MySQL ticket 37735 describes a situation where queries can return unexpected results. If a single connection object is used to create a table, populate that table, then query the rows in the table, it is possible for the query to return unexpected results. In turn, this can cause a problem for Django test cases, as test tables are created, populated and queried using the same connection.
To demonstrate the problem, create a set of tables and test cases that follows the example in the MySQL ticket:
- An Article and a Channel model, with each Article having an m2m relationship with Channel.
- 1 Channel object
- 9 Article objects (with primary keys 1-9), each with the single channel in their m2m table.
Then run:
>>> Article.objects.exclude(pk=9).values_list('id',flat=True)
[1,2,3,4,5,6,7,8,9]
The article with pk=9 should not be in this return set, but under MySQL, it will be. Postgres and SQLite return the expected result.
It's also important to have lots of data - if you only have 1 or 2 articles, the error doesn't occur. 9 seems to be the minimum.
