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A rails plugin to store a collection of boolean attributes in a single ActiveRecord column as a bit field.

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README.rdoc

FlagShihTzu

A rails plugin to store a collection of boolean attributes in a single ActiveRecord column as a bit field.

github.com/xing/flag_shih_tzu

This plugin 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 where 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 “Shih Tzu” means: en.wikipedia.org/wiki/Shih_Tzu

Prerequisites

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).

The plugin has been tested with Rails versions from 2.1 to 3.0 and MySQL, PostgreSQL and SQLite3 databases. It has been tested with ruby 1.9.2 (with Rails 3 only).

Installation

As plugin (Rails 2.x, Rails 3)

cd path/to/your/rails-project
./script/plugin install git://github.com/xing/flag_shih_tzu.git

As gem (Rails 3)

Add following line to your Gemfile:

gem 'flag_shih_tzu', '= 0.1.0.pre'

Make sure to install gem with bundler:

bundle install

Usage

Defining the flags

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.

Using a custom column name

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'

Generated instance methods

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

Spaceship#warpdrive
Spaceship#warpdrive?
Spaceship#warpdrive=
Spaceship#shields
Spaceship#shields?
Spaceship#shields=
Spaceship#electrolytes
Spaceship#electrolytes?
Spaceship#electrolytes=

Generated named scopes

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.

Examples for using the generated methods

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
...

How it stores the values

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

Support for manually building conditions

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))"

Choosing a query mode

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.

Running the plugin tests

  1. (Rails 3 only) Add mysql2, pg and sqlite3 gems to your Gemfile.

  2. Install flag_shih_tzu as plugin inside working Rails application.

  3. Modify test/database.yml to fit your test environment.

  4. If needed, create the test database you configured in test/database.yml.

Then you can run

DB=mysql|postgres|sqlite3 rake test:plugins PLUGIN=flag_shih_tzu

from your Rails project root or

DB=mysql|postgres|sqlite3 rake

from vendor/plugins/flag_shih_tzu.

Authors

Patryk Peszko, Sebastian Roebke, David Anderson and Tim Payton

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

Contributors

TobiTobes, Martin Stannard, Ladislav Martincik, Peter Boling, Daniel Jagszent, Thorsten Boettger, Darren Torpey, Joost Baaij and Musy Bite

License

The MIT License

Copyright © 2009 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:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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