Django ORM question about methods for the QuerySet Object
Question:
Can someone explain to me why you can use a method like count() for a QuerySet Object but something like pop() does not work.
Answers:
The reason is that, unlike a list, a QuerySet is a special type that isn’t evaluated unless you perform any computation on its data. Its purpose is to store query results and can be manipulated using query methods such as .filter(), .order_by(), etc. As opposed to a list of objects that consumes RAM space, a QuerySet won’t do that much. So, it doesn’t support list methods such as remove or pop, etc. that perform manipulation.
The QuerySet is a representation of a database query. It produces a set of model instances from the database.
count() works, because that is a database operation: counting rows matches by the query.
pop() however would require removing an element from that set. But that set exists only in memory, as a representation of a database state. Modifying it in memory would not modify the database – so pop() makes no sense on a QuerySet.
You’d use QuerySet[index] to retrieve a specific element, and then ModelInstance.delete() to remove that instance from the database.
In other words: a QuerySet is a read-only view on database data. It is not a fully mutable list-like collection.
Can someone explain to me why you can use a method like count() for a QuerySet Object but something like pop() does not work.
The reason is that, unlike a list, a QuerySet is a special type that isn’t evaluated unless you perform any computation on its data. Its purpose is to store query results and can be manipulated using query methods such as .filter(), .order_by(), etc. As opposed to a list of objects that consumes RAM space, a QuerySet won’t do that much. So, it doesn’t support list methods such as remove or pop, etc. that perform manipulation.
The QuerySet is a representation of a database query. It produces a set of model instances from the database.
count() works, because that is a database operation: counting rows matches by the query.
pop() however would require removing an element from that set. But that set exists only in memory, as a representation of a database state. Modifying it in memory would not modify the database – so pop() makes no sense on a QuerySet.
You’d use QuerySet[index] to retrieve a specific element, and then ModelInstance.delete() to remove that instance from the database.
In other words: a QuerySet is a read-only view on database data. It is not a fully mutable list-like collection.