populate root_account_id on new LearningOutcomeGroup

* prereq for root_account on ContentTag

refs PLAT-5727
flag=none

test plan:
* specs
* if you want, save a LearningOutcomeGroup and observe that
its root_account and root_account_id attributes are not nil

Change-Id: Id268ea78377574cdb3bd2cef4a53bb6b4ca135b1
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/236478
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Michael Brewer-Davis <mbd@instructure.com>
QA-Review: Xander Moffatt <xmoffatt@instructure.com>
Product-Review: Xander Moffatt <xmoffatt@instructure.com>

app/models/learning_outcome_group.rb

@@ -26,8 +26,10 @@ class LearningOutcomeGroup < ActiveRecord::Base
   has_many :child_outcome_groups, :class_name => 'LearningOutcomeGroup', :foreign_key => "learning_outcome_group_id"
   has_many :child_outcome_links, -> { where(tag_type: 'learning_outcome_association', content_type: 'LearningOutcome') }, class_name: 'ContentTag', as: :associated_asset
   belongs_to :context, polymorphic: [:account, :course]
+  belongs_to :root_account, class_name: 'Account'
 
   before_save :infer_defaults
+  before_save :set_root_account_id
   validates :vendor_guid, length: { maximum: maximum_string_length, allow_nil: true }
   validates_length_of :description, :maximum => maximum_text_length, :allow_nil => true, :allow_blank => true
   validates_length_of :title, :maximum => maximum_string_length, :allow_nil => true, :allow_blank => true
@@ -204,6 +206,17 @@ class LearningOutcomeGroup < ActiveRecord::Base
     scope.select(title_order_by_clause).order(title_order_by_clause)
   end
 
+  def set_root_account_id
+    return if self.root_account_id.present?
+
+    case self.context
+    when Account
+      self.root_account_id = self.context.resolved_root_account_id
+    when Course
+      self.root_account_id = self.context.root_account_id
+    end
+  end
+
   private
 
   def infer_defaults

spec/models/learning_outcome_group_spec.rb

@@ -281,4 +281,23 @@ describe LearningOutcomeGroup do
       expect(active_child_groups).to be_empty
     end
   end
+
+  describe 'before save' do
+    describe 'set_root_account_id' do
+      it 'sets root_account_id using Account context' do
+        group = LearningOutcomeGroup.create!(title: 'group', context: Account.default)
+        expect(group.root_account).to eq Account.default
+      end
+
+      it 'sets root_account_id using Course context' do
+        group = @course.learning_outcome_groups.create!(title: 'group')
+        expect(group.root_account).to eq @course.root_account
+      end
+
+      it 'leaves root_acount_id nil when global (context is nil)' do
+        group = LearningOutcomeGroup.create!(title: 'group', context_id: nil)
+        expect(group.root_account_id).to be_nil
+      end
+    end
+  end
 end