This is the second blog post in a series explaining the topics of my talk Don’t Rewrite your framework at TropitalRails.
Have you heard of value objects? Today, we’re going to see what they are and how to use them in Rails. First of all, let’s take a look at example below:
class Customer < ApplicationRecord
validates :name, :email, :password, :address_street, :address_city, presence: true
end
In this example, we have two related fields: address_street and address_city. They are related because they describe the same thing: an address. We could compose an address with those fields and form an object without identity only a state. It’s called value object.
To clarify, let’s create a class to represent the address:
class Address
attr_reader :street, :city
def initialize(street, city)
@street = street
@city = city
end
def ==(other)
@street == other.street && @city == other.city
end
end
Now we need to use this class in our Customer model. Active Record provides us with the method composed_of:
class Customer < ApplicationRecord
composed_of :address, mapping: { address_street: :street, address_city: :city }
end
Now we are able to use address like as an object:
customer = Customer.new
customer.address_street = "123 Main St"
customer.address_city = "Anytown"
customer.address
#<Address:0x0000ffffb045c6c0 @city="Anytown", @street="123 Main St">
address = Address.new("123 Main St", "Anytown")
customer = Customer.new
customer.address = address
other_address = Address.new("456 Elm St", "Othertown")
customer.address == other_address
# false
We are also able to use the Address class in queries
Customer.where(address: Address.new("123 Main St", "Anytown"))