Post entity:
public class Post {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "id")
private Long id;
#OneToMany(mappedBy = "post")
private List<PostComment> postComments;
...
}
PostComment entity:
public class PostComment {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "id")
private Long id;
#ManyToOne
#JoinColumn(name = "post_id")
private Post post;
#OneToMany(mappedBy = "postComment")
private Set<PostCommentUpvote> postCommentUpvotes;
#OneToMany(mappedBy = "postComment")
private Set<PostCommentDownvote> postCommentDownvotes;
...
}
PostCommentUpvote entity (PostCommentUpvote and PostCommentDownvote have the exact same fields - these entities act like counters)
public class PostCommentUpvote {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Column(name = "id")
private Long id;
#ManyToOne
#JoinColumn(name = "post_comment_id")
private PostComment postComment;
#ManyToOne
#JoinColumn(name = "user_id")
private User user;
...
}
All relations are bi-directional as you can see from the annotations.
The goal: When a user (authenticated) upvotes/downvotes a PostComment I want to do the following:
Check if user already upvoted/downvoted the PostComment.
For this I have Post id (even though this is not needed) and PostComment id and both are indexed.
There are three possible 'states' when User up/downvotes the comment:
User hasn't yet up/downvoted that comment, so it is either new upvote or new downvote
User has already upvoted and if he upvotes again, it will remove the upvote (same with downvote)
User has already upvoted and if he downvotes, upvote is removed and new downvote is added (and vice-versa)
What would be the most optimal way of doing this? Get the PostComment by its id and then loop through the List of PostCommentUpvote/PostCommentDownvote and check the User on every iteration? Or perform a tactical SQL request, which must be faster than looping in Java? If so, what would this SQL query look like? Or any other approach to make this performant. I am open to any suggestion.
Thanks
Assuming you have the post comment id and user id, the following JPA query (or close to it) will return true if the user has upvoted on the post comment and false otherwise:
select case when count(postCommentUpvote) > 0 then 'true' else 'false'
from PostCommentUpvote postCommentUpvote
join postCommentUpvote.postComment postCommnent
where postComment.id = :postCommentId
and user.id = :userId
You would then have to perform the same query using the PostCommentDownVote entity. An alternative would be to remove the up and down vote entities, simply create a PostCommentVote entity which has a boolean attribute that indicates up or down, and helper methods isUpvote() and isDownVote() that would interpret the boolean for you. You could get everything you need with a single query that returns a PostCommentVote if the user has up or down voted and null otherwise.
You did not indicate what you want to do if the user has already commented on the post; ignore the request or update the PostComment. Either way the most optimal way of doing this would be not checking at all. Create a unique index on (user_id, post_comment_id) or drop the the id column and make a composite PK of those columns. Then just insert without checking. Use the On Conflict to either ignore or update the request. You may also want to add an Up/Down vote indicator column.
Related
How can I link an entity that already exists in another table into my important_table? I could insert the ID but then that would require a query. What I want is that the system automatically maps the element in the people_table to the important_table.
#Entity(name = "important_table")
data class ImportantEntity(
#Id #GeneratedValue #Column(name = "id")
val id: Short = 0,
#Embedded
val person: Person
)
Person Entity
#Entity(name = "person_table")
data class PersonEntity(
#Id #GeneratedValue #Column(name = "id")
val id: Long = 0,
...
)
I tried embedded but that creates a duplicate Person in the db. I want the link so that I can find "important" people easy and still get the same data.
Use a #OneToOne mapping to let Hibernate (or any ORM) knows you are linking the tables together.
#Entity(name = "important_table")
data class ImportantEntity(
#Id #GeneratedValue #Column(name = "id")
val id: Short = 0,
#OneToOne(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
val person: Person
)
When you say: I could insert the ID but then that would require a query, indeed, it does... But with the FetchType.EAGER, it's your ORM that will execute the extra query for you... It is transparent.
Nonetheless, be careful with that, if you don't pay attention to your relationships, you can end up loading the entire db in memory...
As explained in this site, and this tuto, #Embedded is used to help having a nice and clean object definition, while storing the data into one table instead of 2... For instance, in your case, you would be storing the data from the class Person into the table ImportantTable.
Hope it helps !
consider following model:
#Entity
#Getter
#Setter
public class Person {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
#ElementCollection
#Column(name = "phone")
private List<String> phoneNumber = new LinkedList<>();
}
In front-end user should be able define order of phone numbers in this sample with drag and drop or something like that. Can you tell me what is the most effective way to handle this use case? I found solution with jpa annotation #OrderColumn which generates additional column. But I guess if I need re-order items, solution is delete all from collection and save it again with new order right? I afraid that is not very elegant solution. Can you give me your advice? Thank you.
