= Migrations

This guide is based on http://guides.rubyonrails.org/migrations.html

== Overview

Migrations make it easy to alter your database's schema in a systematic manner.
They make it easier to coordinate with other developers and make sure that
all developers are using the same database schema.

Migrations are optional, you don't have to use them.  You can always just
create the necessary database structure manually using Sequel's schema
modification methods or another database tool.  However, if you are dealing
with other developers, you'll have to send them all of the changes you are
making.  Even if you aren't dealing with other developers, you generally have
to make the schema changes in 3 places (development, testing, and
production), and it's probably easier to use the migrations system to apply
the schema changes than it is to keep track of the changes manually and
execute them manually at the appropriate time.

Sequel tracks which migrations you have already run, so to apply migrations
you generally need to run Sequel's migrator with <tt>bin/sequel -m</tt>:

  sequel -m path/to/migrations postgres://host/database

Migrations in Sequel use a very simple DSL via the <tt>Sequel.migration</tt>
method, and inside the DSL, use the <tt>Sequel::Database</tt> schema
modification methods such as +create_table+ and +alter_table+.
See the {schema modification guide}[link:files/doc/schema_modification_rdoc.html]
for details on the schema modification methods you can use.

== A Basic Migration

Here is a fairly basic Sequel migration:

  Sequel.migration do
    up do
      create_table(:artists) do
        primary_key :id
        String :name, :null=>false
      end
    end

    down do
      drop_table(:artists)
    end
  end

This migration has an +up+ block which adds an artist table with an integer primary key named id,
and a varchar or text column (depending on the database) named +name+ that doesn't accept +NULL+ values.
Migrations should include both up and +down+ blocks, with the +down+ block reversing
the change made by up.  However, if you never need to be able to migrate down
(i.e. you are one of the people that doesn't make mistakes), you can leave out
the +down+ block.  In this case, the +down+ block just reverses the changes made by up,
dropping the table.

You can simplify the migration given above by using a reversible migration with a +change+
block:

  Sequel.migration do
    change do
      create_table(:artists) do
        primary_key :id
        String :name, :null=>false
      end
    end
  end

The +change+ block acts exactly like an +up+ block.  The only difference is that
it will attempt to create a +down+ block for you, assuming that it knows how to
reverse the given migration.  The +change+ block can usually correctly reverse
the following methods:

* +create_table+
* +add_column+
* +add_index+
* +rename_column+
* +rename_table+
* +alter_table+ (supporting the following methods in the +alter_table+ block):
  * +add_column+
  * +add_constraint+
  * +add_foreign_key+ (with a symbol, not an array)
  * +add_primary_key+ (with a symbol, not an array)
  * +add_index+
  * +add_full_text_index+
  * +add_spatial_index+
  * +rename_column+

If you use any other methods, you should create your own +down+ block.

In normal usage, when Sequel's migrator runs, it runs the +up+ blocks for all
migrations that have not yet been applied.  However, you can use the <tt>-M</tt>
switch to specify the version to which to migrate, and if it is lower than the
current version, Sequel will run the +down+ block on the appropriate migrations.

You are not limited to creating tables inside a migration, you can alter existing tables
as well as modify data.  Let's say your artist database originally only included artists
from Sacramento, CA, USA, but now you want to branch out and include artists in any city:

  Sequel.migration do
    up do
      add_column :artists, :location, String
      self[:artists].update(:location=>'Sacramento')
    end

    down do
      drop_column :artists, :location
    end
  end

This migration adds a +location+ column to the +artists+ table, and sets the +location+ column
to <tt>'Sacramento'</tt> for all existing artists.  It doesn't use a default on the column,
because future artists should not be assumed to come from Sacramento.  In the +down+ block, it
just drops the +location+ column from the +artists+ table, reversing the actions of the up
block.

