FIX: error when sending a private message to a group in some cases

app/models/group.rb

@@ -42,6 +42,7 @@ class Group < ActiveRecord::Base
   }
 
   AUTO_GROUP_IDS = Hash[*AUTO_GROUPS.to_a.flatten.reverse]
+  STAFF_GROUPS = [:admins, :moderators, :staff]
 
   ALIAS_LEVELS = {
     :nobody => 0,
@@ -370,6 +371,10 @@ class Group < ActiveRecord::Base
     Group.mentionable(user).where(id: group_id).exists?
   end
 
+  def staff?
+    STAFF_GROUPS.include?(self.name.to_sym)
+  end
+
   protected
 
     def name_format_validator

spec/components/guardian_spec.rb

@@ -202,6 +202,14 @@ describe Guardian do
       it "returns false if target is not staff" do
         expect(Guardian.new(user).can_send_private_message?(another_user)).to be_falsey
       end
+
+      it "returns true if target is a staff group" do
+        Group::STAFF_GROUPS.each do |name|
+          g = Group[name]
+          g.alias_level = Group::ALIAS_LEVELS[:everyone]
+          expect(Guardian.new(user).can_send_private_message?(g)).to be_truthy
+        end
+      end
     end
   end