Testing uniqueness constraints for models

model_uniqueness_constraint.exs

defmodule Example.User do
  use Example.Web, :model

  schema "users" do
    field :email, :string

    timestamps()
  end

  @doc """
  Builds a changeset based on the `struct` and `params`.
  """
  def changeset(struct, params \\ %{}) do
    struct
    |> cast(params, [:email])
    |> validate_required([:email])
    |> unique_constraint(:email)
  end
end

defmodule Example.UserTest do
  use Example.ModelCase

  alias Example.User

  @valid_attrs %{
    email: "jdoe@example.com"
  }

  test "unique constraint on email" do
    User.changeset(%User{}, @valid_attrs)
    |> Repo.insert!

    result = 
      User.changeset(%User{}, @valid_attrs)
      |> Repo.insert

    assert {:error, _changeset} = result
  end
end

# Make sure to add a unique index to the email field if you don't already have one
defmodule Example.Repo.Migrations.AddUniqueIndexToUsers do
  use Ecto.Migration

  def change do
    create unique_index(:users, [:email])
  end
end