Putting update logic in your migrations
- by Daniel Abrahamsson
A couple of times I've been in the situation where I've wanted to refactor the design of some model and have ended up putting update logic in migrations. However, as far as I've understood, this is not good practice (especially since you are encouraged to use your schema file for deployment, and not your migrations). How do you deal with these kind of problems?
To clearify what I mean, say I have a User model. Since I thought there would only be two kinds of users, namely a "normal" user and an administrator, I chose to use a simple boolean field telling whether the user was an adminstrator or not.
However, after I while I figured I needed some third kind of user, perhaps a moderator or something similar. In this case I add a UserType model (and the corresponding migration), and a second migration for removing the "admin" flag from the user table. And here comes the problem. In the "add_user_type_to_users" migration I have to map the admin flag value to a user type. Additionally, in order to do this, the user types have to exist, meaning I can not use the seeds file, but rather create the user types in the migration (also considered bad practice). Here comes some fictional code representing the situation:
class CreateUserTypes < ActiveRecord::Migration
def self.up
create_table :user_types do |t|
t.string :name, :nil => false, :unique => true
end
#Create basic types (can not put in seed, because of future migration dependency)
UserType.create!(:name => "BASIC")
UserType.create!(:name => "MODERATOR")
UserType.create!(:name => "ADMINISTRATOR")
end
def self.down
drop_table :user_types
end
end
class AddTypeIdToUsers < ActiveRecord::Migration
def self.up
add_column :users, :type_id, :integer
#Determine type via the admin flag
basic = UserType.find_by_name("BASIC")
admin = UserType.find_by_name("ADMINISTRATOR")
User.all.each {|u| u.update_attribute(:type_id, (u.admin?) ? admin.id : basic.id)}
#Remove the admin flag
remove_column :users, :admin
#Add foreign key
execute "alter table users add constraint fk_user_type_id
foreign key (type_id) references user_types (id)"
end
def self.down
#Re-add the admin flag
add_column :users, :admin, :boolean, :default => false
#Reset the admin flag (this is the problematic update code)
admin = UserType.find_by_name("ADMINISTRATOR")
execute "update users set admin=true where type_id=#{admin.id}"
#Remove foreign key constraint
execute "alter table users drop foreign key fk_user_type_id"
#Drop the type_id column
remove_column :users, :type_id
end
end
As you can see there are two problematic parts. First the row creation part in the first model, which is necessary if I would like to run all migrations in a row, then the "update" part in the second migration that maps the "admin" column to the "type_id" column.
Any advice?
