How to test entities which have not null foreign key constraints

I have a dozen of tables like Product, Category, Customer, Order, ...
with not null relationships to one another. I created ORM and right now I am in the middle of tests.

However I find it pretty tedious, because in order to test for example Order entity which has to belong to Customer (namely perform persist operation) I have to create Customer instance as well. Lets go further: because Order cannot exist separately to Product I have to create Product and add it to Order. Product has to be in some Category, and so on. So you can see a chain of mandatory relationships, which makes testing individual entity very difficult.

Natural solution would be to defer constraint check to commit (Oracle):

alter session set constraints=deferred;

However I found a piece of information that Hibernate doesn't care of deferring (support for deferred constraint).

Does it mean, that persistence testing has to be so problematic or I can do it better/different?

I believe that db constraints are sacred, so resigning from them, because hibernate does not support it sounds bad.

You can create factories for test purposes. Their job is to create a graph of objects that are valid. Such class can be used in any kind of testing (not only the repositories):

ProductFactory.withType(...).withOtherImportantOption(...).create()

Such factory could leverage randomization if you wish, but it's not mandatory. Though some amount of randomization will have to be introduced for fields with unique constraints.

PS: not satisfying FK seems like a wrong path - you'll eventually need tests that persist many objects, maybe even commit transactions. And you may also want to test cascades.

Why not just create your test database prepopulated with a bunch of test data.

You can define test data in a file named import.sql, or specified via javax.persistence.sql-load-script-source, and it will be automatically loaded by the schema export tool.

UPDATE: To be clear, you only need a couple of rows of test data in each table, that you'll use in order to set up valid references from the objects you're testing.

For example, if you want to test persisting a Book, you would write:

Book b = new Book();
b.setTitle("Feersum Endjin");
b.setAuthor( em.getReference(Author.class, AUTHOR_ID) );
em.persist(b);

Where AUTHOR_ID is the id of a row of test data.

This is a lot better, IMO, than writing tests that work with objects in an inconsistent state (i.e. with null attributes) and test data that violates the database constraints.

If you're using JPA, then your repositories are tested automatically. If you really need to, then have a look here.

Hope this answers your question.