How to update unittests when migrating from sqlite to postgres (django)

Question

I have a somewhat large Django project with many views/models. I recently migrated my project from sqlite3 to postgres locally, and plan to scale it out even further (put postgres on a separate machine, etc).

I followed these instructions when I migrated from sqlite to postgres, and it appears to have worked perfectly. (ie, my running app looks identical to when the db was sqlite)

My issue is this: When I run my previously-written unittests, the first unittest works and all proceeding unittests fail. Individually, the unittests work fine. I've seen some other posts on stackoverflow that addressed this issue, but the solutions were so unclear. How can I rework my setUp()/ teardown() methods for my unittests so that they will pass with my newly migrated postgres db? Do i need to completely rewrite all unittests?

I've seen the pytest-postgresql library, although I'm not entirely sure how to modify my unittests based off of this.

My testing suite is set up with different classes that test views. So for example,

class View1Tests(TestCase):
    def setUp(self):
        c1 = Category.objects.create(id=55555, leaf_node_name="Test Category 1")
        c2 = Category.objects.create(id=12345, leaf_node_name="Test Category 2")

        s1 = Search.objects.create(category=c1, username="testuser")
        s2 = Search.objects.create(category=c2, username="testuser2")


    def test_view1_success(self):
         #blablabla

    def test_view1_fail(self):
         #blablabla

    def test_view1_something(self):
         #blablabla

I'm getting errors like this:

appname.models.DoesNotExist: Search matching query does not exist.

Again, all of these unittests run perfectly fine when sqlite3 was the db. I think its an issue with the postgres testing settings? But I'm at a loss as to where to even begin. Any help would be appreciated!!

Answer 1

I think I finally figured out the problem. The issue arises when trying to fetch an object by the default primary key (e.g. MyModel.objects.get(pk=1), MyModel.objects.get(id=1)). My assumption is that Postgres uses a serial data type as the AutoField and the value of the increment depends on the sequence generated by Postgres.

The solution I found is really simple. Either fetch by an attribute that has a unique property for that object, or fetch all objects and filter using list indexing. The latter method shouldn't be too cumbersome since in most cases, a few instances of the model are created.

Here's a full example

# models.py
class Book(models.Model):
    isbn = models.CharField(
        max_length=13,
        primary_key=True  # uniqueness is enforced
    )
    book_name = models.CharField(
        max_length=128,
        unique=True  # another unique field
    )
    author_count = models.IntegerField()
    book_genre = models.CharField(max_length=64)


# tests.py
class MyTest(TestCase):
    @classmethod
    def setUpTestData(cls):
        # first object
        Book.objects.create(
            isbn='10101',
            book_name='Binary Code',
            author_count=1,
            book_genre='Fiction'
        )

        # second object
        Book.objects.create(
            isbn='314159',
            book_name='Life of Pi',
            author_count=1,
            book_genre='Philosophy'
        )

    def setUp(self):
        self.book1 = Book.objects.all()[0]
        # or self.book1 = Book.objects.get(isbn='10101')
        # or self.book1 = Book.objects.get(book_name='Binary Code')
        self.book2 = Book.objects.all()[1]
        # or self.book2 = Book.objects.get(isbn='314159')
        # or self.book2 = Book.objects.get(book_name='Life of Pi')

    def test_something(self):
        # lastly, since `setUp` is called before every test_* method
        # you can just access the instance's attributes without calling `get`
        # e.g:
        self.assertEqual(self.book1.book_genre, 'Fiction')

    def test_something_else(self):
        # you an also reload the instance using `refresh_from_db`
        # after making changes
        Book.objects.filter(author_count=1).update(book_genre='Fiction')
        self.book2.refresh_from_db()
        self.assertEqual(self.book2.book_genre, 'Fiction')