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, âpolygonâ 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 disallowsNULL
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
.
Examples
# 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 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 end
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" t.timestamps end
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_at
and updated_at
as datetimes.
#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. The :index
option will also create
an index, similar to calling add_index
. 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' end add_index :taggings, :tag_id, name: 'index_taggings_on_tag_id' add_index :taggings, [:tagger_id, :tagger_type]
Can also be written as follows using references:
create_table :taggings do |t| t.references :tag, index: { name: 'index_taggings_on_tag_id' } t.references :tagger, polymorphic: true, index: true t.references :taggable, polymorphic: { default: 'Photo' } end
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