Added documentation regarding DataCase and made clear that

documentation/how-to/test-resources.livemd

@@ -284,3 +284,28 @@ Finished in 0.08 seconds (0.00s async, 0.08s sync)
 ```
 %{total: 7, failures: 0, excluded: 0, skipped: 0}
 ```
+
+## Testing AshPostgres
+
+Previous examples above use Ash.DataLayer.Ets for testing. This will not work if we use
+AshPostgres, and will need a different setup. We will use DataCase when we have to work
+with AshPostgres, as will be shown in the following tests and examples:
+
+
+```elixir
+# test/my_app/accounts_test.exs
+defmodule MyApp.AccountsTest do
+  use MyApp.DataCase
+
+  test "creates a user" do
+    assert {:ok, user} =
+             MyApp.Accounts.User
+             |> Ash.Changeset.for_create(:create, %{name: "James"})
+             |> Ash.create()
+
+    assert user.name == "James"
+  end
+end
+```
+
+For additional explanation about AshPostgres and DataCase usage, check (/documentation/development/testing.md)

documentation/topics/development/testing.md

@@ -29,6 +29,27 @@ config :ash, :disable_async?, true
 
 This ensures that Ash does not spawn tasks when executing your requests, which is necessary for doing transactional tests with `AshPostgres`.
 
+## DataCase usage
+
+When using `AshPostgres`, you should use `DataCase` instead of `ExUnit.Case`. Without DataCase, the database connection will fail, resulting in `DBConnection.OwnershipError`, and most of the doctests will break for the same reason. 
+
+```elixir
+# test/my_app/accounts_test.exs
+defmodule MyApp.AccountsTest do
+  use MyApp.DataCase
+
+  test "creates a user" do
+    # ...
+  end
+end
+```
+DataCase is a reusable setup module that automatically imports everything necessary from the Ecto library, so you don't have to write alias and import every test file. Before every test, it checks out the database connection, wrapping everything that the test does inside of a database transaction and rolling it back after the test ends, regardless of its result. Checking the connection is what helps us avoid the `DBConnection.OwnershipError`.
+
+For a practical look at writing tests, see [the how-to guide](/documentation/how-to/test-resources.livemd).
+
+For a further explanation on how the sandbox works:
+See [Ash-Postgres guide](https://hexdocs.pm/ash_postgres/readme.html)
+
 ## Missed notifications
 
 If you are using Ecto's transactional features to ensure that your tests all run in a transaction, Ash will detect that it had notifications to send (if you have any notifiers set up) but couldn't because it was still in a transaction. The default behavior when notifications are missed is to warn. However, this can get pretty noisy in tests. So we suggest adding the following config to your `config/test.exs`.