Mapping-Example | This app requests a set of placemarks from the server | User Interface library

For calculating the actual frequencies, you're pretty much at a minimal example already!

To make many2many association work nicely with spring data rest things and provide HAL representation with correct things, you have first know a bit of JPA/Hibernate. There are two approaches (1 and 2 from question, as third is only shortcut for second one and working in Hibernate only.).

Both approaches are shown in proof of concept repository in tags branch. I use given repository to test various set-ups of project.

Approach 1, EmbeddedId. It does use hackish thing in FixConfig class in BackendIdConverter bean, where it uses bookRepository to get Book entity when it parses request id from url onto embedable id class.

Approach 2, IdClass. It is using plain Integers int its IdClass and does seem to be correct solution.

I think that first approach can be modified to work similarly as second, but I am not able to do it for now.

I would mark as solution any answer providing some insight for "why" it is like this.

Unidirectional or bi-directional, it all depends on your requirements. For example :- A and B are two entities : 1>A has one-one unidirectional relationship with B, This is a scenario where you do not need/want to load A when you are loading B. 2>If you want to load A when you are loading B , then you will require One-One bi- directional mapping.

I think you are looking for orphanRemoval = true. This specifies, that objects which are disconnected from the parent will be removed.

In your case the annotation would look something like this:

You shouldn't be using findOne(int) if you already know the ID either way. findOne returns a reference to an object of the target, not an Optional anymore. This is the reason why you can't use map()

Use findById which definitely returns an Òptional which you can then map.

It is possible, but you will need to implement new converter type. The one we have is using JSON. You can implement your own converter that accepts binary data. So, your configuration will looks similar to this:

I'm not sure if you can make it just by method name. But you definitely can use JPA query. Something like this: "SELECT b FROM Book b JOIN b.bookPublishers bp JOIN bp.publisher p WHERE p.id = ?1". and with not equal for the second case

You will need a to many association from Employee to User because User is a so called attributed association with attributes.

The User entity will have a composite ID as primary key.

Please find an excellent article about that topic from Thorben Jansen: https://www.thoughts-on-java.org/many-relationships-additional-properties/

Using a map would be a better approach. Using the Age as the key which corresponds to a counter. If the key exists increment the counter, otherwise insert the key and initiate the counter at 1.