Add spec for testing DB rollback

- Spec sets up alerts
- Rolls back DB and check to see desired behaviour

db/migrate/20200706035141_adjust_unique_index_alert_management_alerts.rb

@@ -16,6 +16,19 @@ class AdjustUniqueIndexAlertManagementAlerts < ActiveRecord::Migration[6.0]
   end
 
   def down
+    # Nullify duplicate fingerprints, except for the newest of each match (project_id, fingerprint).
+    query = <<-SQL
+      UPDATE alert_management_alerts am
+      SET fingerprint = NULL
+      WHERE am.created_at <>
+        (SELECT MAX(created_at)
+        FROM alert_management_alerts am2
+        WHERE am.fingerprint = am2.fingerprint AND am.project_id = am2.project_id)
+      AND am.fingerprint IS NOT NULL;
+    SQL
+
+    execute(query)
+
     remove_index :alert_management_alerts, name: NEW_INDEX_NAME
     add_index(:alert_management_alerts, %w(project_id fingerprint), name: INDEX_NAME, unique: true, using: :btree)
   end

spec/migrations/20200706035141_adjust_unique_index_alert_management_alerts_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('db', 'migrate', '20200706035141_adjust_unique_index_alert_management_alerts.rb')
+
+RSpec.describe AdjustUniqueIndexAlertManagementAlerts, :migration do
+  let(:migration) { described_class.new }
+  let(:alerts) { AlertManagement::Alert }
+  let(:project) { create_project }
+  let(:other_project) { create_project }
+  let(:resolved_state) { 2 }
+  let(:triggered_state) { 1 }
+  let!(:existing_alert) { create_alert(project, resolved_state, '1234', 1) }
+  let!(:p2_alert) { create_alert(other_project, resolved_state, '1234', 1) }
+  let!(:p2_alert_diff_fingerprint) { create_alert(other_project, resolved_state, '4567', 2) }
+
+  it 'can reverse the migration' do
+    expect(existing_alert.fingerprint).not_to eq(nil)
+    expect(p2_alert.fingerprint).not_to eq(nil)
+    expect(p2_alert_diff_fingerprint.fingerprint).not_to eq(nil)
+
+    migrate!
+
+    # Adding a second alert with the same fingerprint now that we can
+    second_alert = create_alert(project, triggered_state, '1234', 2)
+    expect(alerts.count).to eq(4)
+
+    schema_migrate_down!
+
+    # We keep the alerts, but the oldest ones fingerprint is removed
+    expect(alerts.count).to eq(4)
+    expect(second_alert.reload.fingerprint).not_to eq(nil)
+    expect(p2_alert.fingerprint).not_to eq(nil)
+    expect(p2_alert_diff_fingerprint.fingerprint).not_to eq(nil)
+    expect(existing_alert.reload.fingerprint).to eq(nil)
+  end
+
+  def namespace
+    @namespace ||= table(:namespaces).create!(name: 'foo', path: 'foo')
+  end
+
+  def create_project
+    table(:projects).create!(namespace_id: namespace.id)
+  end
+
+  def create_alert(project, status, fingerprint, iid)
+    params = {
+      title: 'test',
+      started_at: Time.current,
+      iid: iid,
+      project_id: project.id,
+      status: status,
+      fingerprint: fingerprint
+    }
+    table(:alert_management_alerts).create!(params)
+  end
+end