title: Find if object is changed in pre-save hook mongoose
body: <p>I am trying to find if the object is changed in pre-save and do some actions accordingly. Followinfg is my code</p>
  
  <pre><code>var eql = require("deep-eql");
  
  OrderSchema.post( 'init', function() {
      this._original = this.toObject();
  });
  
  OrderSchema.pre('save', function(next) {
      var original = this._original;
  
      delete this._original;
      if(eql(this, original)){
          //do some actions
      }
      next();
  });
  </code></pre>
  
  <p>It returns false even when I don't change anything! </p>