Django小技巧16: 数据库访问优化

post.author_id

post.author.id

user.groups.add(administrators, managers)

user.groups.add(administrators)
user.groups.add(managers)

users = User.objects.all()
users.count()

# Or in template...
{{ users.count }}

users = User.objects.all()
len(users)

# Or in template...
{{ users|length }}

groups = Group.objects.all()
if groups.exists():
    # Do something...

groups = Group.objects.all()
if groups:
    # Do something...

review = Review.objects.select_related('author').first()  # Select the Review and the Author in a single query
name = review.author.first_name

review = Review.objects.first()  # Select the Review
name = review.author.first_name  # Additional query to select the Author

# views.py
# If you don't need the model instance, go for:
invoices = Invoice.objects.values('number', 'date', 'value')  # Returns a dict

# If you still need to access some instance methods, go for:
invoices = Invoice.objects.only('number', 'date', 'value')  # Returns a queryset

# invoices.html
<table>
  {% for invoice in invoices %}
    <tr>
      <td>{{ invoice.number }}</td>
      <td>{{ invoice.date }}</td>
      <td>{{ invoice.value }}</td>
    </tr>
  {% endfor %}
</table>

# views.py
invoices = Invoice.objects.all()

# invoices.html
<table>
  {% for invoice in invoices %}
    <tr>
      <td>{{ invoice.number }}</td>
      <td>{{ invoice.date }}</td>
      <td>{{ invoice.value }}</td>
    </tr>
  {% endfor %}
</table>

from django.db.models import F

Product.objects.update(price=F('price') * 1.2)

products = Product.objects.all()
for product in products:
    product.price *= 1.2
    product.save()