Bit fields for ActiveRecord

An extension for ActiveRecord to store a collection of boolean attributes in a single integer column as a bit field.

github.com/xing/flag_shih_tzu

This gem lets you use a single integer column in an ActiveRecord model to store a collection of boolean attributes (flags). Each flag can be used almost in the same way you would use any boolean attribute on an ActiveRecord object.

The benefits:

• No migrations needed for new boolean attributes. This helps a lot if you have very large db-tables, on which you want to avoid ALTER TABLE whenever possible.

• Only the one integer column needs to be indexed.

Using FlagShihTzu, you can add new boolean attributes whenever you want, without needing any migration. Just add a new flag to the has_flags call.

And just in case you are wondering what a “Shih Tzu” is: en.wikipedia.org/wiki/Shih_Tzu

<img src=“https://secure.travis-ci.org/xing/flag_shih_tzu.png” />

The gem is actively being tested with:

• ActiveRecord versions 2.3.x, 3.0.x, 3.1.x

• MySQL, PostgreSQL and SQLite3 databases

• Ruby 1.8.7 and 1.9.2

In environment.rb:

config.gem 'flag_shih_tzu'

Then:

$ rake gems:install # use sudo if necessary

In Gemfile:

gem 'flag_shih_tzu'

Then:

$ bundle install

FlagShihTzu assumes that your ActiveRecord model already has an integer field to store the flags, which should be defined to not allow NULL values and should have a default value of 0 (which means all flags are initially set to false).

class Spaceship < ActiveRecord::Base
  include FlagShihTzu

  has_flags 1 => :warpdrive,
            2 => :shields,
            3 => :electrolytes
end

has_flags takes a hash. The keys must be positive integers and represent the position of the bit being used to enable or disable the flag. The keys must not be changed once in use, or you will get wrong results. That is why the plugin forces you to set them explicitly. The values are symbols for the flags being created.

As said, FlagShihTzu uses a single integer column to store the values for all the defined flags as a bit field.

The bit position of a flag corresponds to the given key.

This way, we can use bit operators on the stored integer value to set, unset and check individual flags.

              +---+---+---+                +---+---+---+
              |   |   |   |                |   |   |   |
Bit position  | 3 | 2 | 1 |                | 3 | 2 | 1 |
(flag key)    |   |   |   |                |   |   |   |
              +---+---+---+                +---+---+---+
              |   |   |   |                |   |   |   |
Bit value     | 4 | 2 | 1 |                | 4 | 2 | 1 |
              |   |   |   |                |   |   |   |
              +---+---+---+                +---+---+---+
              | e | s | w |                | e | s | w |
              | l | h | a |                | l | h | a |
              | e | i | r |                | e | i | r |
              | c | e | p |                | c | e | p |
              | t | l | d |                | t | l | d |
              | r | d | r |                | r | d | r |
              | o | s | i |                | o | s | i |
              | l |   | v |                | l |   | v |
              | y |   | e |                | y |   | e |
              | t |   |   |                | t |   |   |
              | e |   |   |                | e |   |   |
              | s |   |   |                | s |   |   |
              +---+---+---+                +---+---+---+
              | 1 | 1 | 0 | = 4 + 2 = 6    | 1 | 0 | 1 | = 4 + 1 = 5
              +---+---+---+                +---+---+---+

Read more about bit fields here: en.wikipedia.org/wiki/Bit_field

The default column name to store the flags is ‘flags’, but you can provide a custom column name using the :column option. This allows you to use different columns for separate flags:

has_flags 1 => :warpdrive,
          2 => :shields,
          3 => :electrolytes,
          :column => 'features'

has_flags 1 => :spock,
          2 => :scott,
          3 => :kirk,
          :column => 'crew'

Calling has_flags as shown above creates the following instance methods on Spaceship:

Spaceship#warpdrive
Spaceship#warpdrive?
Spaceship#warpdrive=
Spaceship#warpdrive_changed?

Spaceship#shields
Spaceship#shields?
Spaceship#shields=
Spaceship#shields_changed?

Spaceship#electrolytes
Spaceship#electrolytes?
Spaceship#electrolytes=
Spaceship#electrolytes_changed?

