(test): add 3 simple tests covering core usage

- all 3 are taken straight from the README as well, and cover
  whitelists, one-to-one relations, and foreign key relations
  - should add more tests for larger sets of relations, but these do
    cover the vast majority of functionality and most common use cases
    (that are even in the README)

(env): gitignore pytest_cache directory

.gitignore

@@ -1,6 +1,7 @@
 # we use multiple versions of Python and deps, so lockfile doesn't quite fit
 # may add it back in later once Poetry usage stabilizes
 poetry.lock
+.pytest_cache/
 
 # Created by https://www.gitignore.io/api/python
 

tests/test_project/test_app/tests.py

@@ -0,0 +1,57 @@
+import pytest
+from .models import User, Settings, Post
+
+
+@pytest.mark.django_db
+def test_whitelist():
+    new_user = User.objects.create(
+        name='John Doe',
+        email='[email protected]',
+    )
+
+    assert new_user.serialize() == {'name': 'John Doe'}
+
+
+@pytest.mark.django_db
+def test_one_to_one():
+    new_user = User.objects.create(
+        name='John Doe',
+        email='[email protected]',
+    )
+    Settings.objects.create(user=new_user)
+
+    new_user_refreshed = (User.objects.select_related('settings')
+        .get(pk=new_user.pk))
+
+    assert new_user_refreshed.serialize() == {'name': 'John Doe'}
+    # recursively serialize Settings object by passing the join in
+    assert new_user_refreshed.serialize('settings') == \
+        {'name': 'John Doe', 'settings': {'email_notifications': False}}
+
+
+@pytest.mark.django_db
+def test_foreign_key():
+    new_user = User.objects.create(
+        name='John Doe',
+        email='[email protected]',
+    )
+    Post.objects.create(user=new_user, text='wat a nice post')
+    Post.objects.create(user=new_user, text='another nice post')
+
+    serialized_posts = [
+        {'id': 1, 'text': 'wat a nice post', 'user_id': 1},
+        {'id': 2, 'text': 'another nice post', 'user_id': 1}
+    ]
+
+    # called on QuerySet
+    # adds an _id to the foreign key name, just like when using `.values()`
+    assert Post.objects.all().serialize() == serialized_posts
+
+    # called on Manager
+    assert User.objects.get(pk=new_user.pk).post_set.serialize() == \
+        serialized_posts
+
+    # recursively serialize Post objects by passing the join in
+    assert (User.objects.prefetch_related('post_set').get(pk=new_user.pk)
+        .serialize('post_set')) == \
+        {'name': 'John Doe', 'post_set': serialized_posts}