DI example

a.rb

# Without DI:
# It knows what kind of datastore it has
# This way you just mixed URL, HTTP methods and stuff here
class Person
  API_URL = "#{ROOT_API_URL}/api/v2/people"
  
  def save
    return insert if new?
    update
  end
  
  def insert
    @id = datastore.post(attributes)
  end
  
  def update
    datastore.put(id, attributes)
  end
  
  def datastore
    @_datastore ||= RestDataStore.new(API_URL)
  end
end

b.rb

# With DI:
# It doesn't give a shit about what kind of store it has
# Bonus: you can use the same Person class to handle objects from different stores:
#  - database
#  - REST API
#  - xml file
#  etc.
class Person
  def initialize(datastore)
    @datastore = datastore
  end
  
  def save
    return insert if new?
    update
  end
  
  def insert
    @id = datastore.insert(attributes)
  end
  
  def update
    datastore.update(id, attributes)
  end
  
  private
  
  attr_reader :datastore
end

c.rb

# Adding DI not through `initialize`
class Person
  def initialize(lots_of_attributes)
    # ...
  end
  
  def with_datastore(datastore)
    @datastore = datastore
    self
  end
  
  def datastore
    fail "No datastore injected :(" unless @datastore
    @datastore
  end
  
  def save
    # ...
  end
  
  # ...
end