Prepared query for counting the subset of tags for stories with the given tags

story.rb

class Story < Sequel::Model
  STORY_COUNT_BY_TAGS =
    select { [tag, count(tag)] }
    .from(
      select { unnest(tags).as(tag) }
      .from(:stories)
      .where(
        Sequel
        .pg_array(:tags)
        .contains(Sequel.cast(:$filter, :'text[]').pg_array)
      )
    )
    .exclude(tag: Sequel.cast(:$filter, 'text[]').pg_array.any)
    .group(:tag)
    .order(:count).reverse
    .prepare(:all, :tag_counts)

  # SELECT "tag", count("tag")
  # FROM (
  #   SELECT unnest("tags") AS "tag"
  #   FROM "stories"
  #   WHERE ("tags" @> CAST($1 AS text[]))
  # ) AS "t1"
  # WHERE ("tag" != ANY(CAST($1 AS text[])))
  # GROUP BY "tag" 
  # ORDER BY "count" DESC

  def self.count_by_tags(*filter)
    STORY_COUNT_BY_TAGS
      .call(filter: filter)
      .each_with_object({}) do |result, hash|
        hash[result[:tag]] = result[:count]
      end
  end
end

The final exclude is intended to exclude tags in the specified filter from the results. However, the construction

WHERE ("tag" != ANY(CAST($1 AS text[])))

does not function as expected for the case of an empty filter array. It works as expected if the array contains any elements. On the other hand,

WHERE NOT ("tag" = ANY(CAST($1 AS text[])))

works as expected in both cases.

Incidentally, if there is a cleaner way to do the where and exclude calls, I'm all ears :)