How do I add an attachment to a table with a migration? (Ruby on Rails)

I have a posts table and I want to add a video attachment (I'm using ActiveStorage).

I tried to create the following migration:

class AddVideoToPosts < ActiveRecord::Migration[7.0]
  def change
    add_column :posts, :video, :attachment
  end
end

But when I run rails db:migrate I get the following error:

ERROR:  type "attachment" does not exist

Forgive me if this is a simple fix. I'm new to Rails. Thank you!

Solution

You don't need any migrations for some specific model attachments

Just add such line to model

class Post < ApplicationRecord
  has_one_attached :video
end

class Post < ApplicationRecord
  has_many_attached :videos
end

That's all

Of course it will work only if you set up Active Storage and have Active Storage tables

If you haven't such tables — add them with

bin/rails active_storage:install
bin/rails db:migrate

This sets up configuration, and creates the three tables Active Storage uses:

active_storage_blobs stores data about uploaded files, such as filename and content type
active_storage_attachments, a polymorphic join table that connects your models to blobs. If your model's class name changes, you will need to run a migration on this table to update the underlying record_type to your model's new class name
active_storage_variant_records, if variant tracking is enabled, stores records for each variant that has been generated

Please see details in official guide