salix/modules/ticket/back/models/ticket-service.js

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

module.exports = Self => {
    Self.observe('before save', async ctx => {
        let changes = ctx.currentInstance || ctx.instance;
        if (changes) {
            let ticketId = changes.ticketFk;
            let ticket = await Self.app.models.Ticket.findById(ticketId);
            if (ticket.refFk)
                throw new UserError('You cannot add or modify services to an invoiced ticket');
        }
    });
};