Use Trucker to migrate legacy data into your Rails app.
Trucker is designed to help you quickly and repeatedly extract legacy data from an older database. To make this possible, Trucker creates a new database connection that points to your legacy data and then creates optional legacy models which are designed to pair with an existing model in your app.
Let's say you have an existing Post
model and you need to migrate older blog posts into your model. Just create a truck for posts by running rails g truck:post
. This will create a LegacyPost
model which can load data into your Post
model.
Once you've got LegacyPost
configured correctly, you can start running the rake db:migrate:posts
rake task that Trucker created for you to import your legacy content into your new app.
The best part is you don't have to run this migration just once. You can run it again and again in development until you're sure that you've got a rock solid way to import content (and potentially clean it up).
-
Add the trucker gem in to your Gemfile:
gem 'trucker'
-
Bundle install:
bundle install
-
Create a truck for your model:
rails generate truck:model
Important: when running this command, make sure you replace
model
with the singular, lowercase name of your new model where content will be imported.So, if you new model is named
Post
, you should run this command:rails generate truck:post
This will do the following things:
- Add legacy database connection to
database.yml
- Add
app/models/legacy
directory - Add
app/models/legacy
toautoload_paths
inapplication.rb
- Add
app/models/legacy/legacy_base.rb
(from which legacy models will inherit) - Add
app/models/legacy/legacy_post.rb
(for yourLegacyPost
model) - Add
lib/tasks/legacy_post.rake
(to migrate legacy posts)
With Trucker, a new model is always paired with a legacy model that has a matching name.
Post
is matched toLegacyPost
.Product
toLegacyProduct
. And so on.To keep things simple, Trucker purposely ignores whatever your model was called in its original location -- because it's very likely that the original database table was not named using Rails conventions so there's not much benefit in trying to name the legacy model to match table names.
Instead, once the legacy model is created, we can just tweak the model
table_name
inside the model so that it matches the exact table in the legacy database. - Add legacy database connection to
-
Update the legacy database connection in
database.yml
with your legacy database information:legacy: adapter: encoding: database: username: password:
By default, Rails can support connecting to PostgreSQL, MySQL, and SQL Lite databases. If your legacy data is stored in another format like Oracle or SQL Server, you can use a third-party ActiveRecord adapter gem to connect to those data sources.
Rails defaults to
unicode
encoding when connecting to PostgreSQL databases, but be aware that different databases may use alternative encodings likeutf8
,utf8mb4
, and so on. You can often review your data dump or schema to discover which encoding is being used.In Rails development, databases often use the same short name as the app itself and then add a suffix to indicate the environment in which a database should be used. This helps to keep databases organized when you're actively working on a lot of apps. So, if your app is named
avengers
, you could useavengers_legacy
for your legacy database name.However, on the other hand, if you can't easily change your legacy database name, just use the existing name and update the name in your legacy database connection accordingly.
In development, depending on how you installed your database, you may not need to use a username and password. But, if you do, you can edit these settings.
-
If the legacy database doesn't already exist, add it.
rake db:create:all
-
Import your legacy data into the legacy database.
This step will really depend on where your legacy data lives. You may need to connect to a live external database. Or you may need to acquire a database dump and then import that into your database system of choice. It really depends.
-
Tweak your legacy models.
class LegacyPost < LegacyBase self.table_name = "LEGACY_TABLE_NAME_GOES_HERE" end
Since you're migrating data from an old database, your table names may not follow Rails conventions for database table naming. If so, you will need to set the
self.table_name =
value for each of your legacy models to match the name of table from which you will be importing data.For instance, in the example above, if your old posts were stored in an
articles
table, here's how you would custom thetable_name
:class LegacyPost < LegacyBase self.table_name = "articles" end
-
Update legacy model field mappings.
class LegacyPost < LegacyBase self.table_name = "LEGACY_TABLE_NAME_GOES_HERE" def map { :headline => self.title.squish, :body => self.long_text.squish } end end
This is where you will connect your old database attributes with your new ones. The map method is really just a hash which uses your new model attribute names as keys and your legacy model attributes as values.