Note that when updating the +artists+ table in the update, a plain dataset is used, <tt>self[:artists]</tt>.
This looks a little weird, but you need to be aware that inside an up or +down+ block in a migration,
self always refers to the <tt>Sequel::Database</tt> object that the migration is being applied to.
Since <tt>Database#[]</tt> creates datasets, using <tt>self[:artists]</tt> inside the +up+ block creates
a dataset on the database representing all columns in the +artists+ table, and updates it to set the
+location+ column to <tt>'Sacramento'</tt>.  You should avoid referencing the <tt>Sequel::Database</tt>
object directly in your migration, and always use self to reference it, otherwise you may run into problems.

It is possible to use model classes inside migrations, as long as they are loaded into the ruby interpreter,
but it's a bad habit as changes to your model classes can then break old migrations, and this breakage is
often not caught until much later, such as when a new developer joins the team and wants to run all migrations
to create their development database.

== The +migration+ extension

The migration code is not technically part of the core of Sequel.  It's not loaded by default as it
is only useful in specific cases.  It is one of the built-in extensions, which receive the same
level of support as Sequel's core.

If you want to play with Sequel's migration tools without using the <tt>bin/sequel</tt> tool, you
need to load the migration extension manually:

  Sequel.extension :migration

== Schema methods

Migrations themselves do not contain any schema modification methods, but they make it easy to call
any of the <tt>Sequel::Database</tt> modification methods, of which there are many.  The main
ones are +create_table+ and +alter_table+, but Sequel also comes with numerous other schema
modification methods, most of which are shortcuts for +alter_table+ (all of these methods are
described in more detail in the {schema modification guide}[link:files/doc/schema_modification_rdoc.html]):

* add_column
* add_index
* create_view
* drop_column
* drop_index
* drop_table
* drop_view
* rename_table
* rename_column
* set_column_default
* set_column_type

These methods handle the vast majority of cross database schema modification SQL.  If you
need to drop down to SQL to execute some database specific code, you can use the +run+
method:

  Sequel.migration do
    up{run 'CREATE TRIGGER ...'}
    down{run 'DROP TRIGGER ...'}
  end

In this case, we are using { and } instead of do and end to define the blocks. Just as
before, the +run+ methods inside the blocks are called on the +Database+ object,
which just executes the code on the underlying database.

== Errors when running migrations

Sequel attempts to run migrations inside of a transaction if the database supports
transactional DDL statements.  On the databases that don't support transactional DDL
statements, if there is an error while running a migration, it will not rollback the
previous schema changes made by the migration.  In that case, you will
need to update the database by hand.

It's recommended to always run migrations on a test database and ensure they work
before running them on any production database.

== Transactions

You can manually specify to use transactions on a per migration basis.  For example,
if you want to force transaction use for a particular migration, call the transaction
method in the Sequel.migration block:

  Sequel.migration do
    transaction
    change do
      # ...
    end
  end

Likewise, you can disable transaction use via no_transaction:

  Sequel.migration do
    no_transaction
    change do
      # ...
    end
  end

This is necessary in some cases, such as when attempting to use CREATE INDEX CONCURRENTLY
on PostgreSQL (which supports transactional schema, but not that statement inside a
transaction).

You can also override the transactions setting at the migrator level, either by forcing
transactions even if no_transaction is set, or by disabling transactions all together:

  # Force transaction use
  Sequel::Migrator.run(DB, '/path/to/migrations/dir', :use_transactions=>true)

  # Disable use of transactions
  Sequel::Migrator.run(DB, '/path/to/migrations/dir', :use_transactions=>false)

== Migration files

While you can create migration objects yourself and apply them manually, most of the
benefit to using migrations come from using Sequel's +Migrator+, which is what the
<tt>bin/sequel -m</tt> switch does.  Sequel's +Migrator+ expects that each migration
will be in a separate file in a specific directory.  The <tt>-m</tt> switch requires an
argument be specified that is the path to the directory containing the migration files.
For example:

  sequel -m db/migrations postgres://localhost/sequel_test

will look in the <tt>db/migrations</tt> folder relative to the current directory,
and run unapplied migrations on the PostgreSQL database sequel_test running on localhost.

