fix/ensure that latepolicy has unique course_id

Closes CNVS-36864

Runs a fixup to clean up any LatePolicies that have the same course
id, keeping only the newest one. Adds a unique constraint on the db
and activerecord.

Test Plan
 - Before pulling code, create some LatePolicies in the console all
 with the same course.
 - Verify that you have multiple LatePolicies with the same course_id
 - Pull this code and run migrations
 - Check in the console to see if you still have LatePolicies that
 share a course_id with another LatePolicy
 - Attempt to create a LatePolicy that shares a course_id with
 another LatePolicy

Change-Id: I78fd49789dc7f235a399d261a6b6a1a30c42bb8e
Reviewed-on: https://gerrit.instructure.com/112152
Tested-by: Jenkins
Reviewed-by: Neil Gupta <ngupta@instructure.com>
Reviewed-by: Shahbaz Javeed <sjaveed@instructure.com>
QA-Review: Spencer Olson <solson@instructure.com>
Product-Review: Keith T. Garner <kgarner@instructure.com>

app/models/late_policy.rb

@@ -20,7 +20,8 @@ class LatePolicy < ActiveRecord::Base
   belongs_to :course, inverse_of: :late_policy
 
   validates :course_id,
-    presence: true
+    presence: true,
+    uniqueness: true
   validates :late_submission_minimum_percent, :missing_submission_deduction, :late_submission_deduction,
     presence: true,
     numericality: { greater_than_or_equal_to: 0, less_than_or_equal_to: 100 }

db/migrate/20170512184911_make_late_policy_unique.rb

@@ -0,0 +1,35 @@
+#
+# Copyright (C) 2017 - 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 MakeLatePolicyUnique < ActiveRecord::Migration[4.2]
+  # running fixup as a predeploy and add_index without concern of lockup
+  # because we're certain the tables are small. Only one endpoint has been
+  # exposed, in beta, and that endpoint is behind a feature flag. The only way a
+  # duplicate LatePolicy can be created at this point is through a rails console
+  tag :predeploy
+
+  def change
+    reversible do |dir|
+      dir.up do
+        DataFixup::MakeLatePolicyUnique.run
+      end
+    end
+
+    remove_index :late_policies, column: :course_id
+    add_index :late_policies, :course_id, unique: true
+  end
+end

lib/data_fixup/make_late_policy_unique.rb

@@ -0,0 +1,25 @@
+#
+# Copyright (C) 2017 - 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/>.
+
+module DataFixup
+  module MakeLatePolicyUnique
+    def self.run
+      duplicates = LatePolicy.group(:course_id).having("count(id) > 1").pluck("course_id, max(id)").transpose
+      LatePolicy.where(course_id: duplicates[0]).where.not(id: duplicates[1]).delete_all
+    end
+  end
+end

spec/models/late_policy_spec.rb

@@ -52,6 +52,14 @@ describe LatePolicy do
 
     # Inclusion
     it { is_expected.to validate_inclusion_of(:late_submission_interval).in_array(['day', 'hour']) }
+
+    # Uniqueness
+    describe 'uniqueness' do
+      subject { course.create_late_policy }
+      let(:course) { Course.create! }
+
+      it { is_expected.to validate_uniqueness_of(:course_id) }
+    end
   end
 
   describe 'default values' do