loopbackjs: Attach a model to different datasources

Question

I have defined several models that use a Datasource "db" (mysql) for my environment.

Is there any way to have several datasources attached to those models, so I would be able to perform REST operations to different databases?

i.e: GET /api/Things?ds="db"

GET /api/Things?ds="anotherdb"

GET /api/Things (will use default ds)

Answer 1

As @superkhau pointed above, each LoopBack Model can be attached to a single data-source only.

You can create (subclass) a new model for each datasource you want to use. Then you can either expose these per-datasource models via unique REST URLs, or you can implement a wrapper model that will dispatch methods to the correct datasource-specific model.

In my example, I'll show how to expose per-datasource models for a Car model that is attached to db and anotherdb. The Car model is defined in the usual way via common/models/car.json and common/models/car.js.

Now you need to define per-datasource models:

// common/models/car-db.js
{
  "name": "Car-db",
  "base": "Car",
  "http": {
    "path": "/cars:db"
  }
}

// common/models/car-anotherdb.js
{
  "name": "Car-anotherdb",
  "base": "Car",
  "http": {
    "path": "/cars:anotherdb"
  }

}

// server/model-config.json
{
  "Car": {
    "dataSource": "default"
  },
  "Car-db": {
    "dataSource": "db"
  },
  "Car-anotherdb": {
    "dataSource": "anotherdb"
  }
}

Now you have the following URLs available:

GET /api/Cars:db
GET /api/Cars:anotherdb
GET /api/Cars

The solution outlined above has two limitations: you have to define a new model for each datasource and the datasource cannot be selected using a query parameter.

To fix that, you need a different approach. I'll again assume there is a Car model already defined.

Now you need to create a "dispatcher".

// common/models/car-dispatcher.json
{
  "name": "CarDispatcher",
  "base": "Model", //< important!
  "http": {
    "path": "/cars"
  }
}

// common/models/car-dispatcher.js
var loopback = require('loopback').PersistedModel;
module.exports = function(CarDispatcher) {
  Car.find = function(ds, filter, cb) {
    var model = this.findModelForDataSource(ds);
    model.find(filter, cb);
  };

  // a modified copy of remoting metadata from loopback/lib/persisted-model.js
  Car.remoteMethod('find', {
    isStatic: true,
    description: 'Find all instances of the model matched by filter from the data source',
    accessType: 'READ',
    accepts: [
     {arg: 'ds', type: 'string', description: 'Name of the datasource to use' },
     {arg: 'filter', type: 'object', description: 'Filter defining fields, where, orderBy, offset, and limit'}
    ],
    returns: {arg: 'data', type: [typeName], root: true},
    http: {verb: 'get', path: '/'}
  });

  // TODO: repeat the above for all methods you want to expose this way

  Car.findModelForDataSource = function(ds) {
    var app = this.app;
    var ds = ds && app.dataSources[ds] || app.dataSources.default;

    var modelName = this.modelName + '-' + ds;
    var model = loopback.findModel(modelName);
    if (!model) {
      model = loopback.createModel(
        modelName, 
        {},
        { base: this.modelName });
    }

    return model;
  };  
};

The final bit is to remove Car and use CarDispatcher in the model config:

// server/model-config.json
{
  "CarDispatcher": {
    dataSource: null,
    public: true
  }
}