Add Unique Indexes With Ecto as an elixir til

elixir/unique-indexes-with-ecto.md

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+# 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:
+
+```elixir
+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`](https://hexdocs.pm/ecto/Ecto.Migration.html#index/3) when you
+set `unique: true`.