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How to properly do user favorites? JPA

I'm trying to create a favorites mapping and I'm somewhat confused on how to do it,
Here is my user class, for the sake of clarity it has only 2 bindings(1st one(file) - #OneToMany, 2nd one(favorite file) - #ManyToMany.
#OneToMany(mappedBy = "app_user")
#JsonIgnoreProperties("app_user")
private List<File> file;
#ManyToMany(mappedBy = "app_user")
#JsonIgnoreProperties("app_user")
private Set<File> favorite_file;
And I've just realized that intellij complains about it for some reason.
it tries to connect only to 1 field on the other side:
#JsonIgnoreProperties({"file", "profile"})
#ManyToOne
private AppUser app_user;
what would be a better way to do this? because Intellij complains about it and it works but only with native queries(since JPA claims that there is no difference between files and favorite files, but in native query there is a ManyToMany table so it works there)
My goal is to make users have favorite files distinct from files.

It tries to connect only to 1 field on the other side
Well with #OneToMany(mappedBy = "app_user") you explicitly tell it to map it to the same field.
Which doesn't make sense, because
why would you have the same relation represented twice in one entity
The relationship types don't match. You can't have one relationship be ManyToOne from the one side and in the inverse direction be ManyToMany
It looks you have some clarification to do on what you want to represent by your model.
I see multiple possible variants.
The general mapping in AppUser is correct
So an AppUser has multiple File instances but each File is only owned by a single AppUser (if at all).
But an AppUser favorites many files and each File might be favoured by many users.
#Entity
class AppUser {
#OneToMany(mappedBy = "ownedBy")
private List<File> files;
#ManyToMany(mappedBy = "favoredBy")
private Set<File> favoriteFiles;
}
#Entity
class File {
#ManyToOne
private AppUser ownedBy;
#ManyToMany
private Set<AppUser> favoredBy;
}
The important part is that the defining relationships (the one in File) are two distinct relationships.
I removed the JSON annotations, because they are independent of the discussed problem.
Note that it is an interesting discussion if the relationship even should be bidirectional, but that is a different topic.
The favourite files are a subset of the owned files.
Then a more fitting model would be to model favouriting as an attribute of the relationship between AppUser and File
#Entity
class AppUser {
#OneToMany
private List<FileRelation> files;
eFiles;
}
#Entity
class FileRelation {
#ManyToOne
File file;
boolean isFavourite;
}
#Entity
class File {
}
I made the relationships unidirectional and the overall relation between user and file a many to many.
You might want to adapt that to your needs.
Side remark: Please use standard Java naming conventions when coding in Java (no snake case for attributes). Otherwise you are seriously hurting other Java developers.

What is the proper way to create and validate an Entity Model and its DTO in a RESTful API?

I am developing my first RESTful API from scratch and with Spring Boot.
I have already created the endpoints, models and JPA repositories for "standalone" entities. But now that I started linking them together and after doing some research I got to the conclusion that I may have to create DTOs. I don't think everytime I'm creating a new Order with a POST request I should make the client send the whole Customer and Employee objects inside the request as nested objects of Order (if I am also wrong in this please let me know). I am thinking about creating a DTO by just replacing the class relations with just IDs.
This is how my entity is currently defined:
#Data
#Entity
#Table(name = "Orders")
public class Order {
#Id
#GeneratedValue(strategy = GenerationType.SEQUENCE)
private Long id;
#NotBlank
#NotNull
private String description;
#NotBlank
#NotNull
private Status status;
#NotNull
#ManyToOne
#JoinColumn(foreignKey = #ForeignKey(name = "employee_id_fk"))
private Employee employee;
#NotNull
#ManyToOne
#JoinColumn(foreignKey = #ForeignKey(name = "customer_id_fk"))
private Customer customer;
protected Order() {}
public Order(String description) {
this.description = description;
this.status = Status.IN_PROGRESS;
}
}
And my endpoint (this is what I must change):
#PostMapping("/orders")
ResponseEntity<EntityModel<Order>> createOrder(#Valid #RequestBody Order order) {
order.setStatus(Status.IN_PROGRESS);
Order newOrder = repository.save(order);
return ResponseEntity
.created(linkTo(methodOn(OrderController.class).getOrder(newOrder.getId())).toUri())
.body(assembler.toModel(newOrder));
}
Now, how should I validate the requests with this format?
Previously, as you can see, I would simply use #Valid and it would automatically get validated when the endpoint is called against the Order model. However, if I create the DTO, I would have to validate the DTO with the same methodology and duplicate all the annotations from its model (#NotNull, #NotBlank, etc.). Maybe I should validate the entity model after mapping it from the DTO but I don't know how straightforward that would be and whether that is a good practice of validating requests. I also can't remove the validations from the entity model because I'm using Hibernate to map them to tables.

