Fix a modelFactory bug

[T-vK]

I found another bug regarding the modelFactory. Many places in the code were exclusively using the initially provided model instead of the model dynamically provided through modelfactory.getModel.

As we don't have any integration tests for this feature, I created a little test script to verify that it works as expected:

var express = require('express')
var bodyParser = require('body-parser')
var methodOverride = require('method-override')
var mongoose = require('mongoose')
var restify = require('express-restify-mongoose')
const request = require('request-promise')
var app = express()
var router = express.Router()

app.use(bodyParser.json())
app.use(methodOverride())

mongoose.connect('mongodb://localhost/database')

var customerSchema = new mongoose.Schema({
  name: { type: String, required: true },
  comment: { type: String }
})
var userSchema = new mongoose.Schema({
  name: { type: String },
  email: { type: String, required: true }
})

var customerModel = mongoose.model('Customer', customerSchema)
var userModel = mongoose.model('User', userSchema)

restify.serve(router, customerModel, {
  modelFactory: {
    getModel: async (req) => {
      return null // THIS IS EXPECTED TO FAIL
    }
  }
})

restify.serve(router, userModel, {
  modelFactory: {
    getModel: async (req) => {
      return userModel // THIS IS EXPECTED TO WORK
    }
  }
})

app.use(router)

app.listen(3000, async () => {
  try {
    // GET http://localhost:3000/api/v1/User should work
    await testEndpoint('http://localhost:3000/api/v1/User', 200);
    // GET http://localhost:3000/api/v1/Customer should return an error
    await testEndpoint('http://localhost:3000/api/v1/Customer', 400);
    console.log("Success!")
    process.exit(0)
  } catch (e) {
    console.log(e)
    console.log("Failed!")
    process.exit(1)
  }
})

async function testEndpoint(url, expectedStatus) {
  let res = null
  try {
    res = await request({url, resolveWithFullResponse: true})
  } catch (error) {
    res = error
  }
  if (res.statusCode !== expectedStatus)
    throw new Error(`${url} did not return expected status code. Got "${res.statusCode}" instead of "${expectedStatus} - Error: ${res.error}"`)
  else
    console.log(`${url} is behaving as expected.`)
}

We should turn this into an integration test at some point to avoid bugs like this in the future.

[T-vK]

I just made another tiny adjustment to this PR to improve the performance a bit by registering the middleware for the model retrieval on the ERM routes directly (instead of as a global middleware for all routes).