Sequelize - avoid deleting rows of many-to-many association when inserting a new one

mrodo :

My problem is that when I execute this code:

const task = await db.tasks.findByPk(1)
const user = await db.users.findByPk(4)
await user.setTasks(task)

This happens:

Executing (default): SELECT `tasks`.`id`, `tasks`.`start_date` FROM `tasks` AS `tasks` WHERE `tasks`.`id` = '1';
Executing (default): SELECT `id`, `name`, `surname`, `email`, `role` FROM `users` AS `users` WHERE `users`.`id` = '4';
Executing (default): SELECT `id`, `task_id`, `user_id` FROM `users_tasks` AS `users_tasks` WHERE `users_tasks`.`user_id` = 4;
Executing (default): DELETE FROM `users_tasks` WHERE `user_id` = 4 AND `task_id` IN (2)
Executing (default): INSERT INTO `users_tasks` (`id`,`task_id`,`user_id`) VALUES (NULL,1,4);

The user with id 4 had a previous association with a task with id 2. The issue is, I want to keep that association! It is perfectly fine for the purpose of my application for a user to be assigned to more tasks (and vice versa).

I would actually say that that's the normal desired behaviour of a many-to-many relationship... why is Sequelize deleting the row on users_tasks before adding a new one, and how can I prevent this from happening?

Here is my user model:

module.exports = function(sequelize, DataTypes) {
    var User = sequelize.define('users', {
        id: {
            type: DataTypes.INTEGER(10).UNSIGNED,
            allowNull: false,
            primaryKey: true,
            autoIncrement: true
        },
        name: {
            type: DataTypes.STRING(50),
            allowNull: false
        },
        surname: {
            type: DataTypes.STRING(50),
            allowNull: false
        },
        email: {
            type: DataTypes.STRING(256),
            allowNull: false
        },
    }, {
        tableName: 'users',
        timestamps: false,
    })

    User.associate = function (models) {
        User.belongsToMany(models.tasks, {through: 'users_tasks', foreignKey: 'user_id'})
    }

    return User
}

Here is my task model:

module.exports = function(sequelize, DataTypes) {
    var Task = sequelize.define('tasks', {
        id: {
            type: DataTypes.INTEGER(10).UNSIGNED,
            allowNull: false,
            primaryKey: true,
            autoIncrement: true
        },
        start_date: {
            type: DataTypes.DATE,
            allowNull: true,
        },
    }, {
        tableName: 'tasks',
        timestamps: false
    })

    Task.associate = function (models) {
        Task.belongsToMany(models.users, {through: 'users_tasks', foreignKey: 'task_id'})
    }

    return Task
}

And here is my join table:

module.exports = function(sequelize, DataTypes) {
    return sequelize.define('users_tasks', {
        id: {
            type: DataTypes.INTEGER(10).UNSIGNED,
            allowNull: false,
            primaryKey: true,
            autoIncrement: true
        },
        task_id: {
            type: DataTypes.INTEGER(10).UNSIGNED,
            allowNull: false,
            references: {
                model: 'tasks',
                key: 'id'
            }
        },
        user_id: {
            type: DataTypes.INTEGER(10).UNSIGNED,
            allowNull: false,
            references: {
                model: 'users',
                key: 'id'
            }
        }
    }, {
        tableName: 'users_tasks',
        timestamps: false
    })
}

Do I need to change something on the models definitions, or do I need to change something in the way I call setTasks? Or else, what do I need to do? Why is Sequelize enforcing this weird and unwanted behaviour of deleting previously existing associations?

mrodo :

I found out that I should be using add instead of set if I want to keep the preexisting relations.

So, the correct way of achieving the result I want is this:

const task = await db.tasks.findByPk(1)
const user = await db.users.findByPk(4)
await user.addTasks(task)