Represents the schema of an SQL table in an abstract way. This class provides methods for manipulating the schema representation.

Inside migration files, the tobject in create_tableis actually of this type:

class SomeMigration < ActiveRecord::Migration
  def up
    create_table :foo do |t|
      puts t.class  # => "ActiveRecord::ConnectionAdapters::TableDefinition"

  def down

The table definitions The Columns are stored as a ColumnDefinition in the columnsattribute.

[RW] columns

An array of ColumnDefinition objects, representing the column changes that have been defined.

Class Public methods
# File activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb, line 67
def initialize(base)
  @columns = []
  @columns_hash = {}
  @base = base
Instance Public methods

Returns a ColumnDefinition for the column with name name.

# File activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb, line 89
def [](name)
Alias for: references
column(name, type, options = {})

Instantiates a new column for the table. The typeparameter 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 :stringand :textcolumns and number of bytes for :binaryand :integercolumns.

  • :default- The column's default value. Use nil for NULL.

  • :null- Allows or disallows NULLvalues in the column. This option could have been named :null_allowed.

  • :precision- Specifies the precision for a :decimalcolumn.

  • :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 :decimalcolumns:

  • 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 :precisionand :scalemay be used. Internal storage as strings. No default.

  • SQLite3: No restrictions on :precisionand :scale, but the maximum supported :precisionis 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/ :scalefor 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 columndirectly, 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 #timestamps that'll add created_atand updated_atas datetimes.

#references will add an appropriately-named _id column, plus a corresponding _type column if the :polymorphicoption is supplied. If :polymorphic is a hash of options, these will be used when creating the _typecolumn. 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' }
# File activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb, line 227
def column(name, type, options = {})
  name = name.to_s
  type = type.to_sym

  column = self[name] || new_column_definition(@base, name, type)

  limit = options.fetch(:limit) do
    native[type][:limit] if native[type].is_a?(Hash)

  column.limit     = limit
  column.precision = options[:precision]
  column.scale     = options[:scale]
  column.default   = options[:default]
  column.null      = options[:null]

Appends a primary key definition to the table definition. Can be called multiple times, but this is probably not a good idea.

# File activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb, line 84
def primary_key(name)
  column(name, :primary_key)
Also aliased as: belongs_to
# File activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb, line 264
def references(*args)
  options = args.extract_options!
  polymorphic = options.delete(:polymorphic)
  args.each do |col|
    column("#{col}_id", :integer, options)
    column("#{col}_type", :string, polymorphic.is_a?(Hash) ? polymorphic : options) unless polymorphic.nil?

Appends :datetimecolumns :created_atand :updated_atto the table.

# File activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb, line 258
def timestamps(*args)
  options = { :null => false }.merge(args.extract_options!)
  column(:created_at, :datetime, options)
  column(:updated_at, :datetime, options)

Returns a String whose contents are the column definitions concatenated together. This string can then be prepended and appended to to generate the final SQL to create the table.

# File activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb, line 277
def to_sql { |c| c.to_sql } * ', '
# File activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb, line 73
def xml(*args)
  raise NotImplementedError unless %w{
    sqlite mysql mysql2
  }.include? @base.adapter_name.downcase

  options = args.extract_options!
  column(args[0], :text, options)