Implement a way for consumers to add model hooks

[ferjm]

The user story SSB-7 says "As a user I would like to be able to understand the source of data that I am viewing. (LASS, other source, Sensorweb device) so that I can judge the credibility and see the source of the information."

[ferjm]

Copy&paste of an IRC conversation with my proposal for this:

so I think we can use the Datastream/Thing.properties field to add something like a MozillaSensorWebOwner field inside with the client name as value
but it is going to be tricky to do it in a clean way ... :/
because we have the ST API implementation as an independent module
we are not supposed to access the Clients DB from it
so I am thinking that we should probably expose a way for users of the ST npm module to execute some code inside the sequelize model hooks
http://docs.sequelizejs.com/en/latest/docs/hooks/
this way from sensorweb-server we can pass a callback to execute within the beforeCreate hook for example
within this callback we will get the instance of the entity being created
so we need to check at this point if this entity is Thing and in that case add the MozillaSensorWebOwner within its 'properties' field
To get the value of MozillaSensorWebOwner (the Client's name), we can get instance.clientId
and with the clientId value, we can get the Client's name from the Clients DB

I'd like to dedicate this issue to implement the mechanism to allow consumers to add model hooks.

In the end, the consumer should be able to do something like:

const bodyParser    = require('body-parser');
const express          = require('express');
const SensorThings = require('../dist/sensorthings');
const Clients             = require('myclientsmodel');

var app = express();

app.use(bodyParser.urlencoded({ extended: false }));
app.use(bodyParser.json());

const config = {
  db: {
    host: 'localhost',
    port: 5432,
    name: 'sensorthingsexample',
    user: 'postgres',
    password: '12345678'
  }
};

const dbHooks = {
  beforeCreate: (instance, options, fn) => { ... },
  beforeDestroy: (instance, options, fn) => { ... },
  beforeUpdate: null,
  beforeSave: (instance, options, fn) => {
    //XXX Mostly pseudocode!
    if (instance.$modelOptions.name.plural !== 'Things' || !instance.clientId) {
      return;
    }
    return Clients.getById(instance.clientId).then(client => {
      instance.properties = Object.assign({}, instance.properties, {
        MozillaSensorWebOwner: client.name
      });
    });
  },
  beforeUpsert: (values, options, fn) => { ... }
};

app.use('/', SensorThings(config, dbHooks));

app.listen(8080, () => console.log('Running on localhost:8080'));

[ferjm]

Fixed by #235