Django tests: matching query does not exist

I have been working on a blog app that has three regular models: Post, Category, and Comment. And now I am trying to write some tests for these models. When I run tests individually they pass without errors. But when I run tests for the test_models.py as a whole I get four errors telling me that Comment/Category matching query does not exist. I understand that this error means that django can't find the id I want in the database that's created when running the tests. My question is why am I getting these errors only when I run tests on the test_model file but not when I run the test on the test classes individually?

python manage.py test blog.tests.test_models.TestPostModel -v 2 # this command doesn't cause errors
python manage.py test blog.tests.test_models -v 2  # but this one causes 4 errors in TestPostModel

test_models.py

from blog.models import Post, Category, Comment, Recipient
from django.test import TestCase

class TestPostModel(TestCase):
@classmethod
def setUpTestData(cls):
    Post.objects.create(title="creation of the first post", body="i am the body of the first post.")

    for i in range(1, 11):
        Category.objects.create(name=f"post_category{i}")

    for i in range(1,11):
        Comment.objects.create(name=f"user{i}", email=f"user{i}@email.com", comment=f"i am comment number {i}.") 

def test_post_str_method(self):
    post = Post.objects.get(id=1) 
    self.assertTrue(isinstance(post, Post)) 
    self.assertEqual(post.__str__(), post.slug) 

def test_post_get_absolute_url(self):
    post = Post.objects.get(id=1) 
    first_post_url = f"/blog/{post.slug}" 
    self.assertEqual(first_post_url, post.get_absolute_url())  

def test_adding_categories_to_post(self):
    post = Post.objects.get(id=1) 
    for i in range(1, 11):
        post.categories.add(Category.objects.get(id=i)) # Error 1: Category matching query does not exist.
    self.assertEqual(post.categories.count(), 10)

def test_deletting_categories_from_post(self):
    post = Post.objects.get(id=1)
    for i in range(1, 11):
        post.categories.add(Category.objects.get(id=i)) # Error 2: Category matching query does not exist.

    for i in range(1, 6):
        Category.objects.filter(id=i).delete()
    self.assertEqual(post.categories.count(), 5) 

def test_adding_comments_to_post(self):
    post = Post.objects.get(id=1)
    comment =""
    for i in range(1,11):
        comment = Comment.objects.get(id=i) # Error 3: Comment matching query does not exist.
        comment.post = post
        comment.save()

    comments = Comment.objects.filter(post=post)
    self.assertEqual(comments.count(), 10)  

def test_deletting_comments_from_post(self):
    post = Post.objects.get(id=1)
    comment =""
    for i in range(1,11):
        comment = Comment.objects.get(id=i)  # Error 4: Comment matching query does not exist
        comment.post = post
        comment.save()

    Comment.objects.filter(id__in=[1, 2, 3]).delete()
    self.assertEqual(Comment.objects.count(), 7) 

class TestCategoryModel(TestCase):
@classmethod
def setUpTestData(cls):
    Category.objects.create(name="first_category")

def test_category_str_method(self):
    category = Category.objects.get(id=1)
    self.assertTrue(isinstance(category, Category)) 
    self.assertEqual(category.__str__(), category.name) 

def test_category_get_absolute_url(self):
    category = Category.objects.get(id=1)
    first_category_url = f"/blog/hashtag/{category.name}" 
    self.assertEqual(first_category_url, category.get_absolute_url())      

class TestCommentModel(TestCase):
@classmethod
def setUpTestData(cls):
    Comment.objects.create(name=f"user", email=f"user@email.com", comment=f"i am a comment.")

def test_comment_str_method(self):
    comment = Comment.objects.get(id=1)
    self.assertEqual(comment.__str__(), comment.comment) 

Thanks a lot.