mergeCollection(collection) {
    this.models = this.models.concat(collection.models);

    return this;
  }
Modifies the Collection by merging the models from another collection.

    ```js
    user.posts.mergeCollection(newPosts);
    user.posts.save();
    ```

    @method mergeCollection
    @param {Collection} collection
    @return this
    @public