Values

ROADMAP
Why our brains like object-orientation
Examples of value objects
ActiveRecord tips

RUBY IS AN OBJECT-ORIENTED
PROGRAMMING LANGUAGE

class ShippingContainer
# container stuff
end

class Person
# person stuff
end

class Person
attr_accessor :age
end
person = Person.new
person.age = 0
while true do
sleep 31536000
person.age += 1
end

person_1 = Person.new
person_1.age = 40
person_2 = Person.new
person_2.age = 40

hostage = Person.new
hostage.age = 40
captor = Person.new
captor.age = 40

hostage.age = 40
captor = Person.new
captor.age = 40
hostage == captor
# => false

hostage.age = 40
captor = Person.new
captor.age = 40
hostage.object_id
# => 70122276807860
captor.object_id
# => 70122276712780

string_1 = "ABC"
string_2 = "ABC"

string_1 = "ABC"
string_2 = "ABC"
string_1.object_id
# => 70122276804530
string_2.object_id
# => 70122276104871
string_1 == string_2
# => true

string_1 = "ABC"
string_2 = "ABC"
string_1.==(string_2)
# => true

VALUE OBJECTS
Immutable
Easier to reason about
Easier to test
Easier to share
Enable easier method and class naming
Make systems more robust
Enrich semantics

class Person
attr_accessor :name
end
alice = Person.new
alice.name = "Alice Cranston"

class Person
def take_last_name(last_name)
self.name = first_and_middle_names.push(last_name).join(" ")
end
def first_and_middle_names
name.split(" ")[0..-2]
end
end

NAMING IS HARD
Avoid names which are too common
Avoid infamous names

class Person
# ...
COMMON_NAMES = %w( Ben Eric Jane )
def name_common?
COMMON_NAMES.include?(first_name)
end
def first_name
name.split(" ").first
end
end

class Person
def name_similiar_to?(other_name)
FuzzyMatch.matches?(name, other_name)
end
end

dictators = ["Adolf Hitler", "Genghis Khan", "Josef Stalin"]
person = Person.new
["Eric Jones", "Mingus Kan", "Timion Lowry"].reject do |name|
person.name = name
person.name_common? ||
dictators.any? { |dictator| person.name_similiar_to?(dictator) }
end

class Person
COMMON_NAMES = []
def take_last_name
# ..
end
def name_common?
# ...
end
def name_similiar_to?(other_name)
# ...
end
end

class Name
def initialize(@name)
@name = name
end
def ==(other_name)
name == other_name.name
end
protected
attr_reader :name
end

Name.new("Ben Eddy") == Name.new("Ben Eddy")
# => true

class Name
def initialize(name)
@name = name
end
def merge_with(other_name, patronymic: true); end
def common?; end
def similiar_to?(other_name); end
def mononym?; end
def ==(other_name); end
protected
attr_reader :name
end

class Name
def first_and_middle
# ...
end
def last
# ...
end
def merge_with(other_name)
Name.new [first_and_middle, other_name.last].join(" ")
end
end

husband = Person.new
husband.name = Name.new("Melvin Cartwright")
wife = Person.new
wife.name = Name.new("Alice Cranston")
# marriage
wife.name = wife.name.merge_with(husband.name)

dictators = [
Name.new("Adolf Hitler"),
Name.new("Genghis Khan"),
Name.new("Josef Stalin")
]
[
Name.new("Eric Jones"),
Name.new("Mingus Kan"),
Name.new("Timion Lowry")
].reject do |name|
name.common? || dictators.any?(&name.method(:similar_to?))
end

name.common?
name.similar_to?(other_name)
name.merge_with(other_name)

DRAW ON ESTABLISHED
FORMALISMS

class Payment
attr_reader :card_number
def initialize(card_number)
@card_number = card_number
end
end

class Payment
# ...
def charge
PaymentProcessor.charge(card_number)
end
end

class Payment
# ...
def charge
# Add some card number validations
PaymentProcessor.charge(card_number)
end
end

class CreditCardNumber
def initialize(number)
# ...
end
def issuer
# ...
end
def valid?
# validations
end
end
class Payment
def credit_card_number
CreditCardNumber.new(@card_number)
end
end

