I have a one to many relationship namely A and B. A may refer to many B instances. B instances also can be managed independently.
That's why, B class looks like this:
public class B {
#Id
private String id;
private String appId;
}
A class will refer to a list of B instances. So it looks like this:
public class A {
#Id
private String id;
private int age;
private List<B> bInstances;
}
When bInstances are filled with B instances and then A instance is saved, id fields of the B instances are removed from the JSON document since it is annotated with #Id.
I simply need to add this field to JSON when B is embedded into another class.
And when B instance is saved independently, #Id field can be used as the regular key.
How may I do this?
To answer your question specifically, yes the field annotated #Id won't get written to the serialized json. Your best bet here is to duplicate the field and set its value in either the constructor or a setter, for example:
public class B {
#Id
private String metaId;
private String id;
private String appId;
public setMetaId(String metaId) {
this.metaId = metaId;
this.id = metaId;
}
}
You can #JsonIgnore one of them if you don't want to see the duplication in your serializations.
Note: I totally agree with Robin's answer, but there are cases where you want to save the data as-it-was and you want to refer to it in the future.. so you don't care if B instance changed one week after you saved the data. So it really depends on your scenario.
I would strongly advise you to not store the instances of B directly in a document of A. Doing this would result in data inconsistencies if an instance of B is changed in another context than A. Additionally you duplicate data unnecessarily if an instance of B is stored in multiple documents of class A.
The desired way to store instances of one class in another is to save the document ids instead. So your class A should look like this:
public class A {
#Id
private String id;
private int age;
private List<String> bInstanceIds;
}
Related
I see in most of the coders save data(using spring data) as:
savedEntity = repo.save(savedEntity);
Long id = savedEntity.getId();
I am confused about why most of them assign back the returned value to the saved Entity while the following code also works exact(I have tested myself):
repo.save(savedEntity);
Long id = savedEntity.getId();
Did I miss some benefit of assigning back?
for example, let the entity be:
#Entity
public class SavedEntity {
#Id
private int id;
private String name;
//getter, setters, all arg-constructor, and no-arg constructor
}
Consider the object of SavedEntity is
SavedEntity entity = new SavedEntity(1,"abcd");
now for your first question,
SavedUser entity1 = repo.save(entity);
Long id = entity1.getId();
this entity1 object is the return object getting from the database, which means the above entity is saved in the database succesfully.
for the Second Question,
repo.save(entity);
Long id = entity.getId();//which you got it from SavedEntity entity = new SavedEntity(1,"abcd");
here the value of id is the integer you mentioned in place of id(the raw value).
Most of the time the id (primary key) is generated automatically while storing the entity to the database using strategies like AUTO, Sequence etc. So as to fetch those id's or autogenerated primary key values we assign back the saved entity.
For example:
#Entity
public class Customer {
#Id
#GeneratedValue(strategy=GenerationType.AUTO)
private Long id;
private String firstName;
private String lastName;
}
In this case you'll not pass the id externally but it will create a value for it automatically while storing the data to DB.
I am using Spring Data's Rest Repositories from spring-boot-starter-data-rest, with Couchbase being used as the underlining DBMS.
My Pojo for the object is setup as so.
#Document
public class Item{
#Id #GeneratedValue(strategy = UNIQUE)
private String id;
#NotNull
private String name;
//other items and getters and setters here
}
And say the Item has an id of "xxx-xxx-xxx-xxx" and name of "testItem".
Problem is, that when I want to access the item, I need to be accessible by /items/testItem, but instead it is accessible by /items/xxx-xxx-xxx-xxx.
How do I get use its name instead of its generated id, to get the data.
I found out the answer to my own question.
I just need to override the config for the EntityLookup.
#Component
public class SpringDataRestCustomization extends RepositoryRestConfigurerAdapter {
#Override
public void configureRepositoryRestConfiguration(RepositoryRestConfiguration config) {
config.withEntityLookup().forRepository(UserRepository.class).
withIdMapping(User::getUsername).
withLookup(UserRepository::findByUsername);
}
}
Found the info here, though the method name changed slightly.
https://github.com/spring-projects/spring-data-examples/tree/master/rest/uri-customization
If you want query the item by name and want it perform as querying by id,you should make sure the name is unique too.You cant identify a explicit object by name if all objects have a same name,right?
With jpa you could do it like:
#NotNull
#Column(name="name",nullable=false,unique=true)
private String name;
I get an Post request that would give me a List<PersonApi> Objects
class PersonApi {
private String name;
private String age;
private String pincode ;
}
And I have an Entity Object named Person
#Entity
#Table(name = "person_master")
public class Person{
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
Long id;
#Column(name = "name")
String name;
#Column(name = "age")
String age;
#Column(name = "pincode ")
String pincode ;
}
My record from Post request would look something like this (pseudocode representation of the data below)
[
"Arun","33","09876gh"
"James","34","8765468"
]
I need to do a bulk-validation using Spring JPA.. Give the List<PersonApi> and get a True or False based on the condition that all the entries in the PersonApi objects list should be there in the database.
