)
    @waiter = waiter
    @customer = customer
    @total = total
    @tip = tip
    @@all << self
  end

  def self.all
    @@all
  end
end

That looks great! And even better, it's going to give both the customer and waiter instances the ability to get all the information about the meal that they need without having to store it themselves. Let's build some methods.

Building on the Relationship

If you take a look at our customer right now, they aren't capable of doing much. Let's change that and give them the ability to create a meal. To do this, they'll need to take in an instance of a waiter and supply the total and tip, which we'll have defaulted to 0 here as well:

# ./lib/customer.rb

  def new_meal(waiter, total, tip=0)
    Meal.new(waiter, self, total, tip)
  end

As you can see, we don't need to take customer in as an argument, because we're passing in self as a reference to the current instance of customer. This method will allow us to create new meals as a customer, and automatically associate each new meal with the customer that created it. We can do the same thing for the Waiter class:

# ./lib/waiter.rb

  def new_meal(customer, total, tip=0)
    Meal.new(self, customer, total, tip)
  end

Notice that the parameters are different for the new_meal method are different for customer and waiter, but the order of arguments for Meal.new() remains the same - a waiter, a customer, a total and a tip. Great! Now we can create waiters, customers and meals to our heart's content.

  sam = Customer.new("Sam", 27)
  pat = Waiter.new("Pat", 2)
  alex = Waiter.new("Alex", 5)

  sam.new_meal(pat, 50, 10) # A Customer creates a Meal, passing in a Waiter instance
  sam.new_meal(alex, 20, 3) # A Customer creates a Meal, passing in a Waiter instance
  pat.new_meal(sam, 30, 5) # A Waiter creates a Meal, passing in a Customer instance

Reminder: If you would like to practice creating these instances, you can load these classes up using IRB. Run irb from this lesson's main directory, then load up each class into the IRB environment by using require_relative:

require_relative './lib/customer.rb'
require_relative './lib/meal.rb'
require_relative './lib/waiter.rb'

Completing the Has-Many-Through Relationship

This is awesome, but it isn't done yet! To complete our goal of establishing a has-many-through relationship, we need a way for our customer and waiter instances to get information about each other. The only way they can get that information is through the meals they've created.

Relating this to real life, we can imagine a situation where a waiter might want to know who their regular customers are and what meals those customers usually order. Or, a customer might want to know the name of the waiter of their last meal so they can leave a good review. To get our waiters and customers this information, we're going to consult the Meal class from the Customer and Waiter classes. Let's start with the Customer class.

In plain English, the customer is going to look at all of the meals, and then select only the ones that belong to them. Translated into code, that could be written like the following:

# ./lib/customer.rb

def meals
  Meal.all.select do |meal|
    meal.customer == self
  end
end

Boom. We're iterating through every instance of Meal and returning only the ones where the meal's customer matches the current customer instance. If a customer, Rachel, wants to know about all of her meals, all we need to do is call the #meals method on the her Customer instance.

alex = Customer.new("Alex", 30)
rachel = Customer.new("Rachel", 27)
dan = Waiter.new("Dan", 3)

rachel.new_meal(dan, 50, 10)
alex.new_meal(dan, 30, 5)

rachel.meals #=> [#<Meal:0x00007fa23f1575a0 @waiter=#<Waiter:0x00007fa23f14fbe8 @name="Dan", @yrs_experience=22>, @customer=#<Customer:0x00007fa240987468 @name="Rachel", @age=27>, @total=50, @tip=10>]
rachel.meals.length #=> 1

Meal.all.length #=> 2

Above, two meals were created, one for rachel and one for alex, both with the same waiter. However, running rachel.meals only returns the meal rachel is associated with.

So rachel.meals will return an array of all of Rachel's meals, but what if we now want a list of all of the waiters that Rachel has interacted with? Each meal is also associated with a waiter, so to get every waiter from every meal Rachel has had, we need to take the array of all of Rachel's meals, map over it, getting the waiter from each of those meals.

