How can inform Spring Data REST about my desire for link relations of an embedded document? - spring-data-mongodb

Question

In MongoDB, I have a document collection named Customer which embeds customer-defined labels. My domain objects look like this (including Lombok annotations):
#Document(collection = "Customer")
#Getter
#Setter
public class Customer {
#Id
long id;
#Field("name")
String name;
#Field("labels")
List<CustomerLabel> labels;
}
#Getter
#Setter
public class CustomerLabel {
#Id
#Field("label_id")
long labelId;
#Field("label_name")
String labelName;
}
Right now, the response to GET /customers looks like this:
{
"_links": {
"self": {
"href": "http://localhost:8080/app/customers?page=&size=&sort="
}
},
"_embedded": {
"customers": [
{
"name": "Smith, Jones, and White",
"labels": [
{
"labelName": "General label for Smith, Jones, and White"
}
],
"_links": {
"self": {
"href": "http://localhost:8080/app/customers/285001"
}
}
}
]
},
"page": {
"size": 20,
"totalElements": 1,
"totalPages": 1,
"number": 0
}
}
I would like to "call out" the embedded labels document as a separate link relation, so that the response to GET /customers looks more like this:
{
"_links": {
"self": {
"href": "http://localhost:8080/app/customers?page=&size=&sort="
}
},
"_embedded": {
"customers": [
{
"name": "Smith, Jones, and White",
"_links": [
{
"labels": {
"href": "http://localhost:8080/app/customers/285001/labels"
},
{
"self": {
"href": "http://localhost:8080/app/customers/285001"
}
}]
}
]
},
"page": {
"size": 20,
"totalElements": 1,
"totalPages": 1,
"number": 0
}
}
How can I do this in my application?

Answer 1

I'd argue, this is sort of sub-optimal design. When working with MongoDB, the documents you model are basically aggregates in the terms of Domain Driven Design as the concepts align nicely (accepting eventual consistence between related aggregates / documents etc).
Repositories simulate collections of aggregates (read: documents, in the MongoDB) case. So having a repository for embedded documents (CustomerLabel in your case) doesn't make too much sense actually. Also, persisting both Customers and CustomerLabels into the same collection but also let the former embed the latter looks suspicious to me.
So this seems to be more of a MongoDB schema design question than on on how to expose the documents through Spring Data. That said, I am not quite sure you're really going to get a satisfying answer as - as I've indicated - the question you raise seems to mask a more fundamental challenge in your codebase.

Answer 2

I ended up implementing a ResourceProcessor which removes the labels from the Resource object returned from the Repository REST controller. It looks like this:
#Controller
#AllArgsConstructor(onConstructor = #__(#Inject))
class CustomerResourceProcessor implements ResourceProcessor<Resource<Customer>> {
private final #NonNull CustomerLinks customerLinks;
#Override
public Resource<Customer> process(Resource<Customer> resource) {
Customer customer = resource.getContent();
if (customer.getLabels() != null && !customer.getLabels().isEmpty()) {
resource.add(customerLinks.getLabelCollectionLink(resource));
customer.setLabels(null);
}
return resource;
}
}
Then I wrote a LabelController in the manner of the RESTBucks examples I've seen:
#ResponseBody
#RequestMapping(value = "/customers/{customerId}/labels", method = RequestMethod.GET)
Resources<LabelResource> labels(#PathVariable Long customerId) {
List<CustomerLabel> customerLabels = customerRepository.findLabelsByCustomerId(customerId);
return new Resources<>(resourceAssembler.toResources(customerLabels), linkTo(
methodOn(this.getClass()).labels(customerId)).withSelfRel());
}
And the findLabelsByCustomerId method on the CustomerRepository is a custom repository method implementation which only returns the labels field from Mongo.
It all works quite well with a minimal amount of code. The trick was figuring out exactly which code I needed to write. :)