Set todos#author_id to NOT NULL.

app/models/todo.rb

@@ -28,6 +28,7 @@ class Todo < ActiveRecord::Base
   delegate :name, :email, to: :author, prefix: true, allow_nil: true
   validates :action, :project, :target_type, :user, presence: true
+  validates :author, presence: true
   validates :target_id, presence: true, unless: :for_commit?
   validates :commit_id, presence: true, if: :for_commit?

changelogs/unreleased/32282-add-foreign-keys-to-todos.yml

 ---
-title: Add foreign key constraints to todos table.
+title: Add foreign key and NOT NULL constraints to todos table.
 merge_request: 16849
 author:
 type: other

db/post_migrate/20180204200836_change_author_id_to_not_null_in_todos.rb

+class ChangeAuthorIdToNotNullInTodos < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  class Todo < ActiveRecord::Base
+    self.table_name = 'todos'
+    include EachBatch
+  end
+
+  BATCH_SIZE = 1000
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    Todo.where(author_id: nil).each_batch(of: BATCH_SIZE) do |batch|
+      batch.delete_all
+    end
+
+    change_column_null :todos, :author_id, false
+  end
+
+  def down
+    change_column_null :todos, :author_id, true
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
-ActiveRecord::Schema.define(version: 20180202111106) do
+ActiveRecord::Schema.define(version: 20180204200836) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -1707,7 +1707,7 @@ ActiveRecord::Schema.define(version: 20180202111106) do
     t.integer "project_id", null: false
     t.integer "target_id"
     t.string "target_type", null: false
-    t.integer "author_id"
+    t.integer "author_id", null: false
     t.integer "action", null: false
     t.string "state", null: false
     t.datetime "created_at"

spec/models/todo_spec.rb

@@ -20,6 +20,7 @@ describe Todo do
     it { is_expected.to validate_presence_of(:action) }
     it { is_expected.to validate_presence_of(:target_type) }
     it { is_expected.to validate_presence_of(:user) }
+    it { is_expected.to validate_presence_of(:author) }
     context 'for commits' do
       subject { described_class.new(target_type: 'Commit') }