Migrate tags in Rails to PostgreSQL array from ActsAsTaggableOn

ActsAsTaggableOn is a swiss army knife solution if you need to add tags to your ActiveRecord model.
Just by adding one gem to your Gemfile and acts_as_taggable to the model you get everything you need: adding tags, searching for a model by tag, getting top tags, etc. However, sometimes you don't need all these.

In our project, we used acts_as_taggable to store tags for Note model. Then we displayed a list of notes on several pages with assigned tags and had autocompleted input for tags on Note form. Everything worked well, but since we use PostgreSQL, I decided to store tags as an array in Note model.

First of all, I added tags Array<String> column to Note, after this migrated acts_as_taggable tags to notes table with migration.

class MigrateNoteTags < ActiveRecord::Migration[5.2]
  def change
    execute <<-SQL
    UPDATE notes 
    SET tags = grouped_taggings.tags_array 
    FROM
      (
      SELECT
        taggings.taggable_id,
        ARRAY_AGG ( tags.NAME ) tags_array 
      FROM
        taggings
        LEFT JOIN tags ON taggings.tag_id = tags.ID 
      WHERE
        taggable_type = 'Note' 
      GROUP BY
        taggings.taggable_id 
      ) AS grouped_taggings 
    WHERE
      notes.ID = grouped_taggings.taggable_id
    SQL
  end
end

To have backward compatibility, I added Note#tag_list method:

def tag_list
  tags.join(', ')
end

The last thing is to add the ability to search for tags. Since there about 500k records in the Notes table, I decided to create an SQL view:

CREATE OR REPLACE VIEW note_tags AS

SELECT UNNEST
  ( tags ) AS name,
  COUNT ( * ) AS taggings_count 
FROM
  notes 
GROUP BY
  name 

That's it! It takes from 100ms to 150ms to search for tags in this view, which is fine for me.

If you have more significant data sets, then the best would be to create tags table and add triggers to notes table that will update tags on INSERT/UPDATE/DELETE.