= 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 then
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 use 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+.

== 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.

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>.

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 later):

* 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.  Some databases do not support
schema modifications made in transactions, and if the migration raises an error, 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.

== 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.

=== 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
the source control repository, unless the migration contains a 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

== Schema modification methods

Inside your migration's down and +up+ blocks is where you will call the +Database+ schema modification methods.
Here's a brief description of the most common schema modification methods:

=== +create_table+

+create_table+ is the most common schema modification method, and it's used for adding new tables
to the schema.  You provide it with the name of the table as a symbol, as well a block:

  create_table(:artists) do
    primary_key :id
    String :name
  end

Not that if you want a primary key for the table, you need to specify it, Sequel does not create one
by default.

==== Column types

Most method calls inside the create_table block will create columns, since +method_missing+ calls +column+
Columns are generally created by specifying the column type as the method
name, followed by the column name symbol to use, and after that any options that should be used.
If the method is a ruby class name that Sequel recognizes, Sequel will transform it into the appropriate
type for the given database.  So while you specified +String+, Sequel will actually use +varchar+ or
+text+ depending on the underlying database.  Here's a list of all of ruby classes that Sequel will
convert to database types:

  create_table(:columns_types) do       # common database type used
    Integer :a0                         # integer
    String :a1                          # varchar(255)
    String :a2, :size=>50               # varchar(50)
    String :a3, :fixed=>true            # char(255)
    String :a4, :fixed=>true, :size=>50 # char(50)
    String :a5, :text=>true             # text
    File :b,                            # blob
    Fixnum :c                           # integer
    Bignum :d                           # bigint
    Float :e                            # double precision
    BigDecimal :f                       # numeric
    BigDecimal :f2, :size=>10           # numeric(10)
    BigDecimal :f3, :size=>[10, 2]      # numeric(10, 2)
    Date :g                             # date
    DateTime :h                         # timestamp
    Time :i                             # timestamp
    Time :i2, :only_time=>true          # time
    Numeric :j                          # numeric
    TrueClass :k                        # boolean
    FalseClass :l                       # boolean
  end

Note that in addition to the ruby class name, Sequel also pays attention to the column options when
determining which database type to use.

Also note that this conversion is only done if you use a supported ruby class name.  In all other
cases, Sequel uses the type specified verbatim:

  create_table(:columns_types) do  # database type used
    string :a1                     # string
    datetime :a2                   # datetime
    blob :a3                       # blob
    inet :a4                       # inet
  end

In addition to specifying the types as methods, you can use the +column+ method and specify the types
as the second argument, either as ruby classes, symbols, or strings:

  create_table(:columns_types) do  # database type used
    column :a1, :string            # string
    column :a2, String             # varchar(255)
    column :a3, 'string'           # string
    column :a4, :datetime          # datetime
    column :a5, DateTime           # timestamp
    column :a6, 'timestamp(6)'     # timestamp(6)
  end

==== Column options

When using the type name as method, the third argument is an options hash, and when using the +column+
method, the fourth argument is the options hash.  The following options are supported:

:default :: The default value for the column.
:index :: Create an index on this column.
:null :: Mark the column as allowing NULL values (if true),
         or not allowing NULL values (if false).  If unspecified, will default
         to whatever the database default is.
:size :: The size of the column, generally used with string
         columns to specify the maximum number of characters the column will hold.
         An array of two integers can be provided to set the size and the
         precision, respectively, of decimal columns.
:unique :: Mark the column as unique, generally has the same effect as
           creating a unique index on the column.
:unsigned :: Make the column type unsigned, only useful for integer
             columns.

