How can I update a through-NM-attribute in sequelize? - mysql

Question

I've read this question and it didnt' help me, so I'm asking my own:
Let's suppose I have 2 tables joined by a NM through table using Sequelize and MariaDB:
User <-- UserItem --> Item
A single User can have many Items, and a single Item can belong to many Users. But I need a custom through attribute to store the quantity of the Item, let's call it Apples. So, according to the docs, this will be the definition:
var UserItem = Sequelize.define('UserItem', {
quantity: DataTypes.INTEGER
},
timestamps: false
});
models.Item.belongsToMany(models.User, {through: 'UserItem'});
models.User.belongsToMany(models.Item, {through: 'UserItem'});
And then I add a new relationship with the through attribute like this:
User.addItem(item, { quantity: 0 });
This works as expected. But, what if I need to update the quantity of an Item? I could do the following:
User.addItem(item, { quantity: 20 });
And the quantity of my Item will be updated to 20 in case of existing, and inserted otherwise. I don't want this. I want something like that:
User.addItem(item, { quantity: quantity + 1 });
But due to the impossibility to make queries to the join tables, I am unable to get the particular NM row for updating using the previous value.
How can I achieve this? Thanks in advance.

Answer 1

You still have the join table DAO as specified in the docs.
In this case though you need to know if the item is new association or not.
So it will look something like.
User.hasItem( item )
.then( exists => {
if ( !exists ) {
return User.addItem( item, { quantity : 20 } )
} else {
item.UserItem.quantity += 1;
return item.UserItem.save();
}
} )
You can read more about hasAssociation in the docs as well.