Great questions!
I don't think everytime I'm creating a new Order with a POST request I should make the client send the whole Customer and Employee objects inside the request as nested objects of Order (if I am also wrong in this please let me know).
You're right. It's not because we can save bits and bytes (as it may look like), but because the lesser information you can ask from the client, the better the experience he/she would get (whether it's an external integrator or front-end/back-end application within the same company). Fewer amounts of data to encompass = easier to comprehend and less room for an error. It also makes your API cleaner from the design perspective. Is it possible to process your request without the field? Then it shouldn't be in your API.
Now, how should I validate the requests with this format? Previously, as you can see, I would simply use #Valid and it would automatically get validated when the endpoint is called against the Order model. However, if I create the DTO, I would have to validate the DTO with the same methodology and duplicate all the annotations from its model (#NotNull, #NotBlank, etc.).
You can also use #Valid to kick in validation for DTO inside the controller within the method mapped to endpoint. But as you mentioned correctly, all validated fields within DTO should be annotated with #NotNull, #NotBlank, etc. As a solution, to the "duplication" problem, you can create a base class and define all validations in there and inherit DTO and Entity from it. But please, don't do that!
Having the same fields and validation rules within DTO and Enity isn't considered duplication since they are separate concepts and each one of serves its specific purpose within its layer (DTO - top tier, Entity - most often lowest, Data tier). There are a lot of examples demonstrating it (e.g. here and here)
Maybe I should validate the entity model after mapping it from the DTO but I don't know how straightforward that would be and whether that is a good practice of validating requests.
It's a best practice to validate the request and a lot of projects are following it. In your example, it's very straightforward (direct mapping from DTO to Entity), but very often you would have a service layer that does some business logic before handing it off to a data layer (even in your example I recommend moving out your code from controller to a service layer). You don't want malformed request pass beyond the controller to handle it later with excessive if statements, null checks (that leads to a defensive code that's hard to follow and it's also error-prone).
Another note: you shouldn't sacrifice client experience and tell them or force yourself to add two more fields because it allows having one Object serving as DTO and Entity and simplifies development.
The last note: To map fields from DTO to Entity you can use one of the object mapper libraries.

How to maintain bi-directional relationships with Spring Data REST and JPA?

Working with Spring Data REST, if you have a OneToMany or ManyToOne relationship, the PUT operation returns 200 on the "non-owning" entity but does not actually persist the joined resource.
Example Entities:
#Entity(name = 'author')
#ToString
class AuthorEntity implements Author {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
Long id
String fullName
#ManyToMany(mappedBy = 'authors')
Set<BookEntity> books
}
#Entity(name = 'book')
#EqualsAndHashCode
class BookEntity implements Book {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
Long id
#Column(nullable = false)
String title
#Column(nullable = false)
String isbn
#Column(nullable = false)
String publisher
#ManyToMany(fetch = FetchType.LAZY, cascade = [CascadeType.ALL])
Set<AuthorEntity> authors
}
If you back them with a PagingAndSortingRepository, you can GET a Book, follow the authors link on the book and do a PUT with the URI of a author to associate with. You cannot go the other way.
If you do a GET on an Author and do a PUT on its books link, the response returns 200, but the relationship is never persisted.
Is this the expected behavior?

