When do database queries occur when using QuerySet?

Database queries with QuerySet occur when the QuerySet is evaluated. This happens at the following moments:

• Iterating over the QuerySet.
• Using a slice of the QuerySet (e.g., queryset[0]).
• Calling len() on the QuerySet.
• Calling list() on the QuerySet.
• Calling one of the QuerySet methods that do not return a QuerySet (e.g., first(), last(), count(), exists(), aggregate(), annotate() without further filtering).

# Example of iteration
for obj in MyModel.objects.all():
    pass  # Database query will be executed here

# Example with slice
first_obj = MyModel.objects.all()[0] # Database query will be executed here

# Example of len()
count = len(MyModel.objects.all()) # Database query will be executed here

# Example of list()
all_objects_list = list(MyModel.objects.all()) # Database query will be executed here

# Example of first()
one_obj = MyModel.objects.filter(name='test').first() # Database query will be executed here

# Example of count()
total = MyModel.objects.count() # Database query will be executed here

QuerySets are "lazy". They construct an SQL query but do not execute it until the data is actually needed.