I need to store a large amount of nodes with many-to-many relationship to eachother (through parents/childrens). How can I do that in JPA?
I know I can use #ManyToMany on my parents/childrens list but then JPA will load the all parents and their parents and so on right? Is there any way I can do it and only load the parents or children one step away?
The non-working code I have now (the Node.java):
public String name;
#ManyToMany
#JoinTable(name = "NodeParent")
public List<Node> parents;
#ManyToMany
#JoinTable(name = "NodeChilds")
public List<Node> childrens;
For this JPA has the concept op lazy loading.
While iterating over the collection when still within the persistence context, every time you reference a not previously loaded entity, it will be loaded on demand.
Additionally, if you know up-front which entities you need (how deep you want the tree to be fetched) you can write a projection query (JPQL) using the fetch join operator.
Finally, a hibernate specific solution that might be included in JPA 2.1 are so-called fetch profiles. See Fetch profiles and Fetch Profiles in Hibernate 3.5.
Related
When we use SQL with Spring boot, we can use hibernate and add #OneToMany relationships. It helps us to get the reference objects from another entity.
As an example, suppose Order and OrderDetails entities. When I query the Order entity, It automatically maps with the OrderDetails entity and it brings the whole document together. How can I do such a thing with Springboot+mongodb? Is there any easy way to solve this problem? Is it called ORM?
In the past, my DTOs have been a direct map of the entity. However, I am now in a scenario where all we really need is the ID of a nested object so we can then do the DB lookup if needed. It looks something like this
public class UserDto {
Integer id;
String name;
List<Integer> groupIds;
}
But the entity looks like this
public class UserEntity {
Integer id;
String name;
List<UserGroupEntity> userGroups;
}
Is this a common practice? Should I just have the DTO mapped directly from the entity and have embeded UserGroup DTOs?
I usually go with option number one, only including the ids of other entities. The DTO can otherwise become quite bloated after a while, leading to a lot of unnecessary database lookups.
I only go for option number two if the DTO never makes any sense without the containing entities. But this is almost never the case, and it's difficult and dangerous to assume that you know what information all future users of your DTO will want to have.
When it comes to the API endpoint design (assuming you go for option one) you can let the client fetch Users and UserGroups by calling the following endpoints:
/users/:id
/users/:id/usergroups
This is so that the client doesn't have to wait for the User fetch to finish before fetching the UserGroups. This follows RESTful principles and can e.g. be done in parallell from the client.
I am studying JPA in Spring application and I have some doubts related to the #Entity annotation.
So I have a model class like this:
#Entity
#Table(name= “T_CUSTOMER”)
public class Customer {
#Id
#Column(name=“cust_id”)
private Long id;
#Column(name=“first_name”)
private String firstName;
#Transient
private User currentUser;
...........................
...........................
...........................
}
Ok, I know that the #Entity annotation is on the class level and it means that the fields of the object that are instances of this class are to be mapped to the field of the T_CUSTOMER database table.
But why in JPA it is mandatory to use #Entity annotation and I cannot only use the #Table annotation to map a model object to a specific database table? It have some other meaning\behavior that actually I am missing?
What am I missing? What is the exact meaning of the #Entity annotation?
#Entity annotation defines that a class can be mapped to a table. And that is it, it is just a marker, like for example Serializable interface.
And why #Entity annotation is mandatory? ... well, it is the way how JPA is designed. When you create a new entity you have to do at least two things
annotated it with #Entity
create an id field and annotate it with #Id
Anything else is optional, for example table name is derived from entity class name (and therefore #Table annotation can be optional), table's columns are derived from entities variables (and therefore #Column annotation can be optional), and so on ...
JPA is trying to provide a fast and easy start to developers who want to learn/use this API, and giving developers option to configure as few things as possible to make something functional is one of the ways how this API wants to achieve this "easy to use/learn" goal. Hence the #Entity annotation (together with #Id annotation) is the minimum you have to do in order to create an entity.
Entities in JPA are nothing but POJOs representing data that can be persisted to the database. An entity represents a table stored in a database. Every instance of an entity represents a row in the table.
More about the entities:
https://www.baeldung.com/jpa-entities
Entities represent persistent data stored in a relational database automatically using container-managed persistence.They are persistent because their data is stored persistently in some form of data storage system, such as a database: they do survive a server failure, failover, or a network failure. When an entity is reinstantiated, the state of the previous instance is automatically restored.
An entity models a business entity or multiple actions within a single business process. Entities are often used to facilitate business services that involve data and computations on that data. For example, you might implement an entity to retrieve and perform computation on items within a purchase order. Your entity can manage multiple, dependent, persistent objects in performing its tasks.
Entities can represent fine-grained persistent objects, because they are not remotely accessible components.
An entity can aggregate objects together and effectively persist data and related objects using the transactional, security, and concurrency services of a JPA persistence provider.
I have a Question object which has List of Comment objects with #OneToMany mapping. The Question object has a fetchComments(int offset, int pageSize) method to fetch comments for a given question.
I want to paginate the comments by fetching a limited amount of them at a time.