== Two separate migrators

Sequel actually ships with two separate migrators.  One is the +IntegerMigrator+, the other is
the +TimestampMigrator+.  They both have plusses and minuses:

=== +IntegerMigrator+

* Simpler, uses migration versions starting with 1
* Doesn't allow duplicate migrations
* Doesn't allow missing migrations
* Just stores the version of the last migration run
* Good for single developer or small teams with close
  communication
* Lower risk of undetected conflicting migrations
* Requires manual merging of simultaneous migrations

=== +TimeStampMigrator+

* More complex, use migration versions where the version should
  represent a timestamp
* Allows duplicate migrations (since you could have multiple in a given second)
* Allows missing migrations (since you obviously don't have one every second)
* Stores the file names of all applied migrations
* Good for large teams without close communication
* Higher risk of undected conflicting migrations
* Does not require manual merging of simultaneous migrations

=== Filenames

In order for migration files to work with the Sequel, they must be specified as follows:

  version_name.rb

where <tt>version</tt> is an integer and <tt>name</tt> is a string which should be a very brief
description of what the migration does.  Each migration class should contain 1 and only 1
call to <tt>Sequel.migration</tt>.

=== +IntegerMigrator+ Filenames

These are valid migration names for the +IntegerMigrator+:

  1_create_artists.rb
  2_add_artist_location.rb

The only problem with this naming format is that if you have more than 9 migrations, the 10th
one will look a bit odd:

  1_create_artists.rb
  2_add_artist_location.rb
  ...
  9_do_something.rb
  10_do_something_else.rb

For this reasons, it's often best to start with 001 instead of 1, as that means you don't need
to worry about that issue until the 1000th migration:

  001_create_artists.rb
  002_add_artist_location.rb
  ...
  009_do_something.rb
  010_do_something_else.rb

It should be fairly obvious, but migrations start at 1, not 0.  The migration version number 0
is important though, as it is used to mean that all migrations should be unapplied (i.e. all
+down+ blocks run).  In Sequel, you can do that with:

  sequel -m db/migrations -M 0 postgres://localhost/sequel_test

=== +TimestampMigrator+ Filenames

With the +TimestampMigrator+, the version integer should represent a timestamp, though this isn't strictly
required.

For example, for <tt>5/10/2010 12:00:00pm</tt>, you could use any of the following formats:

  # Date
  20100510_create_artists.rb

  # Date and Time
  20100510120000_create_artists.rb

  # Unix Epoch Time Integer
  1273518000_create_artists.rb

The important thing is that all migration files should be in the same format, otherwise when you
update, it'll be difficult to make sure migrations are applied in the correct order, as well as
be difficult to unapply some the affected migrations correctly.

The +TimestampMigrator+ will be used if any filename in the migrations directory has a version
greater than 20000101.  Otherwise, the +IntegerMigrator+ will be used.

You can force the use of the +TimestampMigrator+ in the API by calling TimestampMigrator.apply
instead of Migrator.apply.

=== How to choose

Basically, unless you need the features provided by the +TimestampMigrator+, stick with the
+IntegerMigrator+, as it is simpler and makes it easier to detect possible errors.

For a single developer, the +TimestampMigrator+ has no real benefits, so I would always recommend
the +IntegerMigrator+.  When dealing with multiple developers, it depends on the size of the
development team, the team's communication level, and the level of overlap between developers.

Let's say Alice works on a new feature that requires a migration at the same time Bob works
on a separate feature that requires an unrelated migration.  If both developers are committing
to their own private respositories, when it comes time to merge, the +TimestampMigrator+ will not
require any manually changes.  That's because Alice will have a migration such as
<tt>20100512_do_this.rb</tt> and Bob will have one such as <tt>20100512_do_that.rb</tt>.

If the +IntegerMigrator+ was used, Alice would have <tt>34_do_this.rb</tt> and Bob would have
<tt>34_do_that.rb</tt>.  When the +IntegerMigrator+ was used, it would raise an exception due to
the duplicate migration version.  The only way to fix it would be to renumber one of the two
migrations, and have the affected developer manually modify their database.

