How do I make a column unique and index it in a Ruby on Rails migration?

Question

I would like to make a column unique in Ruby on Rails migration script. What is the best way to do it? Also is there a way to index a column in a table?

I would like to enforce unique columns in a database as opposed to just using :validate_uniqueness_of.

Answer 1

The short answer for old versions of Rails (see other answers for Rails 4+):

add_index :table_name, :column_name, unique: true 

To index multiple columns together, you pass an array of column names instead of a single column name,

add_index :table_name, [:column_name_a, :column_name_b], unique: true 

If you get "index name... is too long", you can add name: "whatever" to the add_index method to make the name shorter.

For fine-grained control, there's a "execute" method that executes straight SQL.

That's it!

If you are doing this as a replacement for regular old model validations, check to see how it works. The error reporting to the user will likely not be as nice without model-level validations. You can always do both.