til/elixir/unique-indexes-with-ecto.md

Unique Indexes With Ecto

You can create a unique index in a migration for one or more columns using the unique_index/3 function.

For example, if you are creating a join table for followers and want to ensure that duplicate follower entries are prevented, you may want to include a unique index like so:

create table(:followers) do
  add :followed_user, references(:users), null: false
  add :following_user, references(:users), null: false
end

create unique_index(:followers, [:followed_user, :following_user])

Keep in mind that unique_index/3 is a shorthand for index/3 when you set unique: true.