So for unrelated migrations, the +TimestampMigrator+ works fine.  However, let's say that the
migrations are related, in such a way that if Bob's is run first, Alice's will fail.  In this
case, the +TimestampMigrator+ would not raise an error when Bob merges Alice's changes, since
Bob ran his migration first.  However, it would raise an error when Alice runs Bob's migration,
and could leave the database in an inconsistant state if the database doesn't support transactional
schema changes.

With the +TimestampMigrator+, you are trading reliability for convenience.  That's possibly a valid
trade, especially if simultaneous related schema changes by separate developers are unlikely, but
you should give it some thought before using it.

== Modifying existing migrations

Just don't do it.

In general, you should not modify any migration that has been run on the database and been committed to
the source control repository, unless the migration contains an error that causes data loss.  As long
as it is possible to undo the migration without losing data, you should just add another migration
that undoes the actions of the previous bad migration, and maybe does the correct action afterward.

The main problem with modifying existing migrations is that you will have to manually modify any
databases that ran the migration before it was modified.  If you are a single developer, that may be
an option, but certainly if you have multiple developers, it's a lot more work.

== Creating a migration

Sequel doesn't come with generators that create migrations for you.  However, creating a migration
is as simple as creating a file with the appropriate filename in your migrations directory that
contains a <tt>Sequel.migration</tt> call.  The minimal do-nothing migration is:

  Sequel.migration{}

However, the migrations you write should contain an +up+ block that does something, and a +down+ block that
reverses the changes made by the +up+ block:

  Sequel.migration do
    up{...}
    down{...}
  end

or they should use the reversible migrations feature with a +change+ block:

  Sequel.migration do
    change{...}
  end

== What to put in your migration's +down+ block

It's usually easy to determine what you should put in your migration's +up+ block,
as it's whatever change you want to make to the database.  The +down+ block is
less obvious.  In general, it should reverse the changes made by the +up+ block, which means
it should execute the opposite of what the +up+ block does in the reverse order in which
the +up+ block does it.  Here's an example where you are switching from having a single
artist per album to multiple artists per album:

  Sequel.migration do
    up do
      # Create albums_artists table
      create_table(:albums_artists) do
        foreign_key :album_id, :albums
        foreign_key :artist_id, :artists
        index [:album_id, :artist_id], :unique=>true
      end

      # Insert one row in the albums_artists table
      # for each row in the albums table where there
      # is an associated artist
      self[:albums_artists].insert([:album_id, :artist_id],
       self[:albums].select(:id, :artist_id).exclude(:artist_id=>nil))

      # Drop the now unnecesssary column from the albums table
      drop_column :albums, :artist_id
    end
    down do
      # Add the foreign key column back to the artists table
      alter_table(:albums){add_foreign_key :artist_id, :artists}

      # If possible, associate each album with one of the artists
      # it was associated with.  This loses information, but
      # there's no way around that.
      self[:albums_artists].
       group(:album_id).
       select{[album_id, max(artist_id).as(artist_id)]}.
       having{artist_id >  0}.
       all do |r|
         self[:artists].
          filter(:id=>r[:album_id]).
          update(:artist_id=>r[:artist_id])
       end

      # Drop the albums_artists table
      drop_table(:albums_artists)
    end
  end

Note that the operations performed in the +down+ block are performed in the
reverse order of how they are performed in the +up+ block.  Also note how it
isn't always possible to reverse exactly what was done in the +up+ block.  You
should try to do so as much as possible, but if you can't, you may want to have
your +down+ block raise a <tt>Sequel::Error</tt> exception saying why the
migration cannot be reverted.

== Running migrations

You can run migrations using the +sequel+ command line program that
comes with Sequel.  If you use the <tt>-m</tt> switch, +sequel+ will
run the migrator instead of giving you an IRB session.  The <tt>-m</tt>
switch requires an argument that should be a path to a directory of migration
files:

  sequel -m relative/path/to/migrations postgres://host/database
  sequel -m /absolute/path/to/migrations postgres://host/database

If you do not provide a <tt>-M</tt> switch, +sequel+ will migrate to the latest
version in the directory.  If you provide a <tt>-M</tt> switch, it should specify
an integer version to which to migrate.

