Allow same email address with different SSO providers

db/migrate/20170406155141_change_users_unique_index_to_uid_provider.rb

@@ -0,0 +1,10 @@
+# A user's email address may not be unique if they use different
+# single-sign-on providers with the same email address.
+# But the combination of the email address and the (SSO)
+# provider should be unique.
+class ChangeUsersUniqueIndexToUidProvider < ActiveRecord::Migration[5.0]
+  def change
+    remove_index 'users', 'email'
+    add_index 'users', ["email", "provider"], :unique => true
+  end
+end