Why does Spring-data-jdbc not save my Car object?

I'm playing around with spring-data-jdbc and discovered a problem, with I can't solve using Google.

No matter what I try to do, I just can't push a trivial object into the database (Bean1.java:25): carRepository.save(new Car(2L, "BMW", "5"));

Both, without one and with a TransactionManager +@Transactional the database (apparently) does not commit the record.

The code is based on a Postgres database, but you might also simply use a H2 below and get the same result.

Here is the (minimalistic) source code: https://github.com/bitmagier/spring-data-jdbc-sandbox/tree/stackoverflow-question

Can somebody tell me, why the car is not inserted into the database?

This is not related to transactions not working. Instead, it's about Spring Data JDBC considering your instance an existing instance that needs updating (instead of inserting).

You can verify this is the problem by activating logging for org.springframework.jdbc.core.namedparam.NamedParameterJdbcTemplate. You should see an update but no insert.

By default, Spring Data JDBC considers an entity as new when it has an id of an object type and a value of null or of a primitive type (e.g. int or long) and a value of 0.

You have the following options in order to make it work:

Set the id to null and configure your database schema so that it will automatically create a new value on insert. After the save your entity instance will contain the generated value from the database.

Note: Spring Data JDBC will set the id even if it is final in your entity.
Leave the id null and set it in a Before-Save listener to the desired value.
Let your entity implement Persistable. This allows you to control when an entity is considered new. You'll probably need a listener as well so you can let the entity know it is not new any longer.
Beginning with version 1.1 of Spring Data JDBC you'll also be able to use a JdbcAggregateTemplate to do a direct insert, without inspecting the id, see https://jira.spring.io/browse/DATAJDBC-282. Of course, you can do that in a custom method of your repository, as is done in this example: https://github.com/spring-projects/spring-data-examples/pull/441