tl;dr
The key to that is not so much anything in Spring Data REST - as you can easily get it to work in your scenario - but making sure that your model keeps both ends of the association in sync.
The problem
The problem you see here arises from the fact that Spring Data REST basically modifies the books property of your AuthorEntity. That itself doesn't reflect this update in the authors property of the BookEntity. This has to be worked around manually, which is not a constraint that Spring Data REST makes up but the way that JPA works in general. You will be able to reproduce the erroneous behavior by simply invoking setters manually and trying to persist the result.
How to solve this?
If removing the bi-directional association is not an option (see below on why I'd recommend this) the only way to make this work is to make sure changes to the association are reflected on both sides. Usually people take care of this by manually adding the author to the BookEntity when a book is added:
class AuthorEntity {
void add(BookEntity book) {
this.books.add(book);
if (!book.getAuthors().contains(this)) {
book.add(this);
}
}
}
The additional if clause would've to be added on the BookEntity side as well if you want to make sure that changes from the other side are propagated, too. The if is basically required as otherwise the two methods would constantly call themselves.
Spring Data REST, by default uses field access so that theres actually no method that you can put this logic into. One option would be to switch to property access and put the logic into the setters. Another option is to use a method annotated with #PreUpdate/#PrePersist that iterates over the entities and makes sure the modifications are reflected on both sides.
Removing the root cause of the issue
As you can see, this adds quite a lot of complexity to the domain model. As I joked on Twitter yesterday:
#1 rule of bi-directional associations: don't use them… :)
It usually simplifies the matter if you try not to use bi-directional relationship whenever possible and rather fall back to a repository to obtain all the entities that make up the backside of the association.
A good heuristics to determine which side to cut is to think about which side of the association is really core and crucial to the domain you're modeling. In your case I'd argue that it's perfectly fine for an author to exist with no books written by her. On the flip side, a book without an author doesn't make too much sense at all. So I'd keep the authors property in BookEntity but introduce the following method on the BookRepository:
interface BookRepository extends Repository<Book, Long> {
List<Book> findByAuthor(Author author);
}
Yes, that requires all clients that previously could just have invoked author.getBooks() to now work with a repository. But on the positive side you've removed all the cruft from your domain objects and created a clear dependency direction from book to author along the way. Books depend on authors but not the other way round.

I faced a similar problem, while sending my POJO(containing bi-directional mapping #OneToMany and #ManyToOne) as JSON via REST api, the data was persisted in both the parent and child entities but the foreign key relation was not established. This happens because bidirectional associations need to be manually maintained.
JPA provides an annotation #PrePersist which can be used to make sure that the method annotated with it is executed before the entity is persisted. Since, JPA first inserts the parent entity to the database followed by the child entity, I included a method annotated with #PrePersist which would iterate through the list of child entities and manually set the parent entity to it.
In your case it would be something like this:
class AuthorEntitiy {
#PrePersist
public void populateBooks {
for(BookEntity book : books)
book.addToAuthorList(this);
}
}
class BookEntity {
#PrePersist
public void populateAuthors {
for(AuthorEntity author : authors)
author.addToBookList(this);
}
}
After this you might get an infinite recursion error, to avoid that annotate your parent class with #JsonManagedReference and your child class with #JsonBackReference. This solution worked for me, hopefully it will work for you too.

