All search fields are not shown in admin side(cms) search field

<%- if @topic_view.topic.tags.present? %>
<%= t 'js.tagging.tags' %>: <%- @topic_view.topic.tags.each do |t| %> <%= t %> <%- end %>
<% end %>

Silverstripe version :4.7

I have migrated a silverstripe project from version 4.1.2 to 4.7.

I faced an issue on,
Object->__call() : the method ‘where’ does not exist on ‘Silverstripe/ORM/UnsavedRelationList’

However I fixed the issue by adding private static $searchable_fields.

But only 5 fields appeared from them in to the admin side search field.

How do I solve this issue?
I am not very familiar with silverstripe framework.

Thank you.