class AddTagsToArticles < ActiveRecord::Migration[4.2]
  def change
    add_column :articles, :tags, :string, default: [].to_json, array: true
  end
end

class AddTagsToArticles < ActiveRecord::Migration[4.2]
  def change
    add_column :articles, :tags, :string
  end
end

Remove default to make it work with both sqlite and postgres
Remove default to make it work with both sqlite and postgres

Postgres doesn't accept array: true and default: string("[]"). So,
simply remove the default.

Anyway, the next migration changes it to a simple string column.
