I couldn’t save Node.js Error objects directly to Mongoose Schema.Types.Mixed fields, so I had to use toString() or _.toPlainObject() before saving them.

Sample Code for Saving Errors to Schema.Types.Mixed

(Example) Sample Code That Failed to Save Error Directly

var err = new Error('bad');

Model.updateOne({
  _id: targetId
}, {
  'mixedField': err ? err.toString() : null
}, function(updatedErr) {
});

(Example) Sample Code That Saved Error Using toString

var err = new Error('good');

Model.updateOne({
  _id: targetId
}, {
  