==== Other methods

In addition to the +column+ method and other methods that create columns, there are a other methods that can be used:

==== +primary_key+

You've seen this one used already.  It's used to create an autoincrementing integer primary key column.

  create_table(:a0){primary_key :id}

If you want to create a primary key column that doesn't use an autoincrementing integer, you should
not use this method.  Instead, you should use the :primary_key option to the +column+ method or type
method:

  create_table(:a1){Integer :id, :primary_key=>true} # Non autoincrementing integer primary key
  create_table(:a2){String :name, :primary_key=>true} # varchar(255) primary key

If you want to create a composite primary key, you should call the +primary_key+ method with an
array of column symbols:

  create_table(:items) do
    Integer :group_id
    Integer :position
    primary_key [:group_id, :position]
  end 

If provided with an array, +primary_key+ does not create a column, it just sets up the primary key constraint.

==== +foreign_key+

+foreign_key+ is used to create a foreign key column that references a column in another table (or the same table).
It takes the column name as the first argument, the table it references as the second argument, and an options hash
as it's third argument.  A simple example is:

  create_table(:albums) do
    primary_key :id
    foreign_key :artist_id, :artists
    String :name
  end

+foreign_key+ accepts some specific options:

:deferrable :: Makes the foreign key constraint checks deferrable, so they aren't checked
               until the end of the transaction.
:key :: For foreign key columns, the column in the associated table
        that this column references.  Unnecessary if this column
        references the primary key of the associated table, at least
        on most databases.
:on_delete :: Specify the behavior of this foreign key column when the row with the primary key
              it references is deleted , can be :restrict, :cascade, :set_null, or :set_default.
:on_update :: Specify the behavior of this foreign key column when the row with the primary key
              it references modifies the value of the primary key, can be
              :restrict, :cascade, :set_null, or :set_default.

Like +primary_key+, if you provide +foreign_key+ with an array of symbols, it will not create a
column, but create a foreign key constraint:

  create_table(:artists) do
    String :name
    String :location
    primary_key [:name, :location]
  end
  create_table(:albums) do
    String :artist_name
    String :artist_location
    String :name
    foreign_key [:artist_name, :artist_location], :artists
  end

==== +index+

+index+ creates indexes on the table.  For single columns, calling index is the same as using the
<tt>:index</tt> option when creating the column:

  create_table(:a){Integer :id, :index=>true}
  # Same as:
  create_table(:a) do
    Integer :id
    index :id
  end

Similar to the +primary_key+ and +foreign_key+ methods, calling +index+ with an array of symbols
will create a multiple column index:

  create_table(:albums) do
    primary_key :id
    foreign_key :artist_id, :artists
    Integer :position
    index [:artist_id, :position]
  end

The +index+ method also accepts some options:

:name :: The name of the index (generated based on the table and column names if not provided).
:type :: The type of index to use (only supported by some databases)
:unique :: Make the index unique, so duplicate values are not allowed.
:where :: Create a partial index (only supported by some databases)

==== +unique+

The +unique+ method creates a unique constraint on the table.  A unique constraint generally
operates identically to a unique index, so the following three +create_table+ blocks are
pretty much identical:

  create_table(:a){Integer :a, :unique=>true}

  create_table(:a) do
    Integer :a
    index :a, :unique=>true
  end

  create_table(:a) do
    Integer :a
    unique :a
  end

Just like +index+, +unique+ can set up a multiple column unique constraint, where the
combination of the columns must be unique:

  create_table(:a) do
    Integer :a
    Integer :b
    unique [:a, :b]
  end

==== +full_text_index+ and +spatial_index+

Both of these create specialized index types supported by some databases.  They
both take the same options as +index+.

==== +constraint+

+constraint+ creates a named table constraint:

  create_table(:artists) do
    primary_key :id
    String :name
    constraint(:name_min_length){char_length(name) > 2}
  end
  
Instead of using a block, you can use arguments that will be handled similarly
to <tt>Dataset#filter</tt>:

  create_table(:artists) do
    primary_key :id
    String :name
    constraint(:name_length_range, :char_length.sql_function(:name)=>3..50)
  end
  
==== +check+

+check+ operates just like +constraint+, except that it doesn't take a name
and it creates an unnamed constraint

  create_table(:artists) do
    primary_key :id
    String :name
    check{char_length(name) > 2}
  end
  
