How do you migrate db structure changes to heroku postgres? - ruby-on-rails

Question

For example, I have a migration file for posts:
class CreatePosts < ActiveRecord::Migration
def change
create_table :posts do |t|
t.text :text
t.integer :ip
t.timestamps
end
end
end
And want to change it to:
class CreatePosts < ActiveRecord::Migration
def change
create_table :posts do |t|
t.text :text
t.integer :ip, :limit => 8
t.timestamps
end
end
end
Would I add a line:
change_column :posts, :ip, :limit => 8
Below so that the file is:
class CreatePosts < ActiveRecord::Migration
def change
create_table :posts do |t|
t.text :text
t.integer :ip, :limit => 8
t.timestamps
change_column :posts, :ip, :limit => 8
end
end
end
And then run heroku run rake --trace db:migrate
I'm having trouble understanding how migrations work, and especially to production, so any help would be greatly appreciated.
There is http://guides.rubyonrails.org/active_record_migrations.html#changing-columns a section 3.5 on column modifiers but it doesn't specify how to pass them.
Thanks!

Answer 1

You should create a separate migration for adding the limit on the ip column.
Generate your migration:
rails generate migration ChangeIpLimitOnPosts
Inside the generated migration file, update the contents:
class ChangeIpLimitOnPosts < ActiveRecord::Migration
def up
change_column :posts, :ip, :integer, limit: 8
end
def down
change_column :posts, :ip, :integer
end
end
Take note here: you need to specify the column type when changing the column, even though in your case, you're not changing the type.
Also, in this case, Active Record will not know how to reverse the transaction if you need to rollback, so you need to explicitly tell Active Record how to do that — this can be done using the up and down methods, rather than change.
Run your migration:
rake db:migrate
On Heroku:
heroku run rake db:migrate
Hope that helps.