use dummy root account for user and site admin content migrations

refs #LS-1361

Change-Id: Ie9ab343027b636cb28169385341aea4829938c7d
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/246934
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: James Williams <jamesw@instructure.com>
Product-Review: James Williams <jamesw@instructure.com>

app/models/account.rb

@@ -2014,4 +2014,8 @@ class Account < ActiveRecord::Base
 
   relation_delegate_class(ActiveRecord::Relation).prepend(DomainRootAccountCache)
   relation_delegate_class(ActiveRecord::AssociationRelation).prepend(DomainRootAccountCache)
+
+  def self.ensure_dummy_root_account
+    Account.find_or_create_by!(id: 0) if Rails.env.test?
+  end
 end

app/models/content_migration.rb

@@ -36,7 +36,6 @@ class ContentMigration < ActiveRecord::Base
   after_save :handle_import_in_progress_notice
   after_save :check_for_blocked_migration
   before_create :set_root_account_id
-  attr_accessor :skip_root_account_assignment
 
   DATE_FORMAT = "%m/%d/%Y"
 
@@ -985,13 +984,15 @@ class ContentMigration < ActiveRecord::Base
   end
 
   def set_root_account_id
-    return if skip_root_account_assignment
     case self.context
     when Course, Group
-      self.root_account_id = self.context.root_account_id
+      self.root_account_id ||= self.context.root_account_id
     when Account
-      self.root_account_id = self.context.resolved_root_account_id
+      self.root_account_id ||= self.context.resolved_root_account_id
+    when User
+      self.root_account_id ||= 0
     end
+    Account.ensure_dummy_root_account if root_account_id == 0
   end
 
   def notification_link_anchor

gems/plugins/academic_benchmark/lib/academic_benchmark.rb

@@ -146,7 +146,7 @@ module AcademicBenchmark
     cm.migration_settings[:migration_options] = options
     cm.strand = "academic_benchmark"
     cm.user = user
-    cm.skip_root_account_assignment = true
+    cm.root_account_id = 0
     cm.save!
     [cm, cm.export_content]
   end

gems/plugins/academic_benchmark/spec_canvas/academic_benchmark_spec.rb

@@ -248,7 +248,7 @@ describe AcademicBenchmark::Converter do
         document: 'document',
         user: @user
       )[0]
-      expect(cm.root_account_id).to be_nil
+      expect(cm.root_account_id).to eq 0
     end
   end
 end