Add models to LDL options

1. Use 'models' to specify the dependencies to other models
2. The 'models' property is an object, such as:

{
    Model1: 'Model1',
    Model2: Model2
}

3. The model classes will be injected into the newly defined class as
static properties using the keys from the models option.

lib/datasource.js

@@ -363,18 +363,10 @@ DataSource.prototype.defineRelations = function(modelClass, relations) {
         for (var rn in relations) {
             var r = relations[rn];
             assert(DataSource.relationTypes.indexOf(r.type) !== -1, "Invalid relation type: " + r.type);
-            var targetModel = isModelClass(r.model) ? r.model : this.getModel(r.model);
-            if (!targetModel) {
-                // The target model doesn't exist, let create a place holder for it
-                targetModel = this.define(r.model, {}, {unresolved: true});
-            }
+            var targetModel = isModelClass(r.model) ? r.model : this.getModel(r.model, true);
             var throughModel = null;
             if (r.through) {
-                throughModel = isModelClass(r.through) ? r.through : this.getModel(r.through);
-                if (!throughModel) {
-                    // The through model doesn't exist, let create a place holder for it
-                    throughModel = this.define(r.through, {}, {unresolved: true});
-                }
+                throughModel = isModelClass(r.through) ? r.through : this.getModel(r.through, true);
             }
             if (!isModelDataSourceAttached(targetModel) || (throughModel && !isModelDataSourceAttached(throughModel))) {
                 // Create a listener to defer the relation set up
@@ -533,8 +525,8 @@ DataSource.prototype.mixin = function (ModelCtor) {
   });
 };
 
-DataSource.prototype.getModel = function(name) {
-    return this.modelBuilder.getModel(name);
+DataSource.prototype.getModel = function(name, forceCreate) {
+    return this.modelBuilder.getModel(name, forceCreate);
 };
 
 DataSource.prototype.getModelDefinition = function(name) {

lib/model-builder.js

@@ -56,8 +56,19 @@ function isModelClass(cls) {
     return cls.prototype instanceof DefaultModelBaseClass;
 }
 
-ModelBuilder.prototype.getModel = function(name) {
-  return this.models[name];
+/**
+ * Get a model by name
+ * @param {String} name The model name
+ * @param {Boolean} forceCreate Indicate if a stub should be created for the
+ * given name if a model doesn't exist
+ * @returns {*} The model class
+ */
+ModelBuilder.prototype.getModel = function(name, forceCreate) {
+  var model = this.models[name];
+  if(!model && forceCreate) {
+   model = this.define(name, {}, {unresolved: true});
+  }
+  return model;
 };
 
 ModelBuilder.prototype.getModelDefinition = function(name) {
@@ -185,6 +196,14 @@ ModelBuilder.prototype.define = function defineClass(className, properties, sett
             ModelClass[i] = ModelBaseClass[i];
         }
     }
+
+    // Load and inject the model classes
+    if(settings.models) {
+      Object.keys(settings.models).forEach(function(m) {
+        var model = settings.models[m];
+        ModelClass[m] = typeof model === 'string' ? modelBuilder.getModel(model, true) : model;
+      });
+    }
     
     ModelClass.getter = {};
     ModelClass.setter = {};

test/loopback-dl.test.js

@@ -906,3 +906,39 @@ describe('Injected methods from connectors', function(){
   });
 
 });
+
+
+describe('ModelBuilder options.models', function(){
+  it('should inject model classes from models', function() {
+    var builder = new ModelBuilder();
+    var M1 = builder.define('M1');
+    var M2 = builder.define('M2', {}, {models: {
+      'M1': M1
+    }});
+
+    assert.equal(M2.M1, M1, 'M1 should be injected to M2');
+  });
+
+  it('should inject model classes by name in the models', function() {
+    var builder = new ModelBuilder();
+    var M1 = builder.define('M1');
+    var M2 = builder.define('M2', {}, {models: {
+      'M1': 'M1'
+    }});
+
+    assert.equal(M2.M1, M1, 'M1 should be injected to M2');
+  });
+
+  it('should inject model classes by name in the models before the class is defined',
+    function() {
+      var builder = new ModelBuilder();
+      var M2 = builder.define('M2', {}, {models: {
+        'M1': 'M1'
+      }});
+      assert(M2.M1, 'M1 should be injected to M2');
+      assert(M2.M1.settings.unresolved, 'M1 is still a proxy');
+      var M1 = builder.define('M1');
+      assert.equal(M2.M1, M1, 'M1 should be injected to M2');
+  });
+
+});