March 2022

14. March 202214. March 2022

Spring Data JPA fixtures

What are fixtures? Fixed data that is always present in the database. You can think of it as the initial dataset.

application.properties

spring.jpa.hibernate.ddl-auto=create
spring.jpa.generate-ddl=true
spring.jpa.open-in-view=false
spring.sql.init.mode=always
spring.sql.init.data-locations=classpath:db/data.sql
spring.jpa.defer-datasource-initialization=true

spring.jpa.hibernate.ddl-auto=create

spring.jpa.generate-ddl=true

spring.jpa.open-in-view=false

spring.sql.init.mode=always

spring.sql.init.data-locations=classpath:db/data.sql

spring.jpa.defer-datasource-initialization=true

The documentation speaks of “the root classpath”, not knowing Java well I had no idea what that means and an answer on Stackoverflow was of course wrong.
It’s not the “src” dir, but the “resources” dir.

So with the above configuration we set spring.sql.init.data-locations=classpath:db/data.sql
which means we create a directory named “db” under src/main/resources.
Also we create a file named “data.sql”.

If you have spring.jpa.hibernate.ddl-auto=create but not spring.jpa.defer-datasource-initialization=true, data.sql will be executed before the schema is created by JPA and then the tables will be destroyed and created.

data.sql obviously contains sql statements like INSERT INTO profiles (name) VALUES('Darko').
data.sql is only for data, don’t use it for schema.
The location setting for schema is spring.sql.init.schema-locations= e.g. spring.sql.init.schema-locations=classpath:db/schema.sql.
Since you’re using schema.sql you don’t need to defer data initialization.

Details at the Spring Boot JPA reference documentation

14. March 202214. March 2022

Spring Data JPA Postgres UUID @Id

I read lots of outdated info on this matter.

Using PostgreSQL v13+ there is a function called gen_random_uuid().
See the manual page of Postgres UUID.

Here’s an example entity called Page

package de.icod.blog.model;

import javax.persistence.*;
import java.util.UUID;

@Entity
@Table(name = "page")
public class Page {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id", nullable = false, columnDefinition = "UUID default gen_random_uuid()")
    private UUID id;

    public UUID getId() {
        return id;
    }

    public void setId(UUID id) {
        this.id = id;
    }
}

package de.icod.blog.model;

import javax.persistence.*;

import java.util.UUID;

@Entity

@Table(name = "page")

public class Page {

@Id

@GeneratedValue(strategy = GenerationType.AUTO)

@Column(name = "id", nullable = false, columnDefinition = "UUID default gen_random_uuid()")

private UUID id;

public UUID getId() {

return id;

}

public void setId(UUID id) {

this.id = id;

}

As you see you can append the columnDefinition to the @Column annotation. “UUID default gen_random_uuid()”
This translates to following SQL

create table page
(
    id uuid default gen_random_uuid() not null
        constraint "primary"
            primary key
);

create table page

(

id uuid default gen_random_uuid() not null

constraint "primary"

primary key

);

And you don’t have to worry about having an empty ID when inserting records.

14. March 202214. March 2022

Easily remember ManyToMany ManyToOne OneToMany

ManyToMany in a Person <> Blog Relationship
A Person can be a subscriber of many blogs.
A Blog can have many subscribers.

const Person = {
  isSubscriberOfBlogs: []
}
const Blog = {
  subscribers = []
}

const Person = {

isSubscriberOfBlogs: []

}

const Blog = {

subscribers = []

}

But since this is an inefficient way to manage usually a proxy table is used with [PersonID], [BlogID], aka the 2 related data models.

ManyToOne and OneToMany in a Person <> Blog Relationship
A Person can be the owner of many Blogs.
A Blog can have one owner Person.

const Person = {
  id: 1
  isOwnerOfBlogs: [10, 20] // OneToMany
}
const BlogA = {
  id: 10,
  owner_id: 1 // ManyToOne
}
const BlogA = {
  id: 20,
  owner_id: 1 // ManyToOne
}

const Person = {

id: 1

isOwnerOfBlogs: [10, 20] // OneToMany

}

const BlogA = {

id: 10,

owner_id: 1 // ManyToOne

}

const BlogA = {

id: 20,

owner_id: 1 // ManyToOne

}

In conclusion, ToMany means it’s an array, ToOne means it’s a single value.

11. March 2022

Vue composition api: ensure variable is not null

I ran into a problem where I was loading data from the backend in App.vue and when loaded that data would be inserted into a vuex store and in another component queried.

However the loading took longer than expected and a data race happened. The route destination rendered faster than the backend could deliver data and it to be stored in the vuex store and returned.

Concretely I loaded a profile of the logged in user with vuex-orm and the axios plugin.
In order to grab the profile data I used

    const profile = computed(() => store.$db().model('profile').query().where('subject_id', sub).first())

1	const profile = computed(() => store.$db().model('profile').query().where('subject_id', sub).first())

I wanted to update location data of that profile if it was different, which was done with a promise, because that’s how the cordova api works.
But profile was always null. And I tried ref() and reactive() but finally solved it with watch().

    const profile = computed(() => store.$db().model('profile').query().where('subject_id', sub).first())
    watch(profile, (cur) => {
      if (typeof cur === "object") {
        updateProfile(cur)
      }
    })

const profile = computed(() => store.$db().model('profile').query().where('subject_id', sub).first())

watch(profile, (cur) => {

if (typeof cur === "object") {

updateProfile(cur)

}

})

So whenever profile changes the location is updated. Of course I have checks to not update if it’s not required.

The initial state of profile is null, then it changes and updateProfile is called with the new value.

4. March 20224. March 2022

o2 DNS Auflösung mit Fehlern oder langsam

Es ist der 04.03.2022 seit ca. 18 Uhr funktioniert die DNS Auflösung bei O2 garnicht oder nur sehr langsam. Vor allem Google scheint probleme zu haben, aber nicht nur Google. Es ist bei mir Rechner- und Domänenübergreifend.

O2 in Göppingen, welches ca. 40km östlich von Stuttgart liegt.

Ich hab sie vorübergehend überbrückt mit meinem Server, als nächstes hätte ich OpenDNS, Google selbst oder Cloudflare versucht (8.8.8.8 8.8.4.4 bzw 1.1.1.1)

Kann jemand diese Beobachtung bestätigen?