This link has a very good tutorial on how you can navigate the recursion problem:Bidirectional Relationships
I was able to use #JsonManagedReference and #JsonBackReference and it worked like a charm

I believe one can also utilize #RepositoryEventHandler by adding a #BeforeLinkSave handler to cross link the bidirectional relation between entities. This seems to be working for me.
#Component
#RepositoryEventHandler
public class BiDirectionalLinkHandler {
#HandleBeforeLinkSave
public void crossLink(Author author, Collection<Books> books) {
for (Book b : books) {
b.setAuthor(author);
}
}
}
Note: #HandlBeforeLinkSave is called based on the first parameter, if you have multiple relations in your equivalent of an Author class, the second param should be Object and you will need to test within the method for the different relation types.

Safe embedded entity with objectify

I have two entities.
#Entity
public class Recipe {
#Id
private Long id;
private List<Step> steps;
}
#Entity
public class Step {
#Id
private Long id;
private String instruction;
}
And the following Clound Endpoint
#ApiMethod(
name = "insert",
path = "recipe",
httpMethod = ApiMethod.HttpMethod.POST)
public Recipe insert(Recipe recipe) {
ofy().save().entities(recipe.getSteps()).now(); //superfluous?
ofy().save().entity(recipe).now();
logger.info("Created Recipe with ID: " + recipe.getId());
return ofy().load().entity(recipe).now();
}
I'm wondering how do I skip the step where I have to save the emebedded entity first. The Id of neither entity is set. I want objectify to automatically create those. But if don't save the embedded entity I get an exception.
com.googlecode.objectify.SaveException: Error saving com.devmoon.meadule.backend.entities.Recipe#59e4ff19: You cannot create a Key for an object with a null #Id. Object was com.devmoon.meadule.backend.entities.Step#589a3afb
Since my object structure will get a lot more complex, I need to find a way to skip this manual step.

I presume you are trying to create real embedded objects, not separate objects stored in the datastore and linked. Your extra save() is actually saving separate entities. You don't want that.
You have two options:
Don't give your embedded object an id. Don't give it #Entity and don't give it an id field (or at least eliminate #Id). It's just a POJO. 90% of the time, this is what people want with embedded objects.
Allocate the id yourself with the allocator, typically in your (non-default) constructor.
Assuming you want a true embedded entity with a real key, #2 is probably what you should use. Keep in mind that this key is somewhat whimsical since you can't actually load it; only the container object can be looked up in the datastore.
I suggest going one step further and never use automatic id generation for any entities ever. Always use the allocator in the (non-default) constructor of your entities. This ensures that entities always have a valid, stable id. If you always allocate the id before a transaction start, it fixes duplicate entities that can be created when a transaction gets retried. Populating null ids is just a bad idea all around and really should not have been added to GAE.

The concept of the embedded is that the embedded content is persisted inside the main entity.
Is this the behaviour you are trying to configure?
The default behaviour of a Collection (List) of #Entity annoted class is to refer them instead of embed them. As you current configuration, the List<Step> variable does not have any annotation to override the default configuration, which is a different entity related to another one.
The error you are getting is because Objectify, when it saves the recipe entity, is trying to get the key of each step to create the relationship (and save them in the recipe entity), but if the entity step is not saved yet on the datastore, does not have a key
If you are trying to persist the steps inside the recipe entity, you need to setup objectify like this
#Entity
public class Recipe {
#Id
private Long id;
private List<Step> steps;
}
public class Step {
private Long id;
private String instruction;
}
As you can see, I removed the #Id annotation (an embedded Entity does not require an ID because is inside another entity) and the #Entity from the Step class. With this configuration, Objectify save the step entities inside the recipe entity
Source: https://code.google.com/p/objectify-appengine/wiki/Entities#Embedded_Object_Native_Representation

Mapping JSON object to Hibernate entity

I'm going to start a project of a REST application managed with Spring and with Hibernate for my model.
I know that Spring allows you to get Java object from the HTTP Request (with #Consumes(JSON) annotation). Is there any conflict if this Java object is also a Hibernate entities? And is nested object working (like #ManyToOne relation)?

