Using Attributes to Override Data Model Conventions

class User < ActiveRecord::Base
  before_save   :compute_updated_at
  before_create :compute_created_at, :compute_updated_at

  def created_at
    read_attribute(:created_at)
  end

  def created_at=(value)
    compute_created_at
  end

  def updated_at
    read_attribute(:updated_at)
  end

  def updated_at=(value)
    compute_updated_at
  end

  private

  def compute_updated_at
    write_attribute(:updated_at, Time.now + 1.month)
  end

  def compute_created_at
    write_attribute(:created_at, Time.now + 2.month)
  end
end

class A:
        x = 111

class B1(A):
        x = 123  # Shadows A.x

assert B1.x == 123

del B1.x            # But after removing B1's own x attribute ...
assert B1.x == 111  # ... B1.x is just an alias to A.x !


# Shadowing can happen at any time:

class B2(A):
        pass

assert B2.x == A.x == 111

B2.x = 5  # shadowing attributes can also be added after the class definition

assert B2.x == 5
assert A.x == 111

del B2.x
assert B2.x == A.x == 111

class C:
        x = 555
        def x(self):
                print('I am x')

C().x()  # outputs "I am x"

del C.x
print(C.x) # AttributeError: 'C' object has no attribute 'x'

a = A()
assert a.x == 111  # instance namespace includes class namespace

a.x = 1000
assert a.x == 1000
assert A.x == 111  # class attribute unchanged

del a.x
assert a.x == 111  # sees A.x again

class ItemModelForm(forms.ModelForm):
    short_description = forms.CharField(max_length=400, min_length=200, widget=TextareaWidget())

    class Meta:
        model = Item
        fields = ['name', 'short_description', 'description']
        widgets = {
            'name': TextInputWidget(attrs={'placeholder': 'Name*'}),
        }