instance method add_column

Add a new type column named column_name to table_name.

The type parameter is normally one of the migrations native types, which is one of the following: :primary_key, :string, :text, :integer, :bigint, :float, :decimal, :numeric, :datetime, :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 the number of characters for a :string column and number of bytes for :text, :binary and :integer columns. This option is ignored by some backends.

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

• :null - Allows or disallows NULL values in the column.

• :precision - Specifies the precision for the :decimal and :numeric columns.

• :scale - Specifies the scale for the :decimal and :numeric columns.

• :comment - Specifies the comment for the column. This option is ignored by some backends.

Note: The precision is the total number of significant digits, and 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.

• 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.

• SqlServer: :precision [1..38], :scale [0..38]. Default (38,0).

Examples

add_column(:users, :picture, :binary, limit: 2.megabytes)
# ALTER TABLE "users" ADD "picture" blob(2097152)

add_column(:articles, :status, :string, limit: 20, default: 'draft', null: false)
# ALTER TABLE "articles" ADD "status" varchar(20) DEFAULT 'draft' NOT NULL

add_column(:answers, :bill_gates_money, :decimal, precision: 15, scale: 2)
# ALTER TABLE "answers" ADD "bill_gates_money" decimal(15,2)

add_column(:measurements, :sensor_reading, :decimal, precision: 30, scale: 20)
# ALTER TABLE "measurements" ADD "sensor_reading" decimal(30,20)

# While :scale defaults to zero on most databases, it
# probably wouldn't hurt to include it.
add_column(:measurements, :huge_integer, :decimal, precision: 30)
# ALTER TABLE "measurements" ADD "huge_integer" decimal(30)

# Defines a column that stores an array of a type.
add_column(:users, :skills, :text, array: true)
# ALTER TABLE "users" ADD "skills" text[]

# Defines a column with a database-specific type.
add_column(:shapes, :triangle, 'polygon')
# ALTER TABLE "shapes" ADD "triangle" polygon