A migration to add unique constraint to a combination of columns

add_index :people, [:firstname, :lastname, :dob], unique: true

You may want to add a constraint without an index. This will depend on what database you're using. Below is sample migration code for Postgres. (tracking_number, carrier) is a list of the columns you want to use for the constraint.

class AddUniqeConstraintToShipments < ActiveRecord::Migration
  def up
    execute <<-SQL
      alter table shipments
        add constraint shipment_tracking_number unique (tracking_number, carrier);
    SQL
  end

  def down
    execute <<-SQL
      alter table shipments
        drop constraint if exists shipment_tracking_number;
    SQL
  end
end

There are different constraints you can add. Read the docs

According to howmanyofme.com, "There are 46,427 people named John Smith" in the United States alone. That's about 127 years of days. As this is well over the average lifespan of a human being, this means that a DOB clash is mathematically certain.

All I'm saying is that that particular combination of unique fields could lead to extreme user/customer frustration in future.

Consider something that's actually unique, like a national identification number, if appropriate.

(I realise I'm very late to the party with this one, but it could help future readers.)