Tagging in Rails 4 using PostgreSQL arrays

Most Rails app has some kind of need for tags. There’s plenty of gems out there and one of the most popular one is acts-as-taggable-on. But if you’re using PostgreSQL for your Rails app you can use PostgreSQLs array data type instead. In this blog post I’m going to show how you can make use of them in Rails 4.

Setup

Make sure you have the Rails 4 gem activated and PostgreSQL installed then run:

rails new pgarrays --database postgresql

# Edit database.yml to connect to your database.

rails generate model document title tags

Next we need to update the migration file named create_documents:

class CreateDocuments < ActiveRecord::Migration
  def change
    create_table :documents do |t|
      t.string :title
      t.string :tags, array: true, default: []
      t.timestamps
    end
  end
end

Note that we have added the array: true option to the tags attribute and that it defaults to an empty array.

Now you’re ready to create the database and run the migrations:

rake db:create
rake db:migrate

If you open up psql, connects to your database and run \d documents;, you should see that your tags column has [] at the end which means it will be treated as an array:

Table "public.documents"
   Column   |            Type             |
------------+-----------------------------+------------------
 id         | integer                     | 
 title      | character varying(255)      | 
 tags       | character varying(255)[]    | 
 created_at | timestamp without time zone | 
 updated_at | timestamp without time zone |

Indexes:
    "documents_pkey" PRIMARY KEY, btree (id)

You can now open the Rails console and start playing with your document model:

rails c

irb(main):001:0> Document.create(title: "PostgreSQL", tags: ["pg","rails"])
=> #<Document id: 1, title: "PostgreSQL", tags: ["pg", "rails"], created_at: "2013-07-28 13:21:43", updated_at: "2013-07-28 13:21:43">

As you can see, the tags property is now represented as an array. If you are curious about how it looks in PostgreSQL, use psql again:

pgarrays_development=# select * from documents;
 id |   title    |    tags    |        created_at         |        updated_at
----+------------+------------+---------------------------+---------------------------
  1 | PostgreSQL | {pg,rails} | 2013-07-28 13:21:43.78282 | 2013-07-28 13:21:43.78282
(1 row)

Querying

To query your documents you can use the Active Record query API as you normally do:

# Find any record that has 'pg' stored in the tags array:
Document.where("'pg' = ANY (tags)")
=> #<ActiveRecord::Relation [#<Document id: 1, title: "PostgreSQL", tags: ["pg", "rails"]>, #<Document id: 2, title: "Rails", tags: ["pg", "rails"]>]>

Indexing

It’s important to add an index for the tags column otherwise it’s going to be slow as hell. You have the option to choose between two types of indexes called GiST and GIN. Which one that will suit you depends on how you want to use the tags. Your can read more about the two index types in the PostgreSQL documentation: GiST and GIN Index Types

To add the index you can create this migration:

class AddTagIndexToDocuments < ActiveRecord::Migration
  def change
    add_index  :documents, :tags, using: 'gin'
  end
end

Richard Nyström