I'm fairly new to ORM. I'm having trouble deciding how exactly I should map the following entities.
DiscussionThread
Post
User
AnonymousUser
DiscussionThread would be something similar to the ones we see in bulletin boards online. It would contain a list of Post which would be posted by User. However, I do not want the User to reveal his/her identity while posting in the DiscussionThread.
In order to achieve that I created a list of proxy usernames denoted by the entity AnonymousUser. Thus, whenever a User decides to make a Post in a DiscussionThread, he would be posting as an AnonymousUser. Any further Post made by the same User in that DiscussionThread would be linked to the same AnonymousUser.The User will have different AnonymousUser names in different DiscussionThreads. An instance of AnonymousUser may be used by two different users on two different threads.
In simpler words, there will be one AnonymousUser for one User in each DiscussionThread.
I have created the following POJO entities, but I'm stuck in how I should map them to each other.
public class AnonymousUser {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
private String name;
private String displayPicture;
//Not sure how to make relationships here
private Set<DiscussionThread> discussionThreads;
private Set<User> users;
}
public class DiscussionThread {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
private String title;
private String description;
}
public class User {
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
private String email;
private String username;
}
Any help will be appreciated.
Thank you!
Well, you basically described:
Don't know if it's right or not but this is one way you could diagram and think about such problems. This is Chen's database notation in Visio.
I'm using #version annotation in spring data so I have a parent entity, and it has list of child entity. when I delete an element from child list the parent version doesn't increase. can anyone clarify for me this #version alternative,
why the versing in this case doesn't increase, is it a good way to manage versioning or should I use trasaction "lock".
in the documentation i read that the version update only on updating a row
in the databse but in my case i put version on parent entity and i want
note: i searched a lot in the internet but i didnt find a clear solution, can any one help me.
I assume you are using Hibernate. Lets say that the "UnderlyingPerTradingAccount" table has a column called "trading_account_id", which is a foreign key to the TradingAccount table. In order to achieve the behavior you described, you need to change the mapping. Can you try this:
public class TradingAccount {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
#OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
#JoinColumn(name="trading_account_id", referencedColumnName = "trading_account_id", insertable = false, updatable = false)
private List<UnderlyingPerTradingAccount> underlyingPerTradingAccounts;
#Version
private Long version;
}
and
public class UnderlyingPerTradingAccount {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
#ManyToOne
#JoinColumn(name="trading_account_id", nullable = false)
private TradingAccount tradingAccount;
private Boolean enableBuy;
private Boolean enableSell;
}
This should mark the parent entity as "dirty" when the child entity is updated and trigger the version increment.
However, I would think of some other method to track "version" changes of the parent entity as this would just cause an additional overhead and update statements to the parent.
I faced the problem when I need to partially udate data in BD.
What I have:
I have three linked entities:
Profile --(1-m)--> Person --(1-1)--> Address
Where Person -> Address is lazy relationship. It was achieved via optional=false option (that allow hibernate to use proxy).
What the problem:
I need to update Profile in such way, that I needn't pull all Addresses that linked with this profile.
When I update Profile (don't work):
profile.setPersons(persons);
session.saveOrUpdate(profile);
throws: org.springframework.dao.DataIntegrityViolationException: not null property references a null or transient value
It happens because Person->Address relationship has optional=false option
I need to do:
//for each person
Address address = requestAddressFromDB();
person.setAddress(address);
persons.add(person)
//and only then
profile.setPersons(persons);
session.saveOrUpdate(profile);
profile.setPerson(person)
But I don't want to pull all address each time I update Profile name.
What is the question:
How can I avoid obligatory Person->(not null)Address constraint to save my profile without pulling all addresses?
ADDITION:
#Entity
public class Person{
#Id
#SequenceGenerator(name = "person_sequence", sequenceName = "sq_person")
#GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "person_sequence")
#Column(name = "id")
private long personID;
#OneToOne(mappedBy="person", cascade=CascadeType.ALL, optional = false, fetch = FetchType.LAZY)
private Address address;
//.. getters, setters
}
#Entity
public class Address {
#Id
#Column(name="id", unique=true, nullable=false)
#GeneratedValue(generator="gen")
#GenericGenerator(name="gen", strategy="foreign", parameters=#Parameter(name="property", value="person"))
private long personID;
#PrimaryKeyJoinColumn
#OneToOne
private FileInfo person;
}
Modify the cascade element on the #OneToOne annotation so that the PERSIST operation is not cascaded. This may require you to manually persist updates to Address in certain areas of your code. If the cascade is not really used however no change is needed.
#OneToOne(mappedBy="person", cascade={CascadeType.MERGE, CascadeType.REMOVE, CascadeType.REFRESH}, optional = false, fetch = FetchType.LAZY)
private Adress address; //Do you know that Address is missing a 'd'?