For those using loopback and changing their database from one to another. This will copy all the data from the given database to the configured database(in server/datasources). This was tested to copy from postgresql to mongodb.

loopback-db-migrate.js

'use strict';

let app = require('../server/server');
let loopback = require('loopback');
let prompt = require('prompt');

console.log(`This will copy all the data from the given database to the 
configured database(in server/datasources)`)

prompt.get(['connector', 'url'], (err, result) => {
  if (err) {
    throw err;
  }

  let ds = app.dataSource('remote', result);
  let models = new Set();
  app.models().forEach(Model => {
    if (isPersisted(Model)) {
      models.add(Model);
      for (let relation in Model.relations) {
        let ModelThrough = Model.relations[relation].modelThrough;
        if (ModelThrough && isPersisted(ModelThrough)) {
          models.add(ModelThrough);
        }
      }
    }
  });

  function isPersisted(Model) {
    return Model.getDataSource() && Model.find;
  }

  Promise.all(Array.from(models).map(Model => {
    let RemoteModel = Model.extend(Model.modelName, {});
    RemoteModel.attachTo(ds);
    return RemoteModel.find()
      .then(instances =>
        // We can't just call Model.create on the array because it will not
        // return a promise in that case
        Promise.all(instances.map((instance) => Model.create(instance))))
      .catch(err => {
        console.error('I got an error for copying the data of', Model.modelName);
        console.error(err);
        console.error(err.stack);
      });
  })).then(() => process.exit());
});