rake db:schema:load vs rake db:migrate • AxiomQ

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March 24, 2016 Programming

Sooner or later every new Ruby developer needs to understand differences between rake db:schema:load and rake db:migrate. Basically, these simple definition tells us everything we need to know:

rake db:migrate runs migrations that have not run yet
rake db:schema:load loads the schema.db file into database.

but the real question is when to use one or the other.

Advice: when you are adding a new migration to an existing app then you need to run rake db:migrate, but when you join to existing application (especially some old application), or when you drop your applications database and you need to create it again, always run rake db:schema:load to load schema.

Example

I am working on application which use globalize gem for ActiveRecord model/data translations. Globalize work this way:

first specify attributes which need to be translatable

class Post < ActiveRecord::Base
translates :title, :text
end

then create translation tables

class CreatePosts < ActiveRecord::Migration
def up
create_table :posts do |t|
t.timestamps
end
Post.create_translation_table! title: :string, text: :text
end
def down
drop_table :posts
Post.drop_translation_table!
end
end

Note that the ActiveRecord model Post must already exist and have listed attributes for translations.

and run rake db:migrate.
Problem comes when you change your mind and decide to leave title to be untranslatable.

remove title from post translations table

class RemoveTitleFromPostTranslations < ActiveRecord::Migration
def up
remove_column :post_translations, :title, :string
end

def down
Entry.add_translation_fields! title: :string
end
end

add title to posts table

class AddTitleToPosts < ActiveRecord::Migration
def change
add_column :posts, :title, :string
end
end

remove title attribute from model translations

class Post < ActiveRecord::Base
translates :text
end

and run rake db:migrate.

Everything looking good, so where is the problem?

Here it is! If you decide to delete your database and create it again you need to use:

rake db:drop
rake db:create
rake db:schema:load

Because, if you try to use rake db:migrate instead of rake db:schema:load you will get BIG ERROR!, because for your first migration “create_posts” it is necessary that you have defined translatable attributes :title and :text in Post model, but you removed :title from Post model translations.

So just follow advice above, and good luck.