How to do this ?
The selected answer is not a right one. (not always right)
You are selecting the whole database to check for existence. Unless your use case is very special, i.e. table is very small, this will kill the performance.
The proper way may start from issuing repository.existsById(id) for each Person, if you never delete the persons, you can even apply some caching on top of it.
exists
Pseudo Code:
List<PersonApi> personsApiList = ...; //from request
List<Person> result = personRepository.findAll();
in your service class you can access your repository to fetch all database entities and check if your list of personapi's is completeley available.
boolean allEntriesExist = result.stream().allMatch(person -> personsApiList.contains(createPersonApiFromPerson(person)));
public PersonApi createPersonApiFromPerson(Person person){
return new PersonApi(person.getName(), person.getAge(), person.getPincode());
}
my problem can be broken down to this little example:
I have a entity class A and a entity class B. A has a List of B objects. Now there is always only one B relevant. So I do not want to load all B's of an A, only to access this one B (last inserted B inside a A).
The question: Can I manipulate an entity without an service, so that there is a #Transient variable, that is always the newest B? And also without saving the newest B separately in A. Is there a way to achieve this?
class B{
#Id
#GeneratedValue
private Long id;
#Column(nullable=false)
private String name;
#Column(nullable=false)
private Date created = new Date();
}
class A{
#Id
#GeneratedValue
private Long id;
#OneToMany
#OrderBy("created ASC")
private List<B> b;
#Transient
private B newestB; // Here should be only the newest B
}
Yes. Forget storing the newest B as a variable and instead simply add a getter for it:
#Transient
public B getNewestB() {
return b.get(b.size() -1);
}
This will solve your problem under the assumption that b is set to FetchType.EAGER. Fetching using b's getter and FetchType.LAZY may not be so straight forward as Spring may rely on an AOP proxy call to trigger the lazy load (you'd need to experiment).
However, I'd discourage both these approaches. You're effectively trying to fit business logic into your Entity. Why not keep your entity clean and perform this query using B's repository?
E.g.
public interface BRepository extends CrudRepository<B, Long> {
#Query(...) //query to get newest B for specified A
B getNewest(A a)
}
I have some problem with Morphia.
Could someone help me?
I am writing web-project on Spring + MongoDB about movies and celebrities.
I have entity class Genre:
#Entity(value="genres")
public class Genre implements IGenre {
#Id
#Indexed
private ObjectId id;
#Indexed
private String name;
private String description;
private long quantity;
private Set <IMovie> movies;
//getters and setters
}
And entity class Movie:
#Entity(value="movies")
public class Movie implements IMovie {
#Id
#Indexed
private ObjectId id;
#Indexed
private String originalTitle;
private String year;
private Set <IGenre> genres;
// getters and setters
}
I have 30 genres. And for example one of them: Comedy.
Also I have 250 000 comedies.
And now I want to do movie pagination by genre = comedy.
How I can get only 20 records from all comedies.
If I use #Embedded or #Reference annotation I will still get the entire list at once. And it's to big for use it in controllers.
You should change your data schema for doing such a query. The schema you use has a circular dependency, in your Genre entity you are holding Movies entity and in movies you hold Genre. Also holding all of the movies according to genre is not easy to query. If I were you I would use such a schema.
#Entity(noClassnameStored = true) // you wouldn't have any problem when you change the class name or package of your class if you don't store the classname through this annotation
public class Movie implements IMovie {
#Id
#Indexed
private ObjectId id;
#Indexed
private String originalTitle;
private String year;
private Set <String> genres; // unique identifier of the genres instead of embedding the whole genre entity
// getters and setters
}
So having such a schema, you can retrieve the movies having a particular genre by writing a simple $in query through genres field. Example query for your case:
datastore.find(Movie.class).field("genres").in(Lists.newArrayList("comedy")).limit(20).asList;
In the below web-page of mongo you can find suggestions about how to design your schema according to diffrerent scenarios.
http://docs.mongodb.org/manual/core/data-modeling/#data-modeling-patterns-and-examples
I'm not too familiar with Mongo, but it looks like you would need to implement a custom query here. Whatever you do, you need to pass a start for your page, as well as a page size (20 in your case).
You can do pagination in Morphia by combining .offset(page_start) and .limit(page_size) on a query. So first you would create a query to get movies that belong to a certain genre, and then apply the pagination.
Looks like it is handled using repositories in Spring http://static.springsource.org/spring-data/data-mongodb/docs/1.0.0.RELEASE/reference/html/#repositories.special-parameters
You'd use a Pageable implementation to pass paging data, without worrying about doing the offset and limit calls yourself. There's an example of "Web pagination" further in the doc.
Hope this helps!