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As explained at https://hexdocs.pm/ecto_sqlite3/Ecto.Adapters.SQLite3.html#module-async-sandbox-testing: The Ecto SQLite3 adapter does not support async tests when used with Ecto.Adapters.SQL.Sandbox. This is due to SQLite only allowing up one write transaction at a time, which often does not work with the Sandbox approach of wrapping each test in a transaction.