If I write a Query object then I can set record offset and maximum records to fetch with Query.setFirstResult(int offset) and Query.setMaxResults(int numberOfResults). But my question is how(if possible) can I achieve the same result without having to write a Query i.e. with simple annotation or property. More clearly, I need to know if there is something like
#OneToMany(cascade = CascadeType.ALL)
#Paginate(offset = x,maxresult = y)//is this kind of annotation available?
private List<Comment> comments;
I have read that #Basic(fetch = FetchType.LAZY) only loads the records needed at runtime, but I won't have control to the number of records fetched there.
I'm new to JPA. So please consider if I've missed something really simple.
No, there is no such a functionality in JPA. Also concept itself is bit confusing. With your example offset (and maxresult as well) is compile time constant and that does not serve pagination purpose too well. Also in general JPA annotations in entities define structure, not the context dependent result (for that need there is queries).
If fetching entities when they are accessed in list is enough and if you are using Hibernate, then closest you can get is extra #LazyCollection:
#org.hibernate.annotations.LazyCollection(LazyCollectionOption.EXTRA)
What is the difference between Unidirectional and Bidirectional associations?
Since the table generated in the db are all the same,so the only difference I found is that each side of the bidiretional assocations will have a refer to the other,and the unidirectional not.
This is a Unidirectional association
public class User {
private int id;
private String name;
#ManyToOne
#JoinColumn(
name = "groupId")
private Group group;
}
public class Group {
private int id;
private String name;
}
The Bidirectional association
public class User {
private int id;
private String name;
#ManyToOne
#JoinColumn(
name = "groupId")
private Group group;
}
public class Group {
private int id;
private String name;
#OneToMany(mappedBy="group")
private List<User> users;
}
The difference is whether the group holds a reference of the user.
So I wonder if this is the only difference? which is recommended?
The main difference is that bidirectional relationship provides navigational access in both directions, so that you can access the other side without explicit queries. Also it allows you to apply cascading options to both directions.
Note that navigational access is not always good, especially for "one-to-very-many" and "many-to-very-many" relationships. Imagine a Group that contains thousands of Users:
How would you access them? With so many Users, you usually need to apply some filtering and/or pagination, so that you need to execute a query anyway (unless you use collection filtering, which looks like a hack for me). Some developers may tend to apply filtering in memory in such cases, which is obviously not good for performance. Note that having such a relationship can encourage this kind of developers to use it without considering performance implications.
How would you add new Users to the Group? Fortunately, Hibernate looks at the owning side of relationship when persisting it, so you can only set User.group. However, if you want to keep objects in memory consistent, you also need to add User to Group.users. But it would make Hibernate to fetch all elements of Group.users from the database!
So, I can't agree with the recommendation from the Best Practices. You need to design bidirectional relationships carefully, considering use cases (do you need navigational access in both directions?) and possible performance implications.
See also:
Deterring “ToMany” Relationships in JPA models
Hibernate mapped collections performance problems
There are two main differences.
Accessing the association sides
The first one is related to how you will access the relationship. For a unidirectional association, you can navigate the association from one end only.
So, for a unidirectional #ManyToOne association, it means you can only access the relationship from the child side where the foreign key resides.
If you have a unidirectional #OneToMany association, it means you can only access the relationship from the parent side which manages the foreign key.
For the bidirectional #OneToMany association, you can navigate the association in both ways, either from the parent or from the child side.
You also need to use add/remove utility methods for bidirectional associations to make sure that both sides are properly synchronized.
Performance
The second aspect is related to performance.
For #OneToMany, unidirectional associations don't perform as well as bidirectional ones.
For #OneToOne, a bidirectional association will cause the parent to be fetched eagerly if Hibernate cannot tell whether the Proxy should be assigned or a null value.
For #ManyToMany, the collection type makes quite a difference as Sets perform better than Lists.
I'm not 100% sure this is the only difference, but it is the main difference. It is also recommended to have bi-directional associations by the Hibernate docs:
http://docs.jboss.org/hibernate/core/3.3/reference/en/html/best-practices.html
Specifically:
Prefer bidirectional associations:
Unidirectional associations are more difficult to query. In a large
application, almost all associations
must be navigable in both directions
in queries.
I personally have a slight problem with this blanket recommendation -- it seems to me there are cases where a child doesn't have any practical reason to know about its parent (e.g., why does an order item need to know about the order it is associated with?), but I do see value in it a reasonable portion of the time as well. And since the bi-directionality doesn't really hurt anything, I don't find it too objectionable to adhere to.
In terms of coding, a bidirectional relationship is more complex to implement because the application is responsible for keeping both sides in synch according to JPA specification 5 (on page 42). Unfortunately the example given in the specification does not give more details, so it does not give an idea of the level of complexity.
When not using a second level cache it is usually not a problem to do not have the relationship methods correctly implemented because the instances get discarded at the end of the transaction.
When using second level cache, if anything gets corrupted because of wrongly implemented relationship handling methods, this means that other transactions will also see the corrupted elements (the second level cache is global).
A correctly implemented bi-directional relationship can make queries and the code simpler, but should not be used if it does not really make sense in terms of business logic.