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Let ActiveRecord serialize and cast your UUIDs to and from binary columns in your database.
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mysql-binuuid-rails lets you define attributes of a UUID type on your models by leveraging the Attributes API that has been available since Rails 5. By doing so, you can store your UUIDs as binary values in your database, and still be able to query using the string representations since the database will take care of the type conversion.

As the name suggests, it only supports MySQL. If you're on PostgreSQL, you can use UUIDs the proper way already.

If you were to store a UUID of 32 characters (without the dashes) as text in your database, it would cost you at least 32 bytes. And that only counts if every character only requires 1 byte. But that completely depends on your encoding. If every character requires 2 bytes, storing it would already cost you 64 bytes. And that's a lot, if you think about the fact that a UUID is only 128 bits.

Being 128 bits, a UUID fits precisely in a column of 16 bytes. Though it won't be really readable it sure saves up a lot of space and it's only 4x bigger than a 32-bit integer, or 2x bigger than a 64-bit integer.

Not to mention the space you'll be saving when you create an index on the column holding your UUID.


You know the drill, add this line to your gemfile:

gem 'mysql-binuuid-rails'


Using binary columns for UUIDs is very easy. There's only two steps you need to perform which are described here.

Adding the column to store your UUID

Suppose you have a model called Book to which you want to add a unique identifier in the form of a UUID. First, make sure your database is able to hold this attribute. So let's create a migration.

$ rails g migration AddUuidColumnToBooks

Open up the migration file and change it as you'd like:

class AddUuidColumnToBooks < ActiveRecord::Migration[5.1]
  def change
    # 'uuid' is the column name, and 'binary' is the column type. You have to
    # specify it as a binary column yourself. And because we know that a UUID
    # takes up 16 bytes, we set can specify its limit.
    add_column :books, :uuid, :binary, limit: 16

Perform the migration:

rails db:migrate

Tell your model how to handle the binary UUID column

All you have to do now, is specify in your Book model how Rails should handle the uuid column. Open up app/models/book.rb and simply add the following single line:

class Book < ApplicationRecord
  attribute :uuid, MySQLBinUUID::Type.new

Migrating from ActiveUUID

There's a couple of things you need to take into consideration when you're migrating from ActiveUUID to mysql-binuuid-rails.

Replace include ActiveUUID::UUID in your models

In your models where you did include ActiveUUID::UUID, you now have to specify the attribute which is a UUID instead:

class Book < ApplicationRecord
  attribute :uuid, MySQLBinUUID::Type.new

No uuid column in database migrations

ActiveUUID comes with a neat column type that you can use in migrations. Since mysql-binuuid-rails does not, you will have to change all migrations in which you leveraged on that migration column if you want your migrations to keep working for new setups.

The idea behind not providing a uuid type for columnns in migrations is that you are aware of what the actual type of the column is you're creating, and that it is not hidden magic.

It's pretty simple:

# Anywhere where you did this in your migrations...

create_table :books do |t|
  t.uuid :reference, ...

# ..you should change these kinds of lines into the kind described
# below. It's what ActiveUUID  did for you, but what you now have
# to do yourself.

create_table :books do |t|
  t.binary :reference, limit: 16, ...

No UUIDTools

ActiveUUID comes with UUIDTools. mysql-binuuid-rails does not. When you retrieve a UUID typed attribute from a model when using ActiveUUID, the result is a UUIDTools::UUID object. When you retrieve a UUID typed attribute from a model when using mysql-binuuid-rails, you just get a String of 36 characters (it includes the dashes).

Migrating shouldn't be that difficult though. UUIDTools::UUID implements #to_s, which returns precisely the same as mysql-binuuid-rails returns by default. But it's good to be aware of this in case you're running into weirdness.

Known issues

  • With Rails 5.0 in combination with uniqueness validations, ActiveRecord generates a wrong query. The x in front of the queried value, which casts the value to the proper data type, is missing.


To start coding on mysql-binuuid-rails, fork the project, clone it locally and then run bin/setup to get up and running. If you want to fool around in a console with the changes you made, run bin/console.

Bug reports and pull requests are welcome on GitHub at https://github.com/nedap/mysql-binuuid-rails


For the most recent major version of ActiveRecord, tests are run against the latest patch level of all minor versions. For earlier major versions, tests are run against the latest minor/patch.

Run tests yourself to verify everything is still working:

$ bundle exec rake




The gem is available as open source under the terms of the MIT License.