The following named scopes become available:

Spaceship.warpdrive         # :conditions => "(spaceships.flags in (1,3,5,7))"
Spaceship.not_warpdrive     # :conditions => "(spaceships.flags not in (1,3,5,7))"
Spaceship.shields           # :conditions => "(spaceships.flags in (2,3,6,7))"
Spaceship.not_shields       # :conditions => "(spaceships.flags not in (2,3,6,7))"
Spaceship.electrolytes      # :conditions => "(spaceships.flags in (4,5,6,7))"
Spaceship.not_electrolytes  # :conditions => "(spaceships.flags not in (4,5,6,7))"

If you do not want the named scopes to be defined, set the :named_scopes option to false when calling has_flags:

has_flags 1 => :warpdrive, 2 => :shields, 3 => :electrolytes, :named_scopes => false

In a Rails 3 application, FlagShihTzu will use scope internally to generate the scopes. The option on has_flags is still named :named_scopes however.

enterprise = Spaceship.new
enterprise.warpdrive = true
enterprise.shields = true
enterprise.electrolytes = false
enterprise.save

if enterprise.shields?
  ...
end

Spaceship.warpdrive.find(:all)
Spaceship.not_electrolytes.count
...

The following class methods may support you when manually building ActiveRecord conditions:

Spaceship.warpdrive_condition         # "(spaceships.flags in (1,3,5,7))"
Spaceship.not_warpdrive_condition     # "(spaceships.flags not in (1,3,5,7))"
Spaceship.shields_condition           # "(spaceships.flags in (2,3,6,7))"
Spaceship.not_shields_condition       # "(spaceships.flags not in (2,3,6,7))"
Spaceship.electrolytes_condition      # "(spaceships.flags in (4,5,6,7))"
Spaceship.not_electrolytes_condition  # "(spaceships.flags not in (4,5,6,7))"

These methods also accept a :table_alias option that can be used when generating SQL that references the same table more than once:

Spaceship.shields_condition(:table_alias => 'evil_spaceships') # "(evil_spaceships.flags in (2,3,6,7))"

While the default way of building the SQL conditions uses an IN() list (as shown above), this approach will not work well for a high number of flags, as the value list for IN() grows.

For MySQL, depending on your MySQL settings, this can even hit the ‘max_allowed_packet’ limit with the generated query.

In this case, consider changing the flag query mode to :bit_operator instead of :in_list, like so:

has_flags 1 => :warpdrive,
          2 => :shields,
          :flag_query_mode => :bit_operator

This will modify the generated condition and named_scope methods to use bit operators in the SQL instead of an IN() list:

Spaceship.warpdrive_condition     # "(spaceships.flags & 1 = 1)",
Spaceship.not_warpdrive_condition # "(spaceships.flags & 1 = 0)",
Spaceship.shields_condition       # "(spaceships.flags & 2 = 2)",
Spaceship.not_shields_condition   # "(spaceships.flags & 2 = 0)",

Spaceship.warpdrive     # :conditions => "(spaceships.flags & 1 = 1)"
Spaceship.not_warpdrive # :conditions => "(spaceships.flags & 1 = 0)"
Spaceship.shields       # :conditions => "(spaceships.flags & 2 = 2)"
Spaceship.not_shields   # :conditions => "(spaceships.flags & 2 = 0)"

The drawback is that due to the bit operator, this query can not use an index on the flags column.

If you need to do mass updates without initializing object for each row, you can use #set_flag_sql method on your class. Example:

Spaceship.set_flag_sql(:warpdrive, true) # "flags = flags | 1"
Spaceship.set_flag_sql(:shields, false)  # "flags = flags & ~2"

And then use it in:

Spaceship.update_all Spaceship.set_flag_sql(:shields, false)

Beware that having multiple flag manipulation sql statements probably will not bring required result (at least on sqlite3, not tested on other databases), so you _should not_ do this:

Spaceship.update_all "#{Spaceship.set_flag_sql(:shields, false)},#{
  Spaceship.set_flag_sql(:warpdrive, true)}"

General rule of thumb: issue only one flag update per update statement.

By default when you call has_flags in your code it will automatically check your database to see if you have correct column defined.

Sometimes this may not be a wanted behaviour (e.g. when loading model without database connection established) so you can set :check_for_column option to false to avoid it.

has_flags 1 => :warpdrive,
          2 => :shields,
          :check_for_column => false

First, make sure all required gems are installed:

$ bundle install

The default rake test task will run the tests against the currently locked ActiveRecord version (see Gemfile.lock):

$ bundle exec rake test

If you want to run the tests against all supported ActiveRecord versions:

$ bundle exec rake test:all

This will internally use bundler to load specific ActiveRecord versions before executing the tests (see gemfiles/), e.g.:

$ BUNDLE_GEMFILE='gemfiles/Gemfile.activerecord-3.1.x' bundle exec rake test

All tests will use an in-memory sqlite database by default. If you want to use a different database, see test/database.yml, install the required adapter gem and use the DB environment variable to specify which config from test/database.yml to use, e.g.:

$ DB=mysql bundle exec rake

Patryk Peszko, Sebastian Roebke, David Anderson, Tim Payton and a helpful group of contributors. Thanks!

Please find out more about our work in our tech blog.

The MIT License

Copyright © 2011 XING AG

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the “Software”), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

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