Make SchoolProject destroy dependent Transitions (#667)

Fix an incorrect `depdendent:` rule between `SchoolProject` and
`SchoolProjectTransition`.

## Status

- Closes RaspberryPiFoundation/digital-editor-issues#1126

## Points for consideration:

- Security
- Performance

## What's changed?

### Database and Model Relations and Nullify Rules

Previously, when a SchoolProject was destroyed, there was a rule that
dependent `school_project_transitions` would be nullified rather than
destroyed.

At the same time, the definition of a `SchoolProjectTransition` states
that the `school_project_id`, which refers to the project being
transitioned, CANNOT be null.

Therefore, when PR
[#637](#637)
allowed deletion of Students and, consequently, Projects, the error in
these relations and nullification rules was exposed.

This leads to an error when deleting a Student who owns projects which
have been transitioned.

This PR changes the `dependent:` rule between SchoolProject and
SchoolProjectTransition to destroy the transition when the Proejct is
destroyed. It also adds a test to ensure this happens.

## Follow on Work

While this PR fixes the immediate problem, there remains an issue that
`SchoolStudentsController#destroy_batch` will return a `200` status code
on both success and failure. I suggest we address this in a follow up
issue.

## Steps to perform after deploying to production

NA

Author: fspeirs

app/models/school_project.rb

@@ -4,7 +4,7 @@ class SchoolProject < ApplicationRecord
   belongs_to :school
   belongs_to :project
   has_many :feedback, dependent: :destroy
-  has_many :school_project_transitions, autosave: false, dependent: :nullify
+  has_many :school_project_transitions, autosave: false, dependent: :destroy
 
   include Statesman::Adapters::ActiveRecordQueries[
     transition_class: ::SchoolProjectTransition,

spec/models/school_project_spec.rb

@@ -20,7 +20,7 @@
   it { is_expected.to belong_to(:school) }
   it { is_expected.to belong_to(:project) }
   it { is_expected.to have_many(:feedback).dependent(:destroy) }
-  it { is_expected.to have_many(:school_project_transitions).dependent(:nullify) }
+  it { is_expected.to have_many(:school_project_transitions).dependent(:destroy) }
 
   describe '#status' do
     it 'defaults to unsubmitted' do

spec/services/student_removal_service_spec.rb

@@ -79,6 +79,22 @@
         # Other student's project should remain
         expect(Project.exists?(other_school_project.id)).to be true
       end
+
+      it 'deletes school project transitions when deleting projects' do
+        project = create(:project, user_id: student.id, school: school)
+        school_project = project.school_project
+
+        # Transition the project to create SchoolProjectTransition records
+        school_project.transition_status_to!(:submitted, student.id)
+        school_project.transition_status_to!(:returned, teacher.id)
+
+        results = service.remove_students
+
+        expect(results.first[:error]).to be_nil
+        expect(Project.exists?(project.id)).to be false
+        expect(SchoolProject.exists?(school_project.id)).to be false
+        expect(SchoolProjectTransition.where(school_project_id: school_project.id).count).to eq(0)
+      end
     end
 
     context 'when student does not have a role in the school' do