  # Migrate all the way down
  sequel -m db/migrations -M 0 postgres://host/database

  # Migrate to version 10 (IntegerMigrator style migrations)
  sequel -m db/migrations -M 10 postgres://host/database

  # Migrate to version 20100510 (TimestampMigrator migrations using YYYYMMDD)
  sequel -m db/migrations -M 20100510 postgres://host/database

Whether or not migrations use the +up+ or +down+ block depends on the version
to which you are migrating.  If you don't provide a <tt>-M</tt> switch, all
unapplied migrations will be migrated up.  If you provide a <tt>-M</tt>, it will
depend on which migrations that have been applied.  Applied migrations greater
than that version will be migrated down, while unapplied migrations less than
or equal to that version will be migrated up.

== Verbose migrations

By default, <tt>sequel -m</tt> operates as a well behaved command line utility
should, printing out nothing if there is no error.  If you want to see the SQL
being executed during a migration, as well as the amount of time that each
migration takes, you can use the <tt>-E</tt> option to +sequel+ to set up a
+Database+ logger that logs to +STDOUT+.  You can also log that same output to
a file using the <tt>-l</tt> option with a log file name.

== Using models in your migrations

Just don't do it.

It can be tempting to use models in your migrations, especially since it's easy
to load them at the same time using the <tt>-L</tt> option to +sequel+.  However,
this ties your migrations to your models, and makes it so that changes in your
models can break old migrations.

With Sequel, it should be easy to use plain datasets to accomplish pretty much
anything you would want to accomplish in a migration.  Even if you have to
copy some code from a model method into a migration itself, it's better than
having your migration use models and call model methods.

== Dumping the current schema as a migration

Sequel comes with a +schema_dumper+ extension that dumps the current schema of
the database as a migration to +STDOUT+ (which you can redirect to a file using
>).  This is exposed in the +sequel+ command line tool with the <tt>-d</tt> and
<tt>-D</tt> switches.  <tt>-d</tt> dumps the schema in database independent
format, while <tt>-D</tt> dumps the schema using a non-portable format, useful
if you are using nonportable columns such as +inet+ in your database.

Let's say you have an existing database and want to create a migration that
would recreate the database's schema:

  sequel -d postgres://host/database > db/migrations/001_start.rb

or using a nonportable format:

  sequel -D postgres://host/database > db/migrations/001_start.rb

The main difference between the two is that <tt>-d</tt> will use the type methods
with the database independent ruby class types, while <tt>-D</tt> will use
the +column+ method with string types.

Note that Sequel cannot dump constraints other than primary key and possibly
foreign key constraints constraints.  If you are using database features such
as constraints or triggers, you should use your database's dump and restore
programs instead of Sequel's schema dumper.

You can take the migration created by the schema dumper to another computer
with an empty database, and attempt to recreate the schema using:

  sequel -m db/migrations postgres://host/database

== Checking for Current Migrations

In your application code, you may want to check that you are up to date in
regards to migrations (i.e. you don't have any unapplied migrations).  Sequel
offers two separate methods to do that.  The first is Sequel::Migrator.check_current.
This method raises an exception if there are outstanding migrations that need to
be run.  The second is Sequel::Migrator.is_current?, which returns true if there
are no outstanding migrations, and false if there are outstanding migrations.

If you want to ensure that your application code is up to date, you may want to
add the following code after connecting to your database:

  Sequel.extension :migration
  Sequel::Migrator.check_current(DB, '/path/to/migrations')

This will cause your application to raise an error when you start it if you have
any outstanding migrations.

== Old-style migration classes

Before the <tt>Sequel.migration</tt> DSL was introduced, Sequel used classes
for Migrations:

  Class.new(Sequel::Migration) do
    def up
    end
    def down
    end
  end

or:

  class DoSomething < Sequel::Migration
    def up
    end
    def down
    end
  end

This usage is discouraged in new code, but will continue to be supported indefinitely.
It is not recommended to convert old-style migration classes to the <tt>Sequel.migration</tt>
DSL, but it is recommended to use the <tt>Sequel.migration</tt> DSL for all new migrations.
