How to create columns like "bigint" or "longtext" in Rails migrations, and what :limit means for column migrations
Rails understands a
:limit options when you create columns in a migration. Its meaning depends on the column type, and sometimes the supplied value.
The documentation states that
:limit sets the column length to the number of characters for
text columns, and to the number of bytes for
This is nice since you may want a
bigint column to store really long numbers in it. You can just create it by saying
integer, :limit => 8 and you'll be able to store 8 bytes of data.
Similarly, you can use it to get a
longtext column by applying the proper limits (65535 bytes would be a
text, up to 16777215 a
mediumtext, and up to 4294967295 a
Or if you know that you'll never store more than a certain value, you might want to choose smaller column sizes for performance (it won't matter until your project becomes big, so don't worry about it for now).
Integer column lengths,
bigint, and all that other stuff
Mind that some values have a special meaning for
create_table 'example' do |t| t.integer :int # int (4 bytes, max 2,147,483,647) t.integer :int1, :limit => 1 # tinyint (1 byte, -128 to 127) t.integer :int2, :limit => 2 # smallint (2 bytes, max 32,767) t.integer :int3, :limit => 3 # mediumint (3 bytes, max 8,388,607) t.integer :int4, :limit => 4 # int (4 bytes) t.integer :int5, :limit => 5 # bigint (8 bytes, max 9,223,372,036,854,775,807) t.integer :int8, :limit => 8 # bigint (8 bytes) t.integer :int11, :limit => 11 # int (4 bytes) end
The first case is probably not surprising, it's just the default: when you say
integer :my_column, you just get an
int(11) since its maximum value has 11 digits. You'll get the same when saying
:limit => 4 just because an
int holds up to 4 bytes of data.
However, mind that while you might think saying
:limit => 11 gives you something like a
superbigint or at least an exception, it will in fact create an
int column, which is smaller than a
These are values for MySQL. I don't know what happens on other database adapters.