Since we already have a #meals method to get an array of meals, we can reuse it here and write a #waiters method like the following:

# ./lib/customer.rb

def waiters
  meals.map do |meal|
    meal.waiter
  end
end

terrance = Customer.new("Terrance", 27)
jason = Waiter.new("Jason", 4)
andrew = Waiter.new("Andrew", 7)
yomi = Waiter.new("Yomi", 10)

terrance.new_meal(jason, 50, 6)
terrance.new_meal(andrew, 60, 8)
terrance.new_meal(yomi, 30, 4)

terrance.waiters #=> [#<Waiter:0x00007fa23f18f860 @name="Jason", @yrs_experience=34>, #<Waiter:0x00007fa23f196818 @name="Andrew", @yrs_experience=27>, #<Waiter:0x00007fa23f19dd20 @name="Yomi", @yrs_experience=20>] 
terrance.waiters.length #=> 3

And to finish out first real-life example, if Terrance wanted to find the name of his last waiter, we can use the #waiters method, then just get the name of the last waiter in the Array.

terrance.waiters.last.name #=> "Yomi"

To reinforce this concept, let's look at the same sort of relationship, but in the other direction. This time, we will build out methods so a waiter can find the customer that tips the the best.

Again to start, just like the customer, the waiter needs a way to get all the meals they have served. We'll create a #meals method again, with a subtle change:

# ./lib/waiter.rb

def meals
  Meal.all.select do |meal|
    meal.waiter == self #checking for waiter now
  end
end

To find the best tipper, we can write another method, #best_tipper, use the array we get from #meals, then return the customer of the meal with the highest tip:

# ./lib/waiter.rb

def best_tipper
  best_tipped_meal = meals.max do |meal_a, meal_b|
    meal_a.tip <=> meal_b.tip
  end

  best_tipped_meal.customer
end

jason = Waiter.new("Jason", 4)
lisa = Customer.new("Lisa", 24)
tim = Customer.new("Tim", 35)
terrance = Customer.new("Terrance", 27)

terrance.new_meal(jason, 50, 3)
lisa.new_meal(jason, 40, 10)
tim.new_meal(jason, 45, 8)

jason.best_tipper #=> #<Customer:0x00007f80829959a8 @name="Lisa", @age=24>
jason.best_tipper.name #=> "Lisa"

And there you have it - customers have access to waiters, and waiters have access to customers. Notice as well that neither the Customer class, nor the Waiter class needed additional attributes - they don't need to keep track of this information; they only need to have the methods that ask the write questions - in this case to the Meal class, the join between customer and waiter.

Conclusion

Why associate customers to waiter objects through meals? By associating meals to waiters, we are not only reflecting the real-world situation that our program is meant to model, but we are also creating clean and re-usable code. Each class only knows about what they specifically need to know about, and we create a single source of truth by keeping our information central in our relationship model.

Further Practice

Below you'll find all the code for the Customer class, including a few new methods. Think about expanding on the Customer and Waiter classes and about what other methods might be possible using the has-many-through relationship. For starters, try some of the following:

A waiter's most frequent customer
The meal of a waiter's worst tipping customer
The average tips for the most experienced waiter and the average tips for the least experienced waiter

class Customer
  attr_accessor :name, :age

  @@all = []

  def initialize(name, age)
    @name = name
    @age = age
    @@all << self
  end

  def self.all
    @@all
  end

  def meals
    Meal.all.select do |meal|
      meal.customer == self
    end
  end

  def waiters
    meals.map do |meal|
      meal.waiter
    end
  end

  def new_meal(waiter, total, tip=0)
    Meal.new(waiter, self, total, tip)
  end

  def new_meal_20_percent(waiter, total)
    tip = total * 0.2
    Meal.new(waiter, self, total, tip)
  end

  def self.oldest_customer
    oldest_age = 0
    oldest_customer = nil
    self.all.each do |customer|
      if customer.age > oldest_age
        oldest_age = customer.age
        oldest_customer = customer
      end
    end
    oldest_customer
  end

end

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