Remove legacy triggers and add not-null constraint

- It removes all ci_triggers with no owner_id
- It adds NOT NULL constraint to owner_id column of ci_triggers

db/post_migrate/20191204114127_delete_legacy_triggers.rb

+# frozen_string_literal: true
+
+class DeleteLegacyTriggers < ActiveRecord::Migration[5.2]
+  DOWNTIME = false
+
+  def up
+    execute <<~SQL
+      DELETE FROM ci_triggers WHERE owner_id IS NULL
+    SQL
+
+    change_column_null :ci_triggers, :owner_id, false
+  end
+
+  def down
+    change_column_null :ci_triggers, :owner_id, true
+  end
+end

db/schema.rb

@@ -994,7 +994,7 @@ ActiveRecord::Schema.define(version: 2020_01_08_233040) do
     t.datetime "created_at"
     t.datetime "updated_at"
     t.integer "project_id"
-    t.integer "owner_id"
+    t.integer "owner_id", null: false
     t.string "description"
     t.string "ref"
     t.index ["owner_id"], name: "index_ci_triggers_on_owner_id"

spec/migrations/20191204114127_delete_legacy_triggers_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20191204114127_delete_legacy_triggers.rb')
+
+describe DeleteLegacyTriggers, :migration, schema: 2019_11_25_140458 do
+  let(:ci_trigger_table) { table(:ci_triggers) }
+  let(:user) { table(:users).create!(name: 'test', email: 'test@example.com', projects_limit: 1) }
+
+  before do
+    @trigger_with_user = ci_trigger_table.create!(owner_id: user.id)
+    ci_trigger_table.create!(owner_id: nil)
+    ci_trigger_table.create!(owner_id: nil)
+  end
+
+  it 'removes legacy triggers which has null owner_id' do
+    expect do
+      migrate!
+    end.to change(ci_trigger_table, :count).by(-2)
+
+    expect(ci_trigger_table.all).to eq([@trigger_with_user])
+  end
+end