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 isEditable = await Self.app.models.Ticket.isEditable(ticketId);
            if (!isEditable)
                throw new UserError(`The current ticket can't be modified`);
        }
    });

    Self.observe('before delete', async ctx => {
        const models = Self.app.models;
        const service = await models.TicketService.findById(ctx.where.id);
        const isEditable = await Self.app.models.Ticket.isEditable(service.ticketFk);

        if (!isEditable)
            throw new UserError(`The current ticket can't be modified`);
    });
};