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Recently, I got a quite wired question on Mongoose, It's about the using of array. There is different use experience but I cannot find out any discussion or issue on this, so I tried to explain here, and ask someone know what's happened.

In javascript, Object is added as reference to array

    let existed;
    let company = { _id: "87" };
    let post = { evaluations: [] };
    let cost = 5;
    let predictDays = 1;
    let content = "8";
    if(!existed)
    {
        existed = {};
        existed.company = company._id;
        post.evaluations.push(existed);
    }

    existed.cost = cost || 99999999;
    existed.predictDays = predictDays || 100;
    existed.content = content || "";
    console.log(existed);
    console.log(post.evaluations);
Output: 
{company: "87", cost: 5, predictDays: 1, content: "8"}
[{company: "87", cost: 5, predictDays: 1, content: "8"}]

In nodejs + mongoose, I got the totally different experience.

post is Document from findById.

    let existed;
    if(!existed)
    {
        existed = {};
        existed.company = company._id;
        post.evaluations.push(existed);
        /* existed = post.evaluations.find((evaluation) => {
            return evaluation.company.toString() === company._id.toString();
        });*/ // Uncomments this would make existed get the working reference as javascript
    }
    existed.cost = cost || 99999999;
    existed.predictDays = predictDays || 100;
    existed.content = content || "";
    console.log(existed);
    console.log(post.evaluations);
Output: 
{
  company: 5d655f7743e25137f8501c38,
  cost: 50,
  predictDays: 3,
  content: '123321'
}
[{"company":"5d6562bfef7d771e4815dd4f"}]

I cannot figure out why It's not working, but If I uncomment the code, that findagain in array, I could get the expected result as below:

{
  company: 5d655f7743e25137f8501c38,
  cost: 50,
  predictDays: 3,
  content: '123321'
}
[{"company":"5d6563391fc9b65250e83988","cost":50,"predictDays":3,"content":"123321"}]

Did mongoose change the push behavior secretly? Hope It's enough infomation to point out my issue, any information is greatly appreciated.

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2
  • Difference between [{company: "87", cost: 5, predictDays: 1, content: "8"}] vs [{"company":"5d6563391fc9b65250e83988","cost":50,"predictDays":3,"content":"123321"}] Commented Aug 27, 2019 at 17:22
  • Make a minimal example in Javascript, so let someone use It conveniently is wrong in your experience? I cannot provide my whole backend implementation. Commented Aug 27, 2019 at 17:29

2 Answers 2

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1

I found out the source code for current version (5.6.11), and It explain why the push is working out of expected.

  /**
   * Wraps [`Array#push`](https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/Array/push) with proper change tracking.
   *
   * @param {Object} [args...]
   * @api public
   * @method push
   * @memberOf MongooseArray
   */

  push() {
    if (this[arraySchemaSymbol] == null) {
      return _basePush.apply(this, arguments);
    }

    _checkManualPopulation(this, arguments);

    let values = [].map.call(arguments, this._mapCast, this);
    values = this[arraySchemaSymbol].applySetters(values, this[arrayParentSymbol], undefined,
      undefined, { skipDocumentArrayCast: true });
    const ret = [].push.apply(this, values);

    this._registerAtomic('$push', values);
    this._markModified();
    return ret;
  }

I can't understand It totally, but I tried to describe here with my conjecture.

  1. Mongoose need to convert javascript object into sub-document when push a object. (I thought It checked on save())

  2. MongooseArray.push() copy object's properties to a new object, so the original reference passed from the parameter is not actually added.

It's just weak explanation, Hope someone could help to explain It better.

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Comments

0

Yes mongoose array's are different they have their own implementation of push , pop etc on their prototype. You can find the documentation here

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4 Comments

Thanks for infomation, I also see this before I post, but you know the documentation nearly write nothing on push, so I decided to ask here :(
I thought you just wanted to know if they change behavior of push secretly. :P
Yeah, but this documentation could not convince me that the issue is exactly caused by this, maybe source code could explain this, but I tried my best and got lost...
MongooseArray.prototype.push() This means each mongoose array have these methods implemented on its prototye. I think you should read about prototypes a little.

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