How to keep the fields order in DB when creating a new doc?

[flight9]

Do you want to request a feature or report a bug?
Maybe a bug. Not sure

What is the current behavior?
When u save a new doc, it's fields order can't be kept as defined in schema or its orignal order.

If the current behavior is a bug, please provide the steps to reproduce.

const mongoose = require('mongoose');
const Schema = mongoose.Schema;
mongoose.connect('mongodb://YOUR_CONNECTION_STRING');

const articleSchema = new Schema({
  'ug_guid': {
    'type': 'String'
  },
  'assetTypeId': {
    'type': Schema.Types.ObjectId,
    'ref': 'assetTypes'
  },
  'mesa_key': {
    'type': 'String'
  },
  'site_specific_name': {
    'type': 'String'
  },
  'serial_number': {
    'type': 'String'
  },
  'mesaTrees': {
    'what': [
      {
        'ns1': {
          'type': 'String'
        },
        'ns2': {
          'type': 'String'
        },
        'what1': {
          'type': 'String',
          'required': true
        },
        'what2': {
          'type': 'String'
        },
        'what3': {
          'type': 'String'
        },
        'what4': {
          'type': 'String'
        },
        'what5': {
          'type': 'String'
        },
        'what6': {
          'type': 'String'
        },
        'what7': {
          'type': 'String'
        },
        'what8': {
          'type': 'String'
        }
      }
    ],
    'ent': [
      {
        'ent1': {
          'type': 'String',
          'required': true
        },
        'ent2': {
          'type': 'String'
        },
        'ent3': {
          'type': 'String'
        }
      }
    ],
    'at': {
      'type': 'String'
    }
  }
});
const Article = mongoose.model('Article', articleSchema);

let data = { 
  ug_guid: '6db93bc6-f0eb-4c31-b4db-4b90a185f927',
  assetTypeId:'5a436c14fb18bba37f8384e3',
  mesa_key: 'mesa_key2_1',
  site_specific_name: 'B Test Site',
  serial_number: '93425726762957156',
  mesaTrees:{ 
    what:
      [ { ns1: 'namespace1',
          ns2: 'namespace2',
          what1: 'what1',
          what2: 'what2',
          what3: 'what3',
          what4: 'what4',
          what5: 'what5',
          what6: 'what6',
          what7: 'what7',
          what8: 'what8',
          _id: '5a436c14fb18bba37f8384e3' } ],
    ent:
      [ { ent1: 'ent1',
          ent2: 'ent2',
          ent3: 'ent3',
          _id: '5a436c14fb18bba37f8384e3' 
        },
        { ent1: 'ent4',
          ent2: 'ent5',
          ent3: 'ent6',
          _id: '5a436c14fb18bba37f8384e3'
        }
      ],
    at: 'at1' 
  } 
};
var art = new Article(data);
art.save(function (err) {
  if (err) {
    console.error('Error:', err);
  }
});

But I got the order in MongoDB like: (NOTE: mesaTrees is promoted to front.)

{
    "_id" : ObjectId("5b878bc0e81523a511b0c42b"),
    "mesaTrees" : {
        "what" : [ 
            {
                "_id" : ObjectId("5a436c14fb18bba37f8384e3"),
                "ns1" : "namespace1",
                "ns2" : "namespace2",
                "what1" : "what1",
                "what2" : "what2",
                "what3" : "what3",
                "what4" : "what4",
                "what5" : "what5",
                "what6" : "what6",
                "what7" : "what7",
                "what8" : "what8"
            }
        ],
        "ent" : [ 
            {
                "_id" : ObjectId("5a436c14fb18bba37f8384e3"),
                "ent1" : "ent1",
                "ent2" : "ent2",
                "ent3" : "ent3"
            }, 
            {
                "_id" : ObjectId("5a436c14fb18bba37f8384e3"),
                "ent1" : "ent4",
                "ent2" : "ent5",
                "ent3" : "ent6"
            }
        ],
        "at" : "at1"
    },
    "ug_guid" : "6db93bc6-f0eb-4c31-b4db-4b90a185f927",
    "assetTypeId" : ObjectId("5a436c14fb18bba37f8384e3"),
    "mesa_key" : "mesa_key2_1",
    "site_specific_name" : "B Test Site",
    "serial_number" : "93425726762957156",
    "__v" : 0
}

What is the expected behavior?
Keep the fields order in DB as defined in schema or its orignal order.

Version
node.js: 8.9.3
mongoose: 5.2.10
MongoDB: 3.6.1

[lineus]

@flight9 if you add a type property to your mesaTrees schema path object like all of your other paths, the order is restored.