(aka
:new_field => self.legacy_field
)Note: make sure to add
self.
to each legacy attribute name. -
Need to tweak some data? Just add some core ruby methods or add a helper method.
class LegacyPost < LegacyBase self.table_name = "NAME_OF_TABLE_WHERE_POST_MODEL_DATA_IS_STORED" def map { :headline => self.title.squish.capitalize, # <= Added capitalize method :body => tweak_body(self.long_text.squish) # <= Added tweak_body method } end # Insert helper methods as needed def tweak_body(body) self .gsub(/<br \//,"\n") # <= Convert break tags into normal line breaks .gsub(/teh/, "the") # <= Fix common typos end end
-
Start migrating!
rake db:migrate:posts
Trucker supports a few command line options when migrating records:
rake db:migrate:posts limit=100 (migrates 100 records)
rake db:migrate:posts limit=100 offset=100 (migrates 100 records, but skip the first 100 records)
You can tweak the default migration output generated by Trucker by using the :label
option.
rake db:migrate:posts
=> Migrating posts
rake db:migrate:posts, :label => "blog posts"
=> Migrating blog posts
Trucker works great for migrating data from many legacy data sources such as apps built with PHP, Perl, Python, or even older versions of Rails (where upgrading an existing Rails code base is not practical). But, if you're migrating data from a large enterprise system, Trucker may not be your best choice.
That said, if you need to pull off a complex migration for a model, you can use a custom helper method to override Trucker's default migrate method in your rake task.
namespace :db do
namespace :migrate do
...
desc 'Migrate pain_in_the_ass model'
task :pain_in_the_ass => :environment do
Trucker.migrate :pain_in_the_ass, :helper => pain_in_the_ass_migration
end
end
end
def pain_in_the_ass_migration
# Custom code goes here
end
If you don't want to write your custom migration method from scratch, you can copy trucker's migrate method method from lib/trucker.rb and tweak accordingly.
As an example, here's a custom helper used to migrate join tables on a bunch of models.
namespace :db do
namespace :migrate do
desc 'Migrates join tables'
task :joins => :environment do
migrate :joins, :helper => :migrate_joins
end
end
end
def migrate_joins
puts "Migrating #{number_of_records || "all"} joins #{"after #{offset_for_records}" if offset_for_records}"
["chain", "firm", "function", "style", "website"].each do |model|
# Start migration
puts "Migrating theaters_#{model.pluralize}"
# Delete existing joins
ActiveRecord::Base.connection.execute("TRUNCATE table theaters_#{model.pluralize}")
# Tweak model ids and foreign keys to match model syntax
if model == 'website'
model_id = "url_id"
send_foreign_key = "url_id".to_sym
else
model_id = "#{model}_id"
send_foreign_key = "#{model}_id".to_sym
end
# Create join object class
join = Object.const_set("Theaters#{model.classify}", Class.new(ActiveRecord::Base))
# Set model foreign key
model_foreign_key = "#{model}_id".to_sym
# Migrate join (unless duplicate)
"LegacyTheater#{model.classify}".constantize.find(:all, with(:order => model_id)).each do |record|
unless join.find(:first, :conditions => {:theater_id => record.theater_id, model_foreign_key => record.send(send_foreign_key)})
attributes = {
model_foreign_key => record.send(send_foreign_key),
:theater_id => record.theater_id
}
# Check if theater chain is current
attributes[:is_current] = {'Yes' => 1, 'No' => 0, '' => 0}[record.current] if model == 'chain'
# Migrate join
join.create(attributes)
end
end
end
end
Check out the Trucker sample app for a working example of Trucker-based legacy data migration. (Note: this app has not been updated to work with trucker 4.0 and above. It should only be used with trucker 0.5.1 and below.)
Trucker is based on a migration technique using legacy models first pioneered by Dave Thomas: http://pragdave.blogs.pragprog.com/pragdave/2006/01/sharing_externa.html
Bug reports and pull requests are welcome on GitHub at https://github.com/mokolabs/trucker. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.
The following folks have made significant contributions to trucker:
The gem is available as open source under the terms of the MIT License.
Everyone interacting in the Trucker project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.