title: mongodb Error mongoose do not push object in array $pushAll
body: <p>I have a simple app with User and Post models,  </p>
  
  <pre><code>var mongoose = require("mongoose");
  mongoose.connect("mongodb://localhost/assoc", {useMongoClient:true});
  mongoose.Promise = global.Promise;
  
  //Post
  var postSchema = new mongoose.Schema({
      title: String,
      content: String
  });
  var Post = mongoose.model("Post", postSchema);
  
  //User
  var userSchema = new mongoose.Schema({
      email: String,
      name: String,
      posts: [postSchema]
  });
  
  var User = mongoose.model("User", userSchema);
  </code></pre>
  
  <p>I Create a user before (name: "gino") and push a post into:</p>
  
  <pre><code>// var newUser = new User({
  //     email: "a.b@c.it",
  //     name: "gino"
  // });
  //
  // newUser.posts.push({
  //     title: "gino's post",
  //     content: "this is content"
  // });
  //
  // newUser.save(function (err, user) {
  //     if (err) {
  //         console.log(err);
  //     } else {
  //         console.log(user);
  //     }
  // });
  </code></pre>
  
  <p>Also create another post to check if Post model works:</p>
  
  <pre><code>// var newPost = new Post({
  //     title: "honky",
  //     content: "tonky"
  // });
  //
  // newPost.save(function (err, post) {
  //     if (err) {
  //         console.log(err);
  //     } else {
  //         console.log(post);
  //     }
  // });
  </code></pre>
  
  <p>When I try to find "gino" and push a new item into the posts array I have an error trying to save user (user.save) with this snippet:</p>
  
  <pre><code>User.findOne({name: "gino"}, function (err, user) {
      if (err) {
          console.log(err);
      } else {
          console.log(user);
          user.posts.push({
              title: "post",
              content: "content"
          });
          user.save(function (err, user) {
              if (err) {
                  console.log(err);
              } else {
                  console.log(user);
              }
          });
      }
  });
  </code></pre>
  
  <p>When I run the app i got this: </p>
  
  <pre><code>{ MongoError: Unknown modifier: $pushAll
      at Function.MongoError.create (appFolder\node_modules\mongodb-core\lib\error.js:31:11)
      at toError (appFolder\node_modules\mongodb\lib\utils.js:139:22)
      at appFolder\node_modules\mongodb\lib\collection.js:1059:67
      at appFolder\node_modules\mongodb-core\lib\connection\pool.js:469:18
      at _combinedTickCallback (internal/process/next_tick.js:131:7)
      at process._tickCallback (internal/process/next_tick.js:180:9)
    name: 'MongoError',
    message: 'Unknown modifier: $pushAll',
    driver: true,
    index: 0,
    code: 9,
    errmsg: 'Unknown modifier: $pushAll' }
  </code></pre>
  
  <p>Someone can help me? </p>