[Answered ]-Does Model.update method in django locks the table before saving the instances?

by

1đź‘Ť

Short Answer:

No, it doesn’t.

The only thing Django does in update process (whether you use F expression or not) is keeping the previous state of your record(s) in case if something goes wrong it can rollback to the previous state.

def update(self, **kwargs):
    """
    Update all elements in the current QuerySet, setting all the given
    fields to the appropriate values.
    """
    self._not_support_combined_queries('update')
    assert not self.query.is_sliced, \
        "Cannot update a query once a slice has been taken."
    self._for_write = True
    query = self.query.chain(sql.UpdateQuery)
    query.add_update_values(kwargs)
    # Clear any annotations so that they won't be present in subqueries.
    query.annotations = {}
    with transaction.mark_for_rollback_on_error(using=self.db):
        rows = query.get_compiler(self.db).execute_sql(CURSOR)
    self._result_cache = None
    return rows

Basically in the line with transaction.mark_for_rollback_on_error(using=self.db), it keeps the previous state of your record, but it does not lock your table or any kind of partial locks.

For example if you have two simultaneous updates at the same time, (suppose one of them is going to take much longer than the other and also slower one hits your database before faster one) then the faster one is going to hit your database regardless of the slower one and does the operation. Then slower one is going to do some other operation on your table (this example is enough for proving that update does not lock your table).

Also note that calling update for updating multiple objects (if this is a doable thing) is the most efficient way for updating multiple objects as far as I know (comparing to calling save on each instance or bulk update).

👤Roham

Leave a comment