Sequelize: Avoid duplicates when create with manyToMany associations - sql

Question

I'm developing a small e-commerce to sell cinema tickets. I'm using Sequelize to design all models and their associations. The issue I'm facing is related with the route defined to create the final order:
Order.create({
userId: req.body.userId, // Number
sessionId: req.body.sessionId, // Number
seats: req.body.seats, // Array of objects
offsiteProducts: req.body.offsiteProducts // Array of objects
},
{
include: [
{
model: Seat,
attributes: ['area', 'number', 'roomId']
},
{
model: OffsiteProduct,
attributes: ['name', 'unitPrice']
}
]
}).then(order => {
res.json(order);
})
The relation between models is as follows:
User.hasMany(Order);
Order.belongsTo(User);
Session.hasMany(Order);
Order.belongsTo(Session);
Seat.belongsToMany(Order, { through: "reserved_seats" });
Order.belongsToMany(Seat, { through: "reserved_seats" });
OffsiteProduct.belongsToMany(Order, { through: ReservedOffsiteProduct });
Order.belongsToMany(OffsiteProduct, { through: ReservedOffsiteProduct });
For the "one-to-many" relationships, passing a foreign key is enough for Sequelize to associate models properly.
But for "many-to-many" associations ("belongsToMany" in Sequelize) it would duplicate the data entered for seats and offsite products and create it both as part of the order and as a new independent seat and offsite product respectively.
How can I avoid this behaviour and include the arrays of seats and offsite products only inside the final order? Thanks.

Answer 1

One way to do this is to execute everything in a transaction. That way, it will be atomic, i.e. "all or nothing".
Sequelize provides some methods for many-to-many associations. In this case, a separate call could be made for each junction table.
With this in mind, the following should accomplish the inserts without any duplicates:
let t = await sequelize.transaction()
try {
let order = await Order.create({
userId: req.body.userId,
sessionId: req.body.sessionId
}, {
transaction: t
})
let seats = await Seat.findAll({
where: {
id: {
[Op.in]: req.body.seatIds
}
}
})
let offsiteProducts = await OffsiteProducts.findAll({
where: {
id: {
[Op.in]: req.body.offsiteProductIds
}
}
})
await order.addSeats(seats, { transaction: t })
await order.addOffsiteProducts(offsiteProducts, { transaction: t })
await t.commit()
} catch (err) {
if (t) {
await t.rollback()
}
}
Alternatively, if the primary keys for the rows in the OffsiteProducts and Seats tables are already known, the above could be shortened to:
let t = await sequelize.transaction()
try {
let order = await Order.create({
userId: req.body.userId,
sessionId: req.body.sessionId
}, {
transaction: t
})
await order.addSeats(req.body.seatIds, { transaction: t })
await order.addOffsiteProducts(req.body.offsiteProductIds, { transaction: t })
await t.commit()
} catch (err) {
if (t) {
await t.rollback()
}
}
There's a bit more explanation for passing arrays of primary keys to the .addSeats and .addOffsiteProducts methods in the docs here.