=== +alter_table+

+alter_table+ is used to alter existing tables, changing their columns, indexes,
or constraints.  It it used just like +create_table+, accepting a block which
is instance_evaled, and providing its own methods:

==== +add_column+

One of the most common methods, +add_column+ is used to add a column to the table.
Its API is similar to that of +create_table+'s +column+ method, where the first
argument is the column name, the second is the type, and the third is an options
hash:

  alter_table(:albums) do
    add_column :copies_sold, Integer, :default=>0
  end

When adding a column, it's a good idea to provide a default value, unless you
want the value for all rows to be set to NULL.

==== +drop_column+

As you may expect, +drop_column+ takes a column name and drops the column.  It's
often used in the +down+ block of a migration to drop a column added in an +up+ block:

  alter_table(:albums) do
    drop_column :copies_sold
  end

==== +rename_column+

+rename_column+ is used to rename a column.  It takes the old column name as the first
argument, and the new column name as the second argument:

  alter_table(:albums) do
    rename_column :copies_sold, :total_sales
  end

==== +add_primary_key+

If you forgot to include a primary key on the table, and want to add one later, you
can use +add_primary_key+.  A common use of this is to make many_to_many association
join tables into real models:

  alter_table(:albums_artists) do
    add_primary_key :id
  end

Just like +create_table+'s +primary_key+ method, if you provide an array of symbols,
Sequel will not add a column, but will add a composite primary key constraint:

  alter_table(:albums_artists) do
    add_primary_key [:album_id, :artist_id]
  end

If you just want to take an existing single column and make it a primary key, call
+add_primary_key+ with an array with a single symbol:

  alter_table(:artists) do
    add_primary_key [:id]
  end

==== +add_foreign_key+

+add_foreign_key+ can be used to add a new foreign key column or constraint to a table.
Like +add_primary_key+, if you provide it with a symbol as the first argument, it
creates a new column:

  alter_table(:albums) do
    add_foreign_key :artist_id, :artists
  end

If you want to add a new foreign key constraint to an existing column, you provide an
array with a single element:

  alter_table(:albums) do
    add_foreign_key [:artist_id], :artists
  end

To set up a multiple column foreign key constraint, use an array with multiple column
symbols:

  alter_table(:albums) do
    add_foreign_key [:artist_name, :artist_location], :artists
  end

==== +add_index+

+add_index+ works just like +create_table+'s +index+ method, creating a new index on
the table:

  alter_table(:albums) do
    add_index :artist_id
  end

It accepts the same options as +create_table+'s +index+ method, and you can set up
a multiple column index using an array:

  alter_table(:albums_artists) do
    add_index [:album_id, :artist_id], :unique=>true
  end

==== +drop_index+

As you may expect, +drop_index+ drops an existing index:

  alter_table(:albums) do
    drop_index :artist_id
  end

Just like +drop_column+, it is often used in the +down+ block of a migration.

==== +add_full_text_index+, +add_spatial_index+

Corresponding to +create_table+'s +full_text_index+ and +spatial_index+ methods,
these two methods create new indexes on the table.

==== +add_constraint+

This adds a named constraint to the table, similar to +create_table+'s +constraint+
method:

  alter_table(:albums) do
    add_constraint(:name_min_length){char_length(name) > 2}
  end

There is no method to add an unnamed constraint, but you can pass nil as the first
argument of +add_constraint+ to do so.  However, it's not recommend to do that
as it is difficult to drop such a constraint.

==== +add_unique_constraint+

This adds a unique constraint to the table, similar to +create_table+'s +unique+
method.  This usually has the same effect as adding a unique index.

  alter_table(:albums) do
    add_unique_constraint [:artist_id, :name]
  end

==== +drop_constraint+

This method drops an existing named constraint:

  alter_table(:albums) do
    drop_constraint(:name_min_length)
  end

There is no database independent method to drop an unnamed constraint.  Generally, the
database will give it a name automatically, and you will have to figure out what it is.
For that reason, you should not add unnamed constraints that you ever might need to remove.

