add ObserverPairingCode model

closes MBL-10642

test plan:
- in the rails console,
  - create an ObserverPairingCode for a student
    @student.generate_observer_pairing_code
  - read pairing codes for a student
    @student.observer_pairing_codes
- generated codes should expire in 1 day
- expired and deleted codes should not be returned
  in the above call to read codes

Change-Id: I1c0bf25bca4ef34016cff94ce721e3d339e251e1
Reviewed-on: https://gerrit.instructure.com/152105
Reviewed-by: Matthew Sessions <msessions@instructure.com>
Product-Review: Matthew Sessions <msessions@instructure.com>
QA-Review: Matthew Sessions <msessions@instructure.com>
Tested-by: Jenkins

app/models/observer_pairing_code.rb

@@ -0,0 +1,29 @@
+#
+# Copyright (C) 2018 - present Instructure, Inc.
+#
+# This file is part of Canvas.
+#
+# Canvas is free software: you can redistribute it and/or modify it under
+# the terms of the GNU Affero General Public License as published by the Free
+# Software Foundation, version 3 of the License.
+#
+# Canvas is distributed in the hope that it will be useful, but WITHOUT ANY
+# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+# A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along
+# with this program. If not, see <http://www.gnu.org/licenses/>.
+#
+
+class ObserverPairingCode < ActiveRecord::Base
+  belongs_to :user, inverse_of: :observer_pairing_codes
+  validates :user_id, :expires_at, :code, presence: true
+
+  scope :active, -> { where("workflow_state<>'deleted' AND expires_at > ?", Time.zone.now) }
+
+  def destroy
+    self.workflow_state = 'deleted'
+    self.save!
+  end
+end

app/models/user.rb

@@ -61,6 +61,8 @@ class User < ActiveRecord::Base
   has_many :observer_enrollments
   has_many :observee_enrollments, :foreign_key => :associated_user_id, :class_name => 'ObserverEnrollment'
 
+  has_many :observer_pairing_codes, -> { where("workflow_state<>'deleted' AND expires_at > ?", Time.zone.now) }, dependent: :destroy, inverse_of: :user
+
   has_many :as_student_observation_links, -> { where.not(:workflow_state => 'deleted') }, class_name: 'UserObservationLink',
     foreign_key: :user_id, dependent: :destroy, inverse_of: :student
   has_many :as_observer_observation_links, -> { where.not(:workflow_state => 'deleted') }, class_name: 'UserObservationLink',
@@ -2843,4 +2845,8 @@ class User < ActiveRecord::Base
     end
     User.where(:id => id_token_map.keys).to_a.select { |u| u.token == id_token_map[u.id] }
   end
+
+  def generate_observer_pairing_code
+    observer_pairing_codes.create(expires_at: 1.day.from_now, code: SecureRandom.hex(3))
+  end
 end

db/migrate/20180531164616_create_observer_pairing_codes.rb

@@ -0,0 +1,31 @@
+#
+# Copyright (C) 2018 - present Instructure, Inc.
+#
+# This file is part of Canvas.
+#
+# Canvas is free software: you can redistribute it and/or modify it under
+# the terms of the GNU Affero General Public License as published by the Free
+# Software Foundation, version 3 of the License.
+#
+# Canvas is distributed in the hope that it will be useful, but WITHOUT ANY
+# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+# A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along
+# with this program. If not, see <http://www.gnu.org/licenses/>.
+#
+
+class CreateObserverPairingCodes < ActiveRecord::Migration[5.1]
+  tag :predeploy
+  def change
+    create_table :observer_pairing_codes do |t|
+      t.belongs_to :user, :null => false, :foreign_key => true
+      t.string :code, :null => false, :limit => 10
+      t.timestamp :expires_at, :null => false, :index => true
+      t.string :workflow_state, :default => 'active', :null => false, :index => true
+
+      t.timestamps
+    end
+  end
+end

spec/models/observer_pairing_code_spec.rb

@@ -0,0 +1,47 @@
+#
+# Copyright (C) 2018 - present Instructure, Inc.
+#
+# This file is part of Canvas.
+#
+# Canvas is free software: you can redistribute it and/or modify it under
+# the terms of the GNU Affero General Public License as published by the Free
+# Software Foundation, version 3 of the License.
+#
+# Canvas is distributed in the hope that it will be useful, but WITHOUT ANY
+# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+# A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along
+# with this program. If not, see <http://www.gnu.org/licenses/>.
+#
+
+require_relative '../spec_helper'
+
+describe ObserverPairingCode do
+  before :once do
+    @student = user_model
+  end
+
+  it 'can pass validations' do
+    code = ObserverPairingCode.create(user: @student, expires_at: 1.day.from_now, code: SecureRandom.hex(3))
+    expect(code.valid?).to eq true
+  end
+
+  it 'can be generated from a user' do
+    code = @student.generate_observer_pairing_code
+    expect(code).not_to be_nil
+  end
+
+  it 'can generate more than one code' do
+    code = @student.generate_observer_pairing_code
+    code2 = @student.generate_observer_pairing_code
+    expect(code2.id).not_to eq code.id
+  end
+
+  it 'ignores expired codes' do
+    ObserverPairingCode.create(user: @student, expires_at: 1.day.ago, code: SecureRandom.hex(3))
+    codes = @student.observer_pairing_codes
+    expect(codes.length).to eq 0
+  end
+end