title: activeadmin and acts_as_taggable_on generates ambiguous_column_name error
body: <p>I am using active_admin and acts_As_taggable_on and I am trying to make a filter.  Here is the model code:</p>
  
  <pre><code>class Person &lt; ApplicationRecord
      acts_as_taggable_on :expertise, :industry
  end
  </code></pre>
  
  <p>Here is the filter:</p>
  
  <pre><code>filter :industry, as: :select, collection: Person.industry_counts.pluck(:name, :name)
  </code></pre>
  
  <p>and here is the error i get when submitting the filter:</p>
  
  <pre><code>SQLite3::SQLException: ambiguous column name: created_at: SELECT COUNT(DISTINCT "people"."id") FROM "people" LEFT OUTER JOIN "taggings" ON "taggings"."taggable_id" = "people"."id" AND "taggings"."context" = ? AND "taggings"."taggable_type" = ? WHERE "taggings"."tag_id" = 0 AND (created_at &gt; '2017-01-17 00:22:53.923894')
  </code></pre>
  
  <p>How do I fix this?</p>