==== +set_column_default+

This modifies the default value of a column:

  alter_table(:albums) do
    set_column_default :copies_sold, 0
  end

==== +set_column_type+

This modifies a column's type.  Most databases will attempt to convert existing values in
the columns to the new type:

  alter_table(:albums) do
    set_column_type :copies_sold, Bignum
  end

You can specify the type as a string or symbol, in which case it is used verbatim, or as a supported
ruby class, in which case it gets converted to an appropriate database type.

==== +set_column_allow_null+

This changes the NULL or NOT NULL setting of a column:

  alter_table(:albums) do
    set_column_allow_null :artist_id, true    # NULL
    set_column_allow_null :copies_sold, false # NOT NULL
  end

=== Other +Database+ schema modification methods

<tt>Sequel::Database</tt> has many schema modification instance methods,
most of which are shortcuts to the same methods in +alter_table+.  The
following +Database+ instance methods just call +alter_table+ with a
block that calls the method with the same name inside the +alter_table+
block with all arguments after the first argument (which is used as
the table name):

* +add_column+
* +drop_column+
* +rename_column+
* +add_index+
* +drop_index+
* +set_column_default+
* +set_column_type+

For example, the following two method calls do the same thing:

  alter_table(:artists){add_column :copies_sold, Integer}
  add_column :artists, :copies_sold, Integer

There are some other schema modification methods that have no +alter_table+
counterpart:

==== +drop_table+

+drop_table+ takes multiple arguments and treats all arguments as a
table name to drop:

  drop_table(:albums_artists, :albums, :artists)

Note that when dropping tables, you may need to drop them in a specific order
if you are using foreign keys and the database is enforcing referential
integrity.  In general, you need to drop the tables containing the foreign
keys before the tables containing the primary keys they reference.

==== +rename_table+

You can rename an existing table using +rename_table+.  Like +rename_column+,
the first argument is the current name, and the second is the new name:

  rename_table(:artist, :artists)

==== <tt>create_table!</tt>

<tt>create_table!</tt> with the bang drops the table unconditionally (swallowing
any errors) before attempting to create it, so:

  create_table!(:artists)
    primary_key :id
  end

is the same as:

  drop_table(:artists) rescue nil
  create_table(:artists)
    primary_key :id
  end

It should not be used inside migrations, as if the table does not exist, it may
mess up the migration.

==== <tt>create_table?</tt>

<tt>create_table?</tt> with a question mark only creates the table if it does
not already exist, so:

  create_table!(:artists)
    primary_key :id
  end

is the same as:

  create_table(:artists)
    primary_key :id
  end unless table_exists?(:artists)

Like <tt>create_table!</tt>, it should not be used inside migrations.

==== +create_view+ and +create_or_replace_view+

These can be used to create views.  The difference between them is that
+create_or_replace_view+ will unconditionally replace an existing view of
the same name, while +create_view+ will probably raise an error.  Both methods
take the name as the first argument, and either an string or a dataset as the
second argument:

  create_view(:gold_albums, DB[:albums].filter{copies_sold > 500000})
  create_or_replace_view(:gold_albums, "SELECT * FROM albums WHERE copies_sold > 500000")

==== +drop_view+

+drop_view+ drops existing views.  Just like +drop_table+, it can accept multiple
arguments:

  drop_view(:gold_albums, :platinum_albums)

== 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
      DB[:albums_artists].insert([:album_id, :artist_id],
       DB[: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.
      DB[:albums_artists].
       group(:album_id).
       select{[album_id, max(artist_id).as(artist_id)]}.
       having{artist_id >  0}.
       all do |r|
         DB[: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 order in which things were done in the +down+ block is in
reverse order to how they were done 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 constraints,
so it dumps foreign key columns as plain integers.  If you are using any real
database features such as foreign keys, 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

== 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.
