Do
post.author_id
Don't
post.author.id
If you have a foreign key named author, Django will automatically store the primary key in the property author_id, while in the author property will be stored a lazy database reference. So if you access the id via the author instance, like this post.author.id, it will cause an additional query.
Do
user.groups.add(administrators, managers)
Don't
user.groups.add(administrators)
user.groups.add(managers)
If you only need the count:
Do
users = User.objects.all()
users.count()
# Or in template...
{{ users.count }}
Don't
users = User.objects.all()
len(users)
# Or in template...
{{ users|length }}
If you only need to know if the queryset is empty:
Do
groups = Group.objects.all()
if groups.exists():
# Do something...
Don't
groups = Group.objects.all()
if groups:
# Do something...
Do
review = Review.objects.select_related('author').first() # Select the Review and the Author in a single query
name = review.author.first_name
Don't
review = Review.objects.first() # Select the Review
name = review.author.first_name # Additional query to select the Author
Let’s say the Invoice model has 50 fields and you want to create a view to display just a summary, with the number, date and value:
Do
# 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>
Don't
# 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>
Do
from django.db.models import F
Product.objects.update(price=F('price') * 1.2)
Don't
products = Product.objects.all()
for product in products:
product.price *= 1.2
product.save()