feat: add transform function

README.md

@@ -88,6 +88,7 @@ Options are:
 * `protocol` - the protocol the Elasticsearch server uses. Defaults to http.
 * `hydrate` - whether or not to replace ES source by mongo document.
 * `filter` - the function used for filtered indexing.
+* `transform` - the function used for transforming a document before indexing it, accepts the document as an argument, expects transformed document to be returned (if returned value is falsy, the original document will be used).
 * `idsOnly` - whether or not returning only mongo ids in `esSearch`.
 * `countOnly` - whether or not returning only the count value in `esCount`.
 * `mappingSettings` - default settings to use with `esCreateMapping`.
@@ -99,7 +100,6 @@ Options are:
 * `bulk.delay` - idle time to wait before calling the `client.bulk` function. Defaults to 1000.
 * `onlyOnDemandIndexing` - whether or not to demand indexing on CRUD operations. If set to true middleware hooks for save, update, delete do not fire. Defaults to false.
 
-
 To have a model indexed into Elasticsearch simply add the plugin.
 
 ```javascript
@@ -378,6 +378,24 @@ MovieSchema.plugin(mexp, {
 });
 ```
 
+### Transforming a document before indexing
+
+You can specify a function to transform a document before indexing it in ElasticSearch.
+
+```javascript
+var MovieSchema = new mongoose.Schema({
+  title: {type: String},
+  genre: {type: String, enum: ['horror', 'action', 'adventure', 'other']}
+});
+
+MovieSchema.plugin(mexp, {
+  transform: function (doc) {
+    delete doc.genre;
+    return doc;
+  }
+});
+```
+
 Instances of Movie model having 'action' as their genre will be indexed to Elasticsearch.
 
 ### Indexing On Demand

lib/index.js

@@ -470,7 +470,13 @@ function indexDoc(update, callback) {
   }
   return utils.run(callback, (resolve, reject) => {
     const esOptions = self.esOptions();
-    const body = utils.serialize(self, esOptions.mapping);
+    let body = utils.serialize(self, esOptions.mapping);
+    if (typeof esOptions.transform === 'function') {
+      const transformedBody = esOptions.transform(body);
+      if (transformedBody) {
+        body = transformedBody;
+      }
+    }
     if (update && update.unset) {
       (typeof update.unset === 'string'
         ? [update.unset]

test/es7/esIndex.js

@@ -17,12 +17,23 @@ describe('esIndex', () => {
         es_type: 'geo_point',
         es_boost: 1.5,
       },
+      doNotIndexMe: Boolean,
     });
 
-    UserSchema.plugin(plugin);
+    UserSchema.plugin(plugin, {
+      transform: document => {
+        delete document.doNotIndexMe;
+        return document;
+      },
+    });
     const UserModel = mongoose.model('User', UserSchema);
 
-    const john = new UserModel({ name: 'John', age: 35, pos: [5.7333, 43.5] });
+    const john = new UserModel({
+      name: 'John',
+      age: 35,
+      pos: [5.7333, 43.5],
+      doNotIndexMe: true,
+    });
 
     return utils
       .deleteModelIndexes(UserModel)