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Herminio Torres
Herminio Torres

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How to use unique_index wisely to grasp our business logic

#ecto #elixir #erlang #phoenix

How do you ensure a unique index when the user tries to get more than one ticket for a paid conference? And the user gets multiple tickets for a free conference?

What are our goals here?

  • sent paid conference to Ticket.changeset/1 to ensure the status: :paid, create one.
  • sent free conference to Ticket.changeset/1 to ensure the status: :free, create many.

How can we apply these to our business logic:

create table(:tickets) do add :conference_id, references(:conferences), null: false add :user_id, references(:users), null: false add :status, :string, null: false, default: "free" end create unique_index(:tickets, [:conference_id, :user_id, :status], where: "status = 'paid'")
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Now, how can we test?

iex> Ticket.changeset(%{conference: %{is_paid: false}, user: %{...}, status: :free}) |> Repo.insert() [debug] QUERY OK {:ok, %Ticket%{id: 1, status: :free, conference_id: 1, user_id: 1} } iex> Ticket.changeset(%{conference: %{is_paid: false}, user: %{...}, status: :free}) |> Repo.insert() [debug] QUERY OK {:ok, %Ticket%{id: 2, status: :free, conference_id: 1, user_id: 1} } iex> Ticket.changeset(%{conference: %{is_paid: true}, user: %{...}, status: :paid}) |> Repo.insert() [debug] QUERY OK {:ok, %Ticket%{id: 3, status: :paid, conference_id: 2, user_id: 1} } iex> Ticket.changeset(%{conference: %{is_paid: true}, user: %{...}, status: :paid}) |> Repo.insert() [debug] QUERY ERROR ** (Ecto.ConstraintError)
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Awesome!

Reference

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