FlagShihTzu

Bit fields for ActiveRecord

Project	FlagShihTzu
gem name	flag_shih_tzu
license	MIT
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version
dependencies
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homepage	https://github.com/pboling/flag_shih_tzu
documentation	http://rdoc.info/github/pboling/flag_shih_tzu/frames
author	Peter Boling
Spread ♡ⓛⓞⓥⓔ♡

Summary

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

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.
• Bitwise Operations are fast!

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.

What is a "Shih Tzu"?

Prerequisites

The gem is actively being tested with:

• MySQL, PostgreSQL and SQLite3 databases
• ActiveRecord versions 2.3.x, 3.0.x, 3.1.x, 3.2.x, 4.0.x, 4.1.x ;)
• Ruby 1.9.2, 1.9.3, 2.0.0, 2.1.2 (see .travis.yml for the matrix)
• Legacy Ruby 1.8.7 compatibility is in the 0.2.X branch

Installation

Rails 2.x

In environment.rb:

config.gem 'flag_shih_tzu'

Then:

$ rake gems:install # use sudo if necessary

Rails 3

In Gemfile:

gem 'flag_shih_tzu'

Then:

$ bundle install

Usage

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.

Defaults (Important)

• Due to the default of 0, all flags are initially set to "false").
• For a default of true it will probably be easier in the long run to negate the flag's meaning / name. ** Such as switched_on => switched_off
• If you really want a different, non-zero, default value for a flag column, proceed adroitly with a different sql default for the flag column.

Database Migration

I like to document the intent of the flags column in the migration when I can...

change_table :spaceships do |t|
  t.integer     :flags, :null => false, :default => 0 # flag_shih_tzu-managed bit field
  # Effective booleans which will be stored on the flags column:
  # t.boolean      :warpdrive
  # t.boolean      :shields
  # t.boolean      :electrolytes
end

Adding to the Model

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 incorrect results. That is why the plugin forces you to set them explicitly. The values are symbols for the flags being created.

Bit Fields: 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][bitfield].

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

This way, we can use [bitwise operators][bit_operation] 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: http://en.wikipedia.org/wiki/Bit_field

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 boolean patterned instance methods

Calling has_flags, as shown above on the 'features' column, creates the following instance methods on Spaceship:

Spaceship#warpdrive
Spaceship#warpdrive?
Spaceship#warpdrive=
Spaceship#not_warpdrive
Spaceship#not_warpdrive?
Spaceship#not_warpdrive=
Spaceship#warpdrive_changed?
Spaceship#all_warpdrives
Spaceship#selected_warpdrives
Spaceship#select_all_warpdrives
Spaceship#unselect_all_warpdrives
Spaceship#selected_warpdrives=
Spaceship#has_warpdrive?

Spaceship#shields
Spaceship#shields?
Spaceship#shields=
Spaceship#not_shields
Spaceship#not_shields?
Spaceship#not_shields=
Spaceship#shields_changed?
Spaceship#all_shields
Spaceship#selected_shields
Spaceship#select_all_shields
Spaceship#unselect_all_shields
Spaceship#selected_shields=
Spaceship#has_shield?

Spaceship#electrolytes
Spaceship#electrolytes?
Spaceship#electrolytes=
Spaceship#not_electrolytes
Spaceship#not_electrolytes?
Spaceship#not_electrolytes=
Spaceship#electrolytes_changed?
Spaceship#all_electrolytes
Spaceship#selected_electrolytes
Spaceship#select_all_electrolytes
Spaceship#unselect_all_electrolytes
Spaceship#selected_electrolytes=
Spaceship#has_electrolyte?

Callbacks and Validations

Optionally, you can set the :bang_methods option to true to also define the bang methods:

Spaceship#electrolytes!     # will save the bitwise equivalent of electrolytes = true on the record
Spaceship#not_electrolytes! # will save the bitwise equivalent of electrolytes = false on the record

which respectively enables or disables the electrolytes flag.

