Add uniqueness constraint to School#creator_id

At present we don't want a user to be able to create more than a single
school (although we have hopes of relaxing this constraint in future).

I've checked the staging and production databases for existing
violations of this constraint and it looks like we're OK: there are 5
schools in staging, each with a unique creator_id, and there's only a
single school in production.

Author: chrisroos

app/models/school.rb

@@ -14,7 +14,7 @@ class School < ApplicationRecord
   validates :municipality, presence: true
   validates :country_code, presence: true, inclusion: { in: ISO3166::Country.codes }
   validates :reference, uniqueness: { case_sensitive: false, allow_nil: true }, presence: false
-  validates :creator_id, presence: true
+  validates :creator_id, presence: true, uniqueness: true
   validates :creator_agree_authority, presence: true, acceptance: true
   validates :creator_agree_terms_and_conditions, presence: true, acceptance: true
 

db/migrate/20240611102154_add_unique_constraint_to_school_creator_id.rb

@@ -0,0 +1,5 @@
+class AddUniqueConstraintToSchoolCreatorId < ActiveRecord::Migration[7.0]
+  def change
+    add_index :schools, :creator_id, unique: true
+  end
+end

db/schema.rb

@@ -111,6 +111,13 @@
       expect(school).to be_invalid
     end
 
+    it 'requires a unique creator_id' do
+      school.save!
+      another_school = build(:school, creator_id: school.creator_id)
+      another_school.valid?
+      expect(another_school.errors[:creator_id]).to include('has already been taken')
+    end
+
     it 'rejects a badly formed url for website' do
       school.website = 'http://.example.com'
       expect(school).to be_invalid