Provide text column length on composite unique index creation

Question

I'm trying to create an index on a new table that requires uniqueness on the agency_id (integer) and the IP address (text). I know that I need to provide a length on the index for IP. But I'm having issues assigning the length to just the IP column.

def up
   create_table :whitelisted_ips do |t|
     t.integer :agency_id
     t.text :ip
     t.timestamps
   end

   add_index :whitelisted_ips, [:agency_id, :ip], unique: true, length: 15
end

This code returns the error

Mysql2::Error: Incorrect prefix key; the used key part isn't a string, the used length is longer than the key part, or the storage engine doesn't support unique prefix keys: CREATE UNIQUE INDEX `index_whitelisted_ips_on_agency_id_and_ip` ON `whitelisted_ips` (`agency_id`(15), `ip`(15))

because it is attempting to put the length on the integer field. Any help would be appreciated.

Answer 1

You should provide a hash for the length parameter instead:

add_index :whitelisted_ips, [:agency_id, :ip], unique: true, length: { ip: 15 }