registry: warn when dataSource is not specified

Modify `registry.configureModel()` to log a warning when `dataSource`
optiont is not specified at all.

Users should provide `dataSource: null` when the model is intentionally
not attached to any data-source.

lib/registry.js

@@ -10,6 +10,7 @@
 var assert = require('assert');
 var extend = require('util')._extend;
 var juggler = require('loopback-datasource-juggler');
+var debug = require('debug')('loopback:registry');
 var DataSource = juggler.DataSource;
 var ModelBuilder = juggler.ModelBuilder;
 
@@ -171,6 +172,18 @@ registry.configureModel = function(ModelCtor, config) {
         'Cannot configure ' + ModelCtor.modelName +
         ': config.dataSource must be an instance of DataSource');
     ModelCtor.attachTo(config.dataSource);
+    debug('Attached model `%s` to dataSource `%s`',
+      ModelCtor.definition.name, config.dataSource.name);
+  } else if (config.dataSource === null) {
+    debug('Model `%s` is not attached to any DataSource by configuration.',
+      ModelCtor.definition.name);
+  } else {
+    debug('Model `%s` is not attached to any DataSource, possibly by a mistake.',
+      ModelCtor.definition.name);
+    console.warn(
+      'The configuration of `%s` is missing `dataSource` property.\n' +
+      'Use `null` or `false` to mark models not attached to any data source.',
+      ModelCtor.definition.name);
   }
 };
 

test/app.test.js

@@ -70,9 +70,14 @@ describe('app', function() {
       });
     });
 
+    it('accepts null dataSource', function() {
+      app.model('MyTestModel', { dataSource: null });
+    });
+
     it('should not require dataSource', function() {
       app.model('MyTestModel', {});
     });
+
   });
 
   describe('app.model(name, config)', function () {

test/loopback.test.js

@@ -185,6 +185,7 @@ describe('loopback', function() {
       var model = loopback.Model.extend(uniqueModelName);
 
       loopback.configureModel(model, {
+        dataSource: null,
         relations: {
           owner: {
             type: 'belongsTo',
@@ -207,6 +208,7 @@ describe('loopback', function() {
       });
 
       loopback.configureModel(model, {
+        dataSource: null,
         relations: {
           owner: {
             model: 'Application'