Maven dependency
The first thing you need to do is to set up the following Hibernate Types Maven dependency in your project pom.xml configuration file:
<dependency>
<groupId>com.vladmihalcea</groupId>
<artifactId>hibernate-types-52</artifactId>
<version>${hibernate-types.version}</version>
</dependency>
Domain model
Now, if you are using PostgreSQL, you need to use the JsonType from Hibernate Types.
In order to use it in your entities, you will have to declare it on either class level or in a package-info.java package-level descriptor, like this:
#TypeDef(name = "json", typeClass = JsonType.class)
And, the entity mapping will look like this:
#Type(type = "json")
#Column(columnDefinition = "json")
private Location location;
If you're using Hibernate 5 or later, then the JSON type is registered automatically by the Postgre92Dialect.
Otherwise, you need to register it yourself:
public class PostgreSQLDialect extends PostgreSQL91Dialect {
public PostgreSQL92Dialect() {
super();
this.registerColumnType( Types.JAVA_OBJECT, "json" );
}
}
The JsonType works with Oracle, SQL Server, PostgreSQL, MySQL, and H2 as well. Check out the project page for more details about how you can map JSON column types on various relational database systems.

Yes, this wouldn't be a problem and is actually a fairly common practice.
In the recent years I have come to realize that sometimes, however, it is not a good idea to always build your views based on your domain directly. You can take a look at this post:
http://codebetter.com/jpboodhoo/2007/09/27/screen-bound-dto-s/
It is also known as "Presentation Model":
http://martinfowler.com/eaaDev/PresentationModel.html
The idea behind that is basically the following:
Imagine you have the domain entry User, who looks like that :
#Entity
#Data
public class User {
#Id private UUID userId;
private String username;
#OneToMany private List<Permission> permissions;
}
Let's now imagine you have a view where you wanna display that user's name, and you totally don't care about the permissions. If you use your approach of immediately returning the User to the view, Hibernate will make an additional join from the Permissions table because event though the permissions are lazily loaded by default, there is no easy way to signal to the jackson serializer or whatever you are using, that you don't care about them in this particular occasion, so jackson will try to unproxy them (if your transaction is still alive by the time your object is put for json serialization, otherwise you get a nasty exception). Yes, you can add a #JsonIgnore annotation on the permissions field, but then if you need it in some other view, you are screwed.
That a very basic example, but you should get the idea that sometimes your domain model can't be immediately used to be returned to the presentation layer, due to both code maintainability and performance issues.

We were using such approach to simplify design and get rid of many dtos (we were abusing them too much). Basically, it worked for us.
However, in our REST model we were trying to do not expose other relations for an object as you can always create another REST resources to access them.
So we just put #JsonIgnore annotations to relations mappings like #OneToMany or #ManyToOnemaking them transient.
Another problem I see that if you still like to return these relations you would have to use Join.FETCH strategy for them or move transaction management higher so that transaction still exists when a response is serialized to JSON (Open Session In View Pattern).
On my opinion these two solutions are not so good.

You can map the json request without using any library at REST web-services (Jersy)
this sample of code:
This hibernate entity called book:
#Entity
#Table(name = "book", schema = "cashcall")
public class Book implements java.io.Serializable {
private int id;
private Author author; // another hibernate entity
private String bookName;
//setters and getters
}
This web-services function
#POST
#Produces(MediaType.APPLICATION_JSON)
#Consumes(MediaType.APPLICATION_JSON)
public String addBook(Book book) {
String bookName=book.getName();
return bookName;
}
This is sample json request:
{
"bookName" : "Head First Java",
"author" : {
"id" : 1
}
}

Since you are just starting, perhaps you could use Spring Data REST?
This is the project: http://projects.spring.io/spring-data-rest/
And here are some simple examples:
https://github.com/spring-projects/spring-data-book/tree/master/rest
https://github.com/olivergierke/spring-restbucks
As you can see in the examples, there are no extra DTOs beyond the #Entity annotated POJOs.