[edited for clarity and to add what itch data migrate scratches]

Data Migrate is a gem I created to migrate data alongside schema changes. As you will see below, it provides a number of rake tasks (with some other support) that allow you to create and migrate data as you would database changes.

Why you want to use this

Its seems when a project hits a certain size, I get to manipulate data outside the application itself. Changing defaults, new validations, one-to-one to one-to-many… I found it a pain and dodgy to have to step up migrations one by one, run a ruby script of some sort, then resume migrations. It tanks a lot of the automation of deploy.

If you don’t use the one off scripts, you could do it as a regular migration. It’d be much better to keep concerns separate. The benefit of having them separate has to do with your data model.

For instance, lets take an absurd example, to illustrate: You have your infamous Rails blog that has posts with many comments. After some use, you decide you are going to be a trend setter, and want only one comment per post, and just the text. “Frist!” rules the day. Given that you:
– write a migration to add a comment column to Post
– write a migration to move the contents of the first comments to the Post
– drop the column_id column from Post
– drop the Comment model
– fix all your test/controller/view mojo.

You’ve just got bit. When you rake setup:development, the mess gets mad at you after it creates your database, and starts cranking through migrations. It gets to the part where you iterate over the comments and it blows up. You don’t have a comment model anymore for it to even try and get ‘all’ from. You think you are smarter, and wrap the AR call in a conditional based on the environment. That’s fine until you get that QA gal, and she wants her own thing. Then the UI people get tired of waiting for the full stack to load on page refreshes, so you have to edit past migrations…

With Data Migrate, you have the control. You can generate your migrations as schema or data as you would as your work flow. For setting tasks that don’t require any intermediate AR activity, like dev and test, you stick with db:migrate. For your prod, and qa, you change their scripts to db:migrate:with_data. Of course you want to test your migration, so you have the choice of db:migrate:with_data or data:migrate to just capture that data change.

So, what’s it going to do?

Once installed you will have a new table that tracks ‘data_migrations’ in the same way Rails handles ‘schema_migrations’ for you. That’s a good thing to point out about my language. When I say ‘data’, I mean just that. Mucking with the models as it were. By ‘schema’, I mean structural changes to the database – new tables, changed columns, etc.

As you use the gem, data migrations are stored in db/data. They act like schema migrations, except they should be reserved for data migrations. For instance, if you realize you need to titleize all yours titles, this is the place to do it.

Data migrations can be created at the same time as schema migrations, or independently. Database (db:) tasks have been added and extended to run on data migrations only, or in conjunction with the schema migration. For instance, `rake db:migrate:with_data` will run both schema and data migrations in the proper order. Generally, db.???.:with_data runs migrations that are aware of data and schema. I am not sure how I feel about the :with_data, and am open to ideas. Tasks prefixed ‘data’ work only with data:migrations.

Note: If a data and schema migration share the same version number, schema gets precedence when migrating up. Data does down.

Still with me? Awesome. Lets dive into my re-hashing of the read me.


The obligatory Bundler shout-out:

  # Gemfile...
  gem 'data_migrate'

After adding Data Migrate to your project,

  rails g data_migrate:install
  rake db:migrate

A table ‘data_migrations’ table will be generated. You are good to go.


Generating Migrations

You can generate a data migration as you would a schema migration:

> rails g data_migration add_this_to_that

    create  db/migrate/20110502022048_add_this_to_that.rb
    create  db/data/20110502022048_add_this_to_that.rb

By default, the migration also generates a schema migration by the same name.
This allows you to do things like:

> rails g data_migration add_this_to_that this:string

If you need a data only migration, either run it as such, with the skip-schema-migration flag:

> rails g data_migration add_this_to_that --skip-schema-migration

    create  db/data/20110502022048_add_this_to_that.rb

Rake Tasks

The list of rake tasks I keep mentioning. You might even find tasks you didn’t know you had.

> rake -T data
    rake data:forward                 # Pushes the schema to the next version (specify steps w/ STEP=n).
    rake data:migrate:down            # Runs the 'down' for a given migration VERSION.
    rake data:migrate:redo            # Rollbacks the database one migration and re migrate up (options: STEP=x, VERSION=x).
    rake data:migrate:status          # Display status of data migrations
    rake data:migrate:up              # Runs the 'up' for a given migration VERSION.
    rake data:rollback                # Rolls the schema back to the previous version (specify steps w/ STEP=n).
    rake data:version                 # Retrieves the current schema version number for data migrations
    rake db:forward:with_data         # Pushes the schema to the next version (specify steps w/ STEP=n).
    rake db:migrate:data              # Migrate the database through scripts in db/data/migrate.
    rake db:migrate:down:with_data    # Runs the 'down' for a given migration VERSION.
    rake db:migrate:redo:with_data    # Rollbacks the database one migration and re migrate up (options: STEP=x, VERSION=x).
    rake db:migrate:status:with_data  # Display status of data and schema migrations
    rake db:migrate:up:with_data      # Runs the 'up' for a given migration VERSION.
    rake db:migrate:with_data         # Migrate the database data and schema (options: VERSION=x, VERBOSE=false).
    rake db:rollback:with_data        # Rolls the schema back to the previous version (specify steps w/ STEP=n).
    rake db:version:with_data         # Retrieves the current schema version numbers for data and schema migrations `

Things you might want to know, but aren’t in the task description:

‘data:?’ tasks work as they would with the ‘vanilla’ db version.

The ‘with_data’ addition to the ‘db’ tasks will run the task in the context of both the data and schema migrations.

`rake db:rollback:with_data` will check to see if it was a schema or data migration invoked last, and do that. Tasks invoked in that space also have an additional line of output, indicating if the action is performed on data or schema.

With ‘up’ and ‘down’, you can specify the option ‘BOTH’, which defaults to false. Using true, will migrate both the data and schema (in the desired direction) if they both match the version provided. Again, going up, schema is given precedence. Down its data.

For more example, assume you have the 2 files:

Running `rake db:migrate:up:with_data VERSION=20110419021211` would execute the ‘db/migrate’ version.
Running `rake db:migrate:up:with_data VERSION=20110419021211` would execute the ‘db/migrate’ version, followed by the ‘db/data’ version.

Going down instead of up would be the opposite.

`rake db:migrate:status:with_data` provides and additional column to indicate which type of migration.

Err. How’d you do that?

The short of it is I hijack the native migration functionality, and point it to my data directory and migrations table for its needs. For the ‘with_data’ tasks, I rewrote the Rails versions to handle both types.

The data migrations themselves should look familiar as they use the ‘self:up/down’ format you’ve grown to know and love.

And… done

Let me know if something here isn’t clear. Though, it all makes perfect sense in my head. You can check out the gem at Ruby Gems or the Git repo if you’d

PS A big thanks to Jeremy Durham for working on the idea with me, and providing guidance.


  1. Great this seems interesting.

    It would be great at some time to drop the dependance on ActiveRecord so that the gem can be used with any ORM.

    • Andrew
      | Permalink

      I agree. Currently, its leaning pretty heavy on both ActiveRecord and Rake, so I don’t think it’ll happen anytime soon. I’d love to get it there, so it could be used more broadly.

  2. Alan
    | Permalink

    It would be great if we could run these locally only, or have some which can run on a prod env. Is there any way to do this, or flag a data migration?

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