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Miguel is a tool for sane management of database schemas. It aims to help with these goals:

  • Have just one up-to-date description of the desired database schema using a concise DSL.
  • Apply that schema to the database anytime, no matter how either may have diverged.
  • Adjust and repeat as often as needed.

To achieve this, it provides the following features:

  • Sequel-like DSL for schema description with some enhancements.
  • Load schema from given description file or from given database.
  • Show changes necessary to turn one schema into another.
  • Render those changes as Sequel's change or up/down migrations.
  • Alternatively apply those changes directly to the database.

Describing the schema

The schema is described using a DSL similar to Sequel's standard schema syntax. It looks like this:

# Example schema for Miguel.
Miguel::Schema.define do

  # The user, the core of every web site.

  table :users do
    primary_key :id

    # The login id, usually user email address.
    String :login
    # The encrypted password.
    String :password

    # First name(s).
    String :first_name
    # Last name(s).
    String :last_name

    # Arbitrary JSON encoded info.
    Text? :info


    unique :login
    index :first_name
    index :last_name
    index :create_time

  # User's emails, as every user can have multiple emails.

  table :user_emails do
    primary_key :id

    # The email address itself.
    String :email
    # To which user does the email belong.
    foreign_key :user_id, :users

    # Flag set when this email is verified.
    False :verified
    # Flag set when this email is marked as their primary email by the user.
    False :primary


    unique :email
    index :user_id
    index :create_time

  # User's profile, collecting various info about the user.

  table :user_profiles do
    primary_key :user_id
    foreign_key [:user_id], :users

    String? :company
    String? :street
    String? :city
    String? :state
    String? :country
    String? :zip
    String? :phone
    String? :fax
    String? :url

    index :country
    index :state

  # User's followers.

  join_table :user_id, :users, :follower_id, :users, :user_followers


One enhancement is that it allows you to define NULL columns simply by adding ? to the type name. Anything else is implicitly NOT NULL, which is a really wise default for many reasons.

Another enhancement is that it allows you to set defaults and define custom shortcuts for types which you use frequently. See documentation of the set_defaults method for details. The preset defaults are like this:

set_defaults :global, null: false

set_defaults :Bool, :TrueClass
set_defaults :True, :TrueClass, default: true
set_defaults :False, :TrueClass, default: false
set_defaults :Signed, :integer, unsigned: false
set_defaults :Unsigned, :integer, unsigned: true
set_defaults :String, text: false
set_defaults :Text, :String, text: true
set_defaults :Time, :timestamp, default: '2000-01-01 00:00:00'
set_defaults :Time?, :timestamp, default: nil

set_defaults :unique, :index, unique: true

set_defaults :Key, :integer, unsigned: false
set_defaults :primary_key, type: :integer, unsigned: false
set_defaults :foreign_key, key: :id, type: :integer, unsigned: false

If you prefer unsigned keys instead and your database engine supports it, you can pass the unsigned_keys: true option to Schema.define to make it happen. If you don't want any of these defaults set up for you, pass the use_defaults: false option to define instead.

Finally, the timestamps helper can be used to create the create_time and update_time timestamps for you. If you pass the mysql_timestamps: true option to define, the update_time timestamp will have the MySQL auto-update feature enabled, and timestamps will use the '0000-00-00 00:00:00' default by default. The latter can be also enabled and disabled explicitly by setting the zero_timestamps option to true or false, respectively.

Using the command

Using the command should be pretty straightforward. Try miguel -h and follow the examples. You can basically:

  • show - show schema loaded from given .rb file or from given database.
  • dump - dump migration which creates such schema.
  • down - dump migration which reverses given schema entirely.
  • diff - dump migration for migrating from one schema to another.
  • apply - apply given schema to given database.
  • clear - entirely wipe out schema of given database.

You don't have to worry about changing things accidentally, the command will always ask for a confirmation before changing anything in the database (unless you use the --force option).

Databases can be specified either by their Sequel URL like sqlite://test.db or mysql2://user:password@localhost/main, or by the common database .yml config file:

# Example db.yml.
adapter: mysql2
user: dev
password: sup3rsecr3t
host: localhost
database: main
encoding: utf8

Note that you can use the --env option to specify an environment other than development if your .yml contains configs for multiple environments.

Use the --migration <format> option to choose how you want the migration displayed. The bare format (the default) shows just the changes themselves, the change format creates the one-way Sequel's change migration, relying on Sequel's ability to reverse it, while the full format creates the two-way Sequel's up/down migration.

It's up to you if you will use diff each time to create the migration files for you, amend them if needed, and then let the sequel command use them normally, or if you will just apply the schema directly and rely on your VCS to keep its previous versions for you, leaving dozens of piecewise migration files finally behind.


The database specific type support is geared towards MySQL and SQLite. Postgres is supported as well, but note that it lacks support for some common types (e.g., unsigned integers) compared to other databases. Generic types should however work with any database, even though your mileage may vary.

Changing primary keys can be as problematic as with normal Sequel migrations, so it's best to set them once and stick with them.

It is currently not possible to describe renaming of columns or tables. If you need that, simply rename them directly in the database or by using standard Sequel migration, and adjust the schema description accordingly.


Copyright © 2015-2019 Patrik Rak

Miguel is released under the MIT license.


Database migrator and migration generator for Sequel.







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