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24 lines
748 B
Markdown
24 lines
748 B
Markdown
# Unique Indexes With Ecto
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You can create a unique index in a migration for one or more columns using
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the
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[`unique_index/3`](https://hexdocs.pm/ecto/Ecto.Migration.html#unique_index/3)
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function.
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For example, if you are creating a join table for `followers` and want to
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ensure that duplicate _follower_ entries are prevented, you may want to
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include a unique index like so:
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```elixir
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create table(:followers) do
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add :followed_user, references(:users), null: false
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add :following_user, references(:users), null: false
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end
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create unique_index(:followers, [:followed_user, :following_user])
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```
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Keep in mind that `unique_index/3` is a shorthand for
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[`index/3`](https://hexdocs.pm/ecto/Ecto.Migration.html#index/3) when you
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set `unique: true`.
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