Use exists() instead of count() in populate_history command

[fabiocapsouza]

Problem Statement
The populate_history commands take a long time to run when history tables have many rows, mainly because of the count() query that checks if the history already exists:

django-simple-history/simple_history/management/commands/populate_history.py

Line 159 in 0f5e159

if history_model.objects.count():

Describe the solution you'd like
I am wondering if the count() query could be changed to exists() instead. Since the count is only used to check if it returns a truthy value, I believe exists would be a better fit and faster to run. If that is ok, I am happy to send a PR.

Describe alternatives you've considered

Additional context

[jeking3]

Sounds reasonable to me. Is there an existing test that covers this code path?

[fabiocapsouza]

I believe so. There is a PopulateHistory TestCase

django-simple-history/simple_history/tests/tests/test_commands.py

Line 37 in 5dfb2f3

class TestPopulateHistory(TestCase):

with a specific test with existing objects:

django-simple-history/simple_history/tests/tests/test_commands.py

Line 120 in 5dfb2f3

def test_existing_objects(self):

[jeking3]

Feel free to open a pull request to make this change.

[Greg-Hamel]

This can probably be closed, it has been released in 3.2.0 through the aforementioned MR.

[ddabble]

I agree, closing 🙂