controller.rb

class FooController
  def index
    # The controller is responsible for gathering the things needed
    # to make the response. This should be a single line of code
    @things = Foo.things 
    
    # The controller is responsible for sending the view back to the user.
    # the view uses our instance variable above to render @things
    render :some_view
  end
end

# in an ideal controller those are the only two things that the action does
# 1. gathers the data
# 2. renders the view
#
# ideally each of these things is 1 line of code, but it might end up being
# more to deal with validation errors etc

model.rb

class Foo
  # the model is responsible for databasey type things
  # relationships are part of that
  belongs_to :bar
  has_many :widgets
  
  # so are validations
  validates_presence_of :some_field
  
  # so are scopes
  scope :active, -> { where(state: 'active') }
  
  # often people put extra functionality into the model
  def activate!
    update(state: 'active')
    send_email_to_admin
  end
  # this is good practice when starting out, but your models
  # can become "fat" with this extra functionality.
  # do this at first but you'll learn more about this later
end