The :bang_methods does not save the records to the database, meaning it cannot engage validations and callbacks.

Alternatively, if you do want to save a flag to the database, while still avoiding validations and callbacks, use update_flag! which:

• sets a flag on a database record without triggering callbacks or validations
• optionally syncs the ruby instance with new flag value, by default it does not.

Example:

update_flag!(flag_name, flag_value, update_instance = false)

Generated class methods

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

Spaceship.flag_columns      # [:features, :crew]

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

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, or the similar query length maximum for PostgreSQL.

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 bitwise operation being done on the SQL side, this query can not use an index on the flags column.

Updating flag column by raw sql

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 using multiple flag manipulation sql statements in the same query probably will not have the desired effect (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.

Skipping flag column check

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

Running the gem tests

First, run some infrequent setup:

$ rvm use 2.0.0
$ bundle install
$ BUNDLE_GEMFILE='gemfiles/Gemfile.activerecord-3.0.x' bundle update --quiet
$ BUNDLE_GEMFILE='gemfiles/Gemfile.activerecord-3.1.x' bundle update --quiet
$ BUNDLE_GEMFILE='gemfiles/Gemfile.activerecord-3.2.x' bundle update --quiet
$ BUNDLE_GEMFILE='gemfiles/Gemfile.activerecord-4.0.x' bundle update --quiet
$ rvm use 2.1.2
$ bundle install
$ BUNDLE_GEMFILE='gemfiles/Gemfile.activerecord-3.2.x' bundle update --quiet
$ BUNDLE_GEMFILE='gemfiles/Gemfile.activerecord-4.0.x' bundle update --quiet
$ BUNDLE_GEMFILE='gemfiles/Gemfile.activerecord-4.1.x' bundle update --quiet

Then just:

$ bundle exec rake test:all

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

$ NOCOVER=true BUNDLE_GEMFILE='gemfiles/Gemfile.activerecord-4.1.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.:

$ NOCOVER=true DB=mysql bundle exec rake

Authors

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

Find out more about Peter Boling's work RailsBling.com.

Find out more about XING Devblog.

How you can help!

Take a look at the reek list, which is the file called REEK, and stat fixing things. Once you complete a change, run the tests. See "Running the gem tests".

If the tests pass refresh the reek list:

bundle exec rake reek > REEK

Follow the instructions for "Contributing" below.

Contributing

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Added some feature')
4. Push to the branch (git push origin my-new-feature)
5. Make sure to add tests for it. This is important so I don't break it in a future version unintentionally.
6. Create new Pull Request

Versioning

This library aims to adhere to Semantic Versioning 2.0.0. Violations of this scheme should be reported as bugs. Specifically, if a minor or patch version is released that breaks backward compatibility, a new version should be immediately released that restores compatibility. Breaking changes to the public API will only be introduced with new major versions.

As a result of this policy, you can (and should) specify a dependency on this gem using the Pessimistic Version Constraint with two digits of precision.

For example:

spec.add_dependency 'flag_shih_tzu', '~> 4.0'

2012 Change of Ownership and 0.3.X Release Notes

FlagShihTzu was originally a XING AG project. Peter Boling was a long time contributor and watcher of the project. In September 2012 XING transferred ownership of the project to Peter Boling. Peter Boling had been maintaining a fork with extended capabilities. These additional features become a part of the 0.3 line. The 0.2 line of the gem will remain true to XING's original. The 0.3 line aims to maintain complete parity and compatibility with XING's original as well. I will continue to monitor other forks for original ideas and improvements. Pull requests are welcome, but please rebase your work onto the current master to make integration easier.

More information on the changes for 0.3.X: pboling/flag_shih_tzu/wiki/Changes-for-0.3.x

License

MIT License

Copyright (c) 2012-3 Peter Boling of RailsBling.com

Copyright (c) 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:

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.