salix/services/loopback/common/methods/sale/updateQuantity.js

let UserError = require('../../helpers').UserError;

module.exports = Self => {
    Self.remoteMethod('updateQuantity', {
        description: 'Changes the quantity of a sale',
        accessType: '',
        accepts: [{
            arg: 'id',
            type: 'number',
            required: true,
            description: 'sale ID',
            http: {source: 'path'}
        }, {
            arg: 'quantity',
            type: 'number',
            required: true,
            description: 'newQuantity'
        }],
        returns: {
            type: 'string',
            root: true
        },
        http: {
            path: `/:id/updateQuantity`,
            verb: 'post'
        }
    });

    Self.updateQuantity = async(id, quantity) => {
        if (isNaN(quantity))
            throw new UserError(`The value should be a number`);

        let currentLine = await Self.app.models.Sale.findOne({where: {id: id}});
        if (quantity > currentLine.quantity)
            throw new UserError('The new quantity should be smaller than the old one');

        return await currentLine.updateAttributes({quantity: quantity});
    };
};