Ecto.Adapter.TestTransactions behaviour

Specifies the adapter test transactions API.

These adapter functions work by starting a transaction and storing the connection back in the pool with an open transaction. At the end of the test, the transaction is rolled back, reverting all data added during tests.

Note this approach only works if the connection pool has size of 1 and does not support any overflow.

Postgres example

The first step is to configure your database pool to have size of 1 and no max overflow. You set those options in your config/config.exs:

config :my_app, Repo,
  size: 1,
  max_overflow: 0

Since you don't want those options in your production database, we typically recommend to create a config/test.exs and add the following to the bottom of your config/config.exs file:

import_config "config/#{Mix.env}.exs"

Now with the test database properly configured, you can write transactional tests:

# All tests in this module will be wrapped in transactions
defmodule PostTest do
  # Tests that use the shared repository cannot be async
  use ExUnit.Case
  alias Ecto.Adapters.Postgres

  setup do
    Postgres.begin_test_transaction(TestRepo)

    on_exit fn ->
      Postgres.rollback_test_transaction(TestRepo)
    end
  end

  test "create comment" do
    assert %Post{} = TestRepo.insert(%Post{})
  end
end