Why use association in sequelize orm? - node.js

Question

I made some api system with nodejs.
Also I use sequelize(version 4) for communicates with MySQL.
Below is my model structure.
[user]
no(PK)
userid
userpw
[article]
no(PK)
subject
content
writer(FK to user's userid)
I made with sequelize orm define like this.
import Sequelize from 'sequelize';
const sequelize = new Sequelize('db', 'user', 'pw', {
host: '127.0.0.1',
dialect: 'mysql'
})
const User = sequelize.define('user', {
no: {
type: Sequelize.INTEGER,
primaryKey: true,
autoIncrement: true
},
userid: {
type: Sequelize.STRING(30),
allowNull: false,
unique: true
},
userpw: {
type: Sequelize.STRING(30),
allowNull: false
}
}, {
freezeTableName: true,
timestamps: true,
underscored: true
});
const Article = sequelize.define('article', {
no: {
type: Sequelize.INTEGER,
primaryKey: true
},
subject: {
type: Sequelize.STRING(50),
allowNull: false
},
content: {
type: Sequelize.STRING(100),
allowNull: false
},
writer: {
type: Sequelize.STRING(30),
references: {
model: 'user',
key: 'userid'
}
}
}, {
freezeTableName: true,
timestamps: true,
underscored: true
})
sequelize.sync({
force: true
});
export default { User, Article };
In Article model, I define write and refer to User's userid.
So Article's writer column is Foreign Key refer to User's userid.
Because If user was deleted, related article have to delete.
Run that code, table will be created and structure is like this.
mysql> desc user;
+------------+-------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+----------------+
| no | int(11) | NO | PRI | NULL | auto_increment |
| userid | varchar(30) | NO | UNI | NULL | |
| userpw | varchar(30) | NO | | NULL | |
| created_at | datetime | NO | | NULL | |
| updated_at | datetime | NO | | NULL | |
+------------+-------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)
mysql> desc article;
+------------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+--------------+------+-----+---------+-------+
| no | int(11) | NO | PRI | NULL | |
| subject | varchar(50) | NO | | NULL | |
| content | varchar(100) | NO | | NULL | |
| writer | varchar(30) | YES | MUL | NULL | |
| created_at | datetime | NO | | NULL | |
| updated_at | datetime | NO | | NULL | |
+------------+--------------+------+-----+---------+-------+
6 rows in set (0.00 sec)
After do that, I searched more information about association in sequelize orm.
And I found that sequelize has many association like hasMany, BelongsTo, 'hasMany`, ...
So I add User.hasMany(Article); to my previous code and run it, it add more columns.
mysql> desc article;
+------------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------+--------------+------+-----+---------+-------+
| no | int(11) | NO | PRI | NULL | |
| subject | varchar(50) | NO | | NULL | |
| content | varchar(100) | NO | | NULL | |
| writer | varchar(30) | YES | MUL | NULL | |
| created_at | datetime | NO | | NULL | |
| updated_at | datetime | NO | | NULL | |
| user_no | int(11) | YES | MUL | NULL | |
+------------+--------------+------+-----+---------+-------+
7 rows in set (0.00 sec)
user_no column was added.
I think it automatically added by hasMany function.
But in my case, I do not have to create user_no, Because I already refer to User's user_id column.
Overall, my question is:
I think I do not have to set hasMany in my case. Is it right?
Why ORM offer assocation function? Just for convenience? or Is there any reason for it?
Thanks.