Bit fields for ActiveRecord
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.
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
The gem is actively being tested with:
ActiveRecord versions 2.3.x, 3.0.x, 3.1.x, 3.2.x
MySQL, PostgreSQL and SQLite3 databases
Ruby 1.8.7, 1.9.2 and 1.9.3
$ rake gems:install # use sudo if necessary
$ 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).
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.
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
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#warpdrive_changed? Spaceship#shields Spaceship#shields? Spaceship#shields= Spaceship#shields_changed? Spaceship#electrolytes Spaceship#electrolytes? Spaceship#electrolytes= Spaceship#electrolytes_changed?
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.
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 gem tests
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
Please find out more about our work in our Devblog.
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