Add Unique Indexes With Ecto as an elixir til

README.md

@@ -7,7 +7,7 @@ variety of languages and technologies. These are things that don't really
 warrant a full blog post. These are mostly things I learn by pairing with
 smart people at [Hashrocket](http://hashrocket.com/).
 
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+_500 TILs and counting..._
 
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 - [Run ExUnit Tests In A Deterministic Order](elixir/run-exunit-tests-in-a-deterministic-order.md)
 - [Same Functions Should Be Grouped Together](elixir/same-functions-should-be-grouped-together.md)
 - [String Interpolation With Just About Anything](elixir/string-interpolation-with-just-about-anything.md)
+- [Unique Indexes With Ecto](elixir/unique-indexes-with-ecto.md)
 - [Updating Values In A Map](elixir/updating-values-in-a-map.md)
 - [Virtual Fields With Ecto Schemas](elixir/virtual-fields-with-ecto-schemas.md)
 - [Word Lists For Atoms](elixir/word-lists-for-atoms.md)

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`.