Make collection propertie render as relation instead of property in json HAL representaion - spring-data-rest

Question

I got hal formatted response as this:
{
"name": "Publisher A",
"bookPublishers": [
{
"publishedDate": "2019-07-12T08:19:04.583+0000",
"_links": {
"publisher": {
"href": "http://localhost:8080/api/publishers/1"
},
"book": {
"href": "http://localhost:8080/api/books/2"
}
}
},
{
"publishedDate": "2019-07-12T08:19:04.564+0000",
"_links": {
"publisher": {
"href": "http://localhost:8080/api/publishers/1"
},
"book": {
"href": "http://localhost:8080/api/books/1"
}
}
}
],
"_links": {
"self": {
"href": "http://localhost:8080/api/publishers/1"
},
"publisher": {
"href": "http://localhost:8080/api/publishers/1"
},
"friends": {
"href": "http://localhost:8080/api/publishers/1/friends"
},
"createdBy": {
"href": "http://localhost:8080/api/publishers/1/contact"
}
}
}
I see there property bookPublishers and also in links friends. Imho they should be both association links (see 2.4. Creating the Associations) where can I "put" another resources.
I would like to make spring render bookPublishers same as friends.
Sample project is here: https://github.com/luvarqpp/poc-springHalRelations
You can do:
git clone https://github.com/luvarqpp/poc-springHalRelations.git
cd poc-springHalRelations
mvn clean spring-boot:run
And than open http://localhost:8080/api
PS: Bonus question, what is easiest way to provide own relation for business logic, like relation "renameAuthor" for example.

Answer 1

For collection relationships, Spring Data will provide a link when a repository exists for the relevant type. Where no repository exists then the collection will be in-lined in the response, otherwise, how else will the client get the data.
Therefore, create a repository for your BookPublisher type.
Relevant documentation part citation:
the component responsible for creating the links to referenced entities (such as those objects under the _links property in the object’s JSON representation). It takes an #Entity and iterates over its properties, creating links for those properties that are managed by a Repository and copying across any embedded or simple properties.
You can also create a projection that would in-line the data when required. Clients could specify this projection in the request therefore preventing an additional server call.
e.g.
/publisher/1?projection=withBookPublishers.
https://docs.spring.io/spring-data/rest/docs/current/reference/html/#projections-excerpts.projections