A :limit of Rails’ Migrations

Posted by Samuel Mullen in Blog, Database, Ruby on Rails

Mar
1

Migrations in Ruby on Rails use the “:limit” symbol to set the maximum length of the underlying field’s data type. Take for example, the following example migration:

create_table :things do |t|
  t.string :name, :limit => 32
  t.string :description
  t.timestamps
end

By default, Rails will create :description as data type “varchar(255)” and :name as “varchar(32)” in a MySQL database. But did you know you can set :limit to be greater than 255?

For whatever reason, many of us have gained the impression that 255 is the longest :string can be, but that just isn’t the case. If I wanted the :description field in the example above to be greater than 255, I could just define it as follows:

t.string :description, :limit => 1024

In fact, strings (i.e. varchars) in MySQL can hold up to 65,535 bytes of data.

The opinionated nature of Ruby on Rails is a great asset in most instances, but we have to be careful not to let its opinions :limit us.

Note: I’m pretty sure Rails sets the default limit of strings to be 255 for two reasons: 1) cross database compatability, and 2) MySQL’s InnoDB (utf-8) engine can’t index varchar fields exceeding 255 characters.