Change to ActiveRecord's associations.rb (around line 1500)

associations.rb

if reflection.options[:conditions]
    reflection_table = (reflection.options[:class_name] || reflection.options[:source] || reflection.name).gsub('::', '_').tableize
    
    if reflection.options[:conditions].to_s.include?("#{reflection_table}")
        send(through.name).select("DISTINCT #{through.quoted_table_name}.#{primary_key}").joins(reflection.options[:source]).where(reflection.options[:conditions]).map! { |r| r.send(primary_key) }
    else
        send(through.name).select("DISTINCT #{through.quoted_table_name}.#{primary_key}").where(reflection.options[:conditions]).map! { |r| r.send(primary_key) }
    end
else
    send(through.name).select("DISTINCT #{through.quoted_table_name}.#{primary_key}").map! { |r| r.send(primary_key) }
end