docs(add-model): create tips for many-to-many relationships

docs/how-to/add-model-and-api-endpoints.md

@@ -51,6 +51,23 @@ Let's start!
             assert str(recurring_event) == payload["name"]
         ```
 
+        For testing many-to-many relationships, we can add
+
+        ```python title="app/core/tests/test_models.py" linenums="1"
+        def test_project_recurring_event_relationship(project):
+            recurring_event = RecurringEvent.objects.get(name="{Name of Recurring Event}")
+
+            project.recurring_events.add(recurring_event)
+            assert project.recurring_events.count() == 1
+            assert project.recurring_events.contains(recurring_event)
+            assert recurring_event.projects.contains(project)
+
+            project.sdgs.remove(recurring_event)
+            assert project.recurring_events.count() == 0
+            assert not project.recurring_events.contains(recurring_event)
+            assert not recurring_event.projects.contains(project)
+        ```
+
     1. See it fail
 
         ```bash
@@ -99,6 +116,45 @@ class RecurringEvent(AbstractBaseModel):  # (1)!
 1. Try to add the relationships to non-existent models, but comment them out. Another developer will complete them when they go to implement those models.
 1. Always override the `__str__` function to output something more meaningful than the default. It lets us do a quick test of the model by calling `str([model])`. It's also useful for the admin site model list view.
 
+??? note "Updating models.py for many-to-many relationships"
+    For adding many-to-many relationships with additional fields, such as `ended_on`, we can add
+
+    ```python title="app/core/tests/test_models.py" linenums="1"
+    class Project(AbstractBaseModel):
+        ...
+        recurring_events = models.ManyToManyField(
+            "RecurringEvent",
+            related_name="projects",
+            blank=True,
+            through="ProjectRecurringEventXref",
+        )
+        ...
+
+
+    class ProjectRecurringEventXref(AbstractBaseModel):
+        """
+        Joins a recurring event to a project
+        """
+
+        recurring_event_id = models.ForeignKey(RecurringEvent, on_delete=models.CASCADE)
+        project_id = models.ForeignKey(Project, on_delete=models.CASCADE)
+        ended_on = models.DateField("Ended on", null=True, blank=True)
+    ```
+
+    For adding many-to-many relationships without additional fields, we can just add
+
+    ```python title="app/core/tests/test_models.py" linenums="1"
+    class Project(AbstractBaseModel):
+        ...
+        recurring_events = models.ManyToManyField(
+            "RecurringEvent",
+            related_name="projects",
+            blank=True,
+            through="ProjectRecurringEventXref",
+        )
+        ...
+    ```
+
 ### Run migrations
 
 This generates the database migration files
@@ -247,6 +303,67 @@ This is code that serializes objects into strings for the API endpoints, and des
 
 In `app/core/api/serializers.py`
 
+??? note "Updating serializers.py for many-to-many relationships"
+    Following the many-to-many relationship between project and recurring event from above,
+
+    Update the existing serializer classes
+
+    ```python title="app/core/api/serializers.py" linenums="1"
+    class ProjectSerializer(serializers.ModelSerializer):
+        """Used to retrieve project info"""
+
+        recurring_events = serializers.StringRelatedField(many=True)
+
+        class Meta:
+            model = Project
+            fields = (
+                "uuid",
+                "name",
+                "description",
+                "created_at",
+                "updated_at",
+                "completed_at",
+                "github_org_id",
+                "github_primary_repo_id",
+                "hide",
+                "google_drive_id",
+                "image_logo",
+                "image_hero",
+                "image_icon",
+                "recurring_events",
+            )
+            read_only_fields = (
+                "uuid",
+                "created_at",
+                "updated_at",
+                "completed_at",
+            )
+
+
+    class RecurringEventSerializer(serializers.ModelSerializer):
+        """Used to retrieve recurring_event info"""
+
+        projects = serializers.StringRelatedField(many=True)
+
+        class Meta:
+            model = RecurringEvent
+            fields = (
+                "uuid",
+                "name",
+                "start_time",
+                "duration_in_min",
+                "video_conference_url",
+                "additional_info",
+                "project",
+                "projects",
+            )
+            read_only_fields = (
+                "uuid",
+                "created_at",
+                "updated_at",
+            )
+    ```
+
 1. Import the new model
 
     ```python title="app/core/api/serializers.py" linenums="1"
@@ -491,6 +608,35 @@ In `app/core/api/urls.py`
         ./scripts/test.sh
         ```
 
+??? note "Test many-to-many relationships"
+    In `app/core/tests/test_api.py`
+
+    1. Import API URL
+
+        ```python title="app/core/tests/test_api.py" linenums="1"
+        PROJECT_URL = reverse("project-list")
+        ```
+
+    1. Add test case
+
+        ```python title="app/core/tests/test_api.py" linenums="1"
+        def test_project_sdg_xref(auth_client, project, sdg):
+            project.sdgs.add(sdg)
+            project.save()
+
+            proj_res = auth_client.get(PROJECT_URL)
+            sdg_res = auth_client.get(SDG_URL)
+
+            assert filter(lambda proj: str(proj["uuid"]) == str(project.pk), proj_res.data)
+            assert filter(lambda _sdg: str(_sdg["uuid"]) == str(sdg.pk), sdg_res)
+        ```
+
+    1. Run the test script to show it passing
+
+        ```bash
+        ./scripts/test.sh
+        ```
+
 ??? note "Check and commit"
     This is a good place to pause, check, and commit progress.