Add an array column in Rails

You can use following steps

rails g migration add_subjects_to_book subjects:text

And the migration file:

class AddSubjectsToBook < ActiveRecord::Migration
  def change
    add_column :books, :subjects, :text, array: true, default: []
  end
end

We can check it now:

2.1.2 :001 > b = Book.create
   (0.2ms)  BEGIN
  SQL (2.0ms)  INSERT INTO "books" ("created_at", "updated_at") VALUES ($1, $2) RETURNING "id"  [["created_at", "2014-10-17 08:21:17.870437"], ["updated_at", "2014-10-17 08:21:17.870437"]]
   (0.5ms)  COMMIT
 => #<Book id: "39abef75-56af-4ad5-8065-6b4d58729ee0", title: nil, created_at: "2014-10-17 08:21:17", updated_at: "2014-10-17 08:21:17", description: {}, metadata: {}, subjects: []>

2.1.2 :002 > b.subjects.class
 => Array

If you want to add array while creating table you can do as follows

create_table :products do |t|
  t.string :name, null: false
  t.references :category, null: false
  t.text :tags, array: true, default: []
end

Note: array column is supported by PostgreSQL

If you are using Postgres, then this post http://blog.plataformatec.com.br/2014/07/rails-4-and-postgresql-arrays/ suggests using array: true option in migration script:

create_table :users do |t|
  # other columns
  t.text :addresses, array: true, default: []
end

In case you are not using Postgres, this answer might be helpful: Storing arrays in database : JSON vs. serialized array

Add an array column in Rails

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