column(name, type, options = {}) public

Instantiates a new column for the table. The type parameter is normally one of the migrations native types, which is one of the following: :primary_key, :string, :text, :integer, :float, :decimal, :datetime, :timestamp, :time, :date, :binary, :boolean.

You may use a type not in this list as long as it is supported by your database (for example, "polygon" in MySQL), but this will not be database agnostic and should usually be avoided.

Available options are (none of these exists by default):

  • :limit - Requests a maximum column length. This is number of characters for :string and :text columns and number of bytes for :binary and :integer columns.
  • :default - The column’s default value. Use nil for NULL.
  • :null - Allows or disallows NULL values in the column. This option could have been named :null_allowed.
  • :precision - Specifies the precision for a :decimal column.
  • :scale - Specifies the scale for a :decimal column.

For clarity’s sake: the precision is the number of significant digits, while the scale is the number of digits that can be stored following the decimal point. For example, the number 123.45 has a precision of 5 and a scale of 2. A decimal with a precision of 5 and a scale of 2 can range from -999.99 to 999.99.

Please be aware of different RDBMS implementations behavior with :decimal columns:

  • The SQL standard says the default scale should be 0, :scale <= :precision, and makes no comments about the requirements of :precision.
  • MySQL: :precision [1..63], :scale [0..30]. Default is (10,0).
  • PostgreSQL: :precision [1..infinity], :scale [0..infinity]. No default.
  • SQLite2: Any :precision and :scale may be used. Internal storage as strings. No default.
  • SQLite3: No restrictions on :precision and :scale, but the maximum supported :precision is 16. No default.
  • Oracle: :precision [1..38], :scale [-84..127]. Default is (38,0).
  • DB2: :precision [1..63], :scale [0..62]. Default unknown.
  • Firebird: :precision [1..18], :scale [0..18]. Default (9,0). Internal types NUMERIC and DECIMAL have different storage rules, decimal being better.
  • FrontBase?: :precision [1..38], :scale [0..38]. Default (38,0). WARNING Max :precision/:scale for NUMERIC is 19, and DECIMAL is 38.
  • SqlServer?: :precision [1..38], :scale [0..38]. Default (38,0).
  • Sybase: :precision [1..38], :scale [0..38]. Default (38,0).
  • OpenBase?: Documentation unclear. Claims storage in double.

This method returns self.


 # Assuming td is an instance of TableDefinition
 td.column(:granted, :boolean)
 # granted BOOLEAN

 td.column(:picture, :binary, :limit => 2.megabytes)
 # => picture BLOB(2097152)

 td.column(:sales_stage, :string, :limit => 20, :default => 'new', :null => false)
 # => sales_stage VARCHAR(20) DEFAULT 'new' NOT NULL

 td.column(:bill_gates_money, :decimal, :precision => 15, :scale => 2)
 # => bill_gates_money DECIMAL(15,2)

 td.column(:sensor_reading, :decimal, :precision => 30, :scale => 20)
 # => sensor_reading DECIMAL(30,20)

 # While <tt>:scale</tt> defaults to zero on most databases, it
 # probably wouldn't hurt to include it.
 td.column(:huge_integer, :decimal, :precision => 30)
 # => huge_integer DECIMAL(30)

 # Defines a column with a database-specific type.
 td.column(:foo, 'polygon')
 # => foo polygon

Short-hand examples

Instead of calling column directly, you can also work with the short-hand definitions for the default types. They use the type as the method name instead of as a parameter and allow for multiple columns to be defined in a single statement.

What can be written like this with the regular calls to column:

  create_table "products", :force => true do |t|
    t.column "shop_id",    :integer
    t.column "creator_id", :integer
    t.column "name",       :string,   :default => "Untitled"
    t.column "value",      :string,   :default => "Untitled"
    t.column "created_at", :datetime
    t.column "updated_at", :datetime

Can also be written as follows using the short-hand:

  create_table :products do |t|
    t.integer :shop_id, :creator_id
    t.string  :name, :value, :default => "Untitled"

There’s a short-hand method for each of the type values declared at the top. And then there’s TableDefinition#timestamps that’ll add created_at and updated_at as datetimes.

TableDefinition#references will add an appropriately-named _id column, plus a corresponding _type column if the :polymorphic option is supplied. If :polymorphic is a hash of options, these will be used when creating the _type column. So what can be written like this:

  create_table :taggings do |t|
    t.integer :tag_id, :tagger_id, :taggable_id
    t.string  :tagger_type
    t.string  :taggable_type, :default => 'Photo'

Can also be written as follows using references:

  create_table :taggings do |t|
    t.references :tag
    t.references :tagger, :polymorphic => true
    t.references :taggable, :polymorphic => { :default => 'Photo' }
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