4321 5678 9012 1234
4321-5678-9012-1234
4321567890121234

Be conservative in what you do, be liberal
in what you accept from others.

class Payment
CreditCardNumber.new(@card_number.gsub(/D/, ''))
end
end

def self.coerce(number)
return number if number.is_a?(CreditCardNumber)
new(number.to_s.gsub(/D/, ''))
end
end
class Payment
CreditCardNumber.coerce(@card_number)
end
end

COERCE AT THE BOUNDARY
Money.coerce(10)
Money.coerce(10.0)
Money.coerce("10")
Money.coerce("$10.00")
Money.coerce(:vegetables)
# => TypeError

payments = [
Payment.new(CreditCardNumber.new("371449635398431")),
Payment.new(CreditCardNumber.new("4012888888881881"))
]
payments.group_by(&:credit_card_number)

# => {
CreditCardNumber.new("371449635398431") => [payment]
}

def eql?(other)
number == other.number
end
end

# => {
CreditCardNumber.new("4012888888881881") => [payment, payment]
}

payments = [
]
grouped = payments.group_by(&:credit_card_number)
# => {
}
grouped[CreditCardNumber.new("4012888888881881")]
# => nil

def hash
number.hash
end
end

payments = [
]
grouped = payments.group_by(&:credit_card_number)
# => {
}
grouped[CreditCardNumber.new("4012888888881881")]
# => [payment, payment]

RECAP
==
eql?
hash
# optionally
<=>

class User
def active?
status == "invited" || status == "email_confirmed"
end
end

class User
def active?
status.active?
end
def status
Status.new(@status)
end
end

dangerous_symptoms = Symptoms.new(
Symptom.new(:coughing),
Symptom.new(:fever)
)
patients.each do |patient|
if patient.symptoms.match?(dangerous_symptoms)
raise "OUTBREAK DETECTED"
end
end

duration_1 = Duration.new(60)
duration_2 = Duration.from_minutes(1)
duration_1 == duration_2
# => true
duration_1.longer_than?(Duration.new(120))
# => false

query = "SELECT * FROM users"
query += " WHERE last_name = 'Williams'"
query = SqlQuery.new("SELECT * FROM users")
query = query.where(last_name: "Williams")

THE VIRTUES OF VALUES
Make the implicit explicit
Segregate domain generality from specificity
Segregate imperative code from functional code
Easy and safe to share
Simplify transactions
Few dependencies
Easier to test

ACTIVERECORD AND VALUE
OBJECTS

DATABASES CAN ONLY STORE
PRIMITIVES

READING/WRITING
class Payment < ActiveRecord::Base
CreditCardNumber.new(super)
end
def credit_card_number=(number)
super CreditCardNumber.coerce(number).to_s
end
end

READING/WRITING
class Person < ActiveRecord::Base
def address
Address.new({
street: street,
city: city,
state: state
})
end
def address=(attributes)
Address.coerce(attributes).tap do |address|
self.street = address.street
self.city = address.city
self.state = address.state
end
end
end

FORMS
<%= form_for @person do |f| %>
<%= f.fields_for :address, f.object.address do |ff| %>
<%= ff.input :street %>
<%= ff.input :city %>
<%= ff.input :statee %>
<% end %>
<% end %>
{
person: {
address: {
street: "123 Main",
city: "Boulder",
state: "CO"
}
}
}

FORMS
<%= simple_form_for @person do |f| %>
<%= f.input :address, as: :address %>
<% end %>

QUERYING
def self.with_credit_card_number(number)
where(card_number: number)
end
end
number = CreditCardNumber.new("4012888888881881")
Payment.with_credit_card_number(number)
# => TypeError: Cannot visit CreditCardNumber

QUERYING
where(card_number: number.to_s)
end
end

QUERYING
handler = proc do |column, credit_card_number|
column.eq(credit_card_number.to_s)
end
ActiveRecord::PredicateBuilder.register_handler(
CreditCardNumber, handler)
where(card_number: number)
end
end

GETTING STARTED
Wrap primitives
Use internally
Get help
Values
Good

FURTHER READING
Domain Driven Design - Eric Evans
Are We There Yet? - Rich Hickey
Value of Values - Rich Hickey
Boundaries - Gary Bernhardt

Values

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