Ruby on Rails

Some of my tests would not pass if the test database was not cleared after the previous tests.
For instance my application, based on the Michael Hartl's tutorial, gives users the opportunity to create an association between two users.
The web interface is configured to use or not use Ajax.
There are usually two tests, one for Ajax and the other for standard html requests, and both of them check that the association between the same two users user1 and user2 was created by counting the number of user1 relationships, that should be increased by 1. Now user1 could not create twice the same association with user2, unless that association was cleared after the first test. Is that so?
If this is Rails behaviour, which I only guess is such because I could not find documentation about it, I would rely on this for all my future tests.
In particular I would like to test a background job that populates my database by counting that the number of saved records (100) is correct as expected.
If the database is cleared after this test I could re run the test and I would still count 100 records.
If the database is not cleared, the second time I run the test I would count 200 records, so the test would fail.