salix/modules/invoiceIn/back/models/invoice-in-tax.js

const UserError = require('vn-loopback/util/user-error');

module.exports = Self => {
    Self.observe('before save', async function(ctx) {
        if (ctx.isNewInstance) return;

        const models = Self.app.models;
        const invoiceIn = await models.InvoiceIn.findById(ctx.currentInstance.invoiceInFk, null, ctx.options);
        if (invoiceIn.isBooked) throw new UserError('InvoiceIn is already booked');
    });

    Self.observe('before delete', async function(ctx) {
        const models = Self.app.models;
        const invoiceInTax = await Self.findById(ctx.where.id, null, ctx.options);
        const invoiceIn = await models.InvoiceIn.findById(invoiceInTax.invoiceInFk, null, ctx.options);
        if (invoiceIn.isBooked) throw new UserError('InvoiceIn is already booked');
    });
};