Here is an example using your schema with the suggested change:

6944.js

#!/usr/bin/env node
'use strict';

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost:27017/test', { useNewUrlParser: true });
const conn = mongoose.connection;
const Schema = mongoose.Schema;

const articleSchema = new Schema({
  'ug_guid': {
    'type': 'String'
  },
  'assetTypeId': {
    'type': Schema.Types.ObjectId,
    'ref': 'assetTypes'
  },
  'mesa_key': {
    'type': 'String'
  },
  'site_specific_name': {
    'type': 'String'
  },
  'serial_number': {
    'type': 'String'
  },
  'mesaTrees': {
    type: {
      'what': [
        {
          'ns1': {
            'type': 'String'
          },
          'ns2': {
            'type': 'String'
          },
          'what1': {
            'type': 'String',
            'required': true
          },
          'what2': {
            'type': 'String'
          },
          'what3': {
            'type': 'String'
          },
          'what4': {
            'type': 'String'
          },
          'what5': {
            'type': 'String'
          },
          'what6': {
            'type': 'String'
          },
          'what7': {
            'type': 'String'
          },
          'what8': {
            'type': 'String'
          }
        }
      ],
      'ent': [
        {
          'ent1': {
            'type': 'String',
            'required': true
          },
          'ent2': {
            'type': 'String'
          },
          'ent3': {
            'type': 'String'
          }
        }
      ],
      'at': {
        'type': 'String'
      }
    }
  }
});

const Article = mongoose.model('Article', articleSchema);

let data = {
  ug_guid: '6db93bc6-f0eb-4c31-b4db-4b90a185f927',
  assetTypeId: '5a436c14fb18bba37f8384e3',
  mesa_key: 'mesa_key2_1',
  site_specific_name: 'B Test Site',
  serial_number: '93425726762957156',
  mesaTrees: {
    what:
      [{
        ns1: 'namespace1',
        ns2: 'namespace2',
        what1: 'what1',
        what2: 'what2',
        what3: 'what3',
        what4: 'what4',
        what5: 'what5',
        what6: 'what6',
        what7: 'what7',
        what8: 'what8',
        _id: '5a436c14fb18bba37f8384e3'
      }],
    ent:
      [{
        ent1: 'ent1',
        ent2: 'ent2',
        ent3: 'ent3',
        _id: '5a436c14fb18bba37f8384e3'
      },
      {
        ent1: 'ent4',
        ent2: 'ent5',
        ent3: 'ent6',
        _id: '5a436c14fb18bba37f8384e3'
      }
      ],
    at: 'at1'
  }
};

var art = new Article(data);

async function run() {
  await conn.dropDatabase();
  await art.save();
  let doc = await Article.collection.findOne({});
  console.log(doc);
  return conn.close();
}

run();

Output:

issues: ./6944.js
{ _id: 5b87bd3a80feb70f7ae23e5a,
  ug_guid: '6db93bc6-f0eb-4c31-b4db-4b90a185f927',
  assetTypeId: 5a436c14fb18bba37f8384e3,
  mesa_key: 'mesa_key2_1',
  site_specific_name: 'B Test Site',
  serial_number: '93425726762957156',
  mesaTrees: { what: [ [Object] ], ent: [ [Object], [Object] ], at: 'at1' },
  __v: 0 }
issues:

I'm not sure if this is a bug, I'll leave this open and we'll investigate. Thanks for reporting this!

[flight9]

@lineus thanks

[vkarpov15]

@lineus be careful, your solution converts mesaTrees to a Mixed type. As a workaround, do this instead:

const articleSchema = new Schema({
  'ug_guid': {
    'type': 'String'
  },
  'assetTypeId': {
    'type': Schema.Types.ObjectId,
    'ref': 'assetTypes'
  },
  'mesa_key': {
    'type': 'String'
  },
  'site_specific_name': {
    'type': 'String'
  },
  'serial_number': {
    'type': 'String'
  },
  'mesaTrees': new Schema({
      'what': [
        {
          'ns1': {
            'type': 'String'
          }
        }
      ]
  });
});

That should work better.

The issue here is that Mongoose's steps for creating a new document via new Article() are:

1. $__buildDoc()
2. $__applyDefaults()
3. Initial set

$__buildDoc() sets all nested paths, like mesaTrees, to an empty object {} by default. Mongoose then modifies the default empty object in place during initial set, so nested paths always come first in key order. This key order switch is very confusing and something we're going to have to improve on in general, but the above fix includes a reasonable solution that should cover the vast majority of cases.