Checking if an object exists in a list Java Stream-CodePudding

I have 2 entity. One of them User, and the other one is Followers. I'm trying to make a follower system like Twitter. User can follow another user but, i want to check if user followed, don't do it again.

This is my Follower Entity :

@Entity
public class Followers {

    @Id
    @GeneratedValue(strategy= GenerationType.AUTO)
    private long id;

    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name="from_user_fk")
    @JsonIgnore
    private User from;

    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name="to_user_fk")
    @JsonIgnoreProperties({ "password", "balance","id","mail" })
    private User to;

    public Followers() {};

    public Followers(User from, User to) {
        this.from = from;
        this.to = to;
    }

    public User getFrom() {
        return from;
    }

    public void setFrom(User from) {
        this.from = from;
    }

    public User getTo() {
        return to;
    }

    public void setTo(User to) {
        this.to = to;
    }
}

And this is the Service class :

public ResponseEntity<?> followUser(String username, User user) {
        User byUsername = getByUsername(username);
        List<Followers> followers1 = byUsername.getFollowers();
        List<Followers> collect = followers1.stream().filter(p -> p.getTo().getId().equals(user.getId())).collect(Collectors.toList());
        if(followers1.size()>0){
            return ResponseEntity.status(HttpStatus.FORBIDDEN).body("e");
        }
        Followers followers = new Followers();
        followers.setFrom(user);
        followers.setTo(byUsername);
        followersRepository.save(followers);
        return ResponseEntity.ok(new GenericResponse("Followed"));

    }

    public List<Followers> getUserFollowers(String username) {
        User byUsername = getByUsername(username);
        List<Followers> followers = byUsername.getFollowers();
        return followers;

    }

As you can see, I got the followers of the user I want to follow, and try to check if its following or not. But I couldn't.

Any help would be appreciated

CodePudding user response：

What you've built is incredibly inefficient:

Go to the DB and fetch all followers for a given user.
Then check through these if the person you'd like to add already exists.
If no, add it.
If yes, don't do anything or show an error.

The fail whale is in your near future with this kind of inefficiency.

There's a much, much simpler way. Just.. add it! Don't do any check at all.

Your DB should be configured to disallow having the same user/follower pair, so if you attempt to pull that stunt when that user already has that follower, the DB will refuse and throw an exception that indicates that there's a DB constraint violation. That's your cue to render whatever error you please.

Note that 'check if X is allowed, if yes, do X' is fundamentally broken when talking about multicore architecture. What you're forgetting: What if the user 'double clicks' the 'add follower' link? Then 2 requests start, simultaneously. They both check if X is already a follower (they are not), then they both add X as follower (and now X is a double-follower which you did not want).

Generally if a DB is involved, it is better at data consistency and transactions support, so use the best tool for the job: A DB constraint.