added tutorial to getters-setters

docs/tutorials/getters-setters.md

@@ -70,6 +70,19 @@ below.
 [require:getters/setters.*setters.*update skip]
 ```
 
+## Passing Parameters using `$locals`
+
+You can't pass parameters to your getter and setter functions like you do to normal function calls.
+To configure or pass additional properties to your getters and setters, you can use the document's `$locals` property.
+
+The `$locals` property is the preferred place to store any program-defined data on your document without conflicting with schema-defined properties.
+In your getter and setter functions, `this` is the document being accessed, so you set properties on `$locals` and then access those properties in your getters examples.
+For example, the following shows how you can use `$locals` to configure the language for a custom getter that returns a string in different languages.
+
+```javascript
+[require:getters/setters.*localization.*locale]
+```
+
 Differences vs ES6 Getters/Setters
 ----------------------------------
 

test/docs/getters-setters.test.js

@@ -174,4 +174,53 @@ describe('getters/setters', function() {
       // acquit:ignore:end
     });
   });
+  describe('localization', function() {
+    it('locale', async function() {
+      const internationalizedStringSchema = new Schema({
+        en: String,
+        es: String
+      });
+
+      const ingredientSchema = new Schema({
+        // Instead of setting `name` to just a string, set `name` to a map
+        // of language codes to strings.
+        name: {
+          type: internationalizedStringSchema,
+          // When you access `name`, pull the document's locale
+          get: function(value) {
+            return value[this.$locals.language || 'en'];
+          }
+        }
+      });
+
+      const recipeSchema = new Schema({
+        ingredients: [{ type: mongoose.ObjectId, ref: 'Ingredient' }]
+      });
+
+      const Ingredient = mongoose.model('Ingredient', ingredientSchema);
+      const Recipe = mongoose.model('Recipe', recipeSchema);
+
+      // Create some sample data
+      const { _id } = await Ingredient.create({
+        name: {
+          en: 'Eggs',
+          es: 'Huevos'
+        }
+      });
+      await Recipe.create({ ingredients: [_id] });
+
+      // Populate with setting `$locals.language` for internationalization
+      const language = 'es';
+      const recipes = await Recipe.find().populate({
+        path: 'ingredients',
+        transform: function(doc) {
+          doc.$locals.language = language;
+          return doc;
+        }
+      });
+
+      // Gets the ingredient's name in Spanish `name.es`
+      assert.equal(recipes[0].ingredients[0].name, 'Huevos'); // 'Huevos'
+    });
+  })
 });