Why Does Looping Through a Mongoose Object Display Metadata?

Question

Why does looping through a mongoose object with nunjucks display metadata?

I am using mongodb and nunjucks in an app I am writing.

I am trying to iterate through a model called persona, but doing so displays mongoose metadata associated with the record.

If I simply display the persona variable by writing {{persona}}.

My output is as follows. Just the keys/values defined in my schema.

{ _id: 582f186df1f05603132090d5, name: 'Alex', name_lower: 'alex', __v: 0, 
meta: { validated: null, contributors: 'Research Team', sources: '4 Interviews' }, 
pain_points: { points: 'Debugging' }, 
ideal_day: { responsibilities: 'Coding websites.', goals: 'Finish the research site.', joys: 'Good code, Good food.', hobbies: 'Dance, Hiking, Eating' }, 
environment: { workspace: 'Desk', tools: 'Atom, Sketch', info_from: null, info_to: null, coworkers_relationship: null, technology_relationship: null }, 
basic_info: { jobtitle: 'FED', experience: '2', education: 'CS', company: '' } }

However, if I loop through the persona

    {% for name, item in persona %}
        {{ name }} : {{ item }}
    {% endfor %}

In addition to displaying the keys in my schema, all mongoose metadata associated with the record will also be displayed. I would like to understand why different information is displayed when I am looping over the object.

    $__
    isNew
    errors
    _doc
    $__original_save
    save
    _pres
    _posts
    $__original_validate
    validate
    $__original_remove
    remove
    db
    discriminators
    __v
    id
    _id
    meta
    pain_points
    ideal_day
    environment
    basic_info
    updated_at
    created_at
    name_lower
    name
    schema
    collection
    $__handleSave
    $__save
    $__delta
    $__version
    increment
    $__where

I was able to fix this problem by using Mongoose's lean(), but still don't understand why I experienced this behavior.

Answer 1

When you call {{persona}} then result is persona.toString().
If object doesn't have override method toString then result will be [Object object] (by default toString method).

When you use loop {% for key, value in persona %} then it's equals to

for(var key in obj)
  print(key + ' - ' + obj[key]);

This code prints all object properties and methods.

To exclude methods you must use next loop

for(var key in obj)
  if (typeof(obj) != 'function') // or obj.hasOwnProperty(key)
      print(key + ' ' + obj[key]);

So, to avoid your problem you must "clear" data before pass it to nunjucks or before output.
You can do it define custom filter

var env = nunjucks.configure(...

env.addFilter('lean', function(obj) {
    var res = {};
    for(var key in obj)
        if (typeof(obj) != 'function') // or obj.hasOwnProperty(key)
           res[key] = obj[key];
    return res;
});
...
{% for key, value in persona | lean %}
{{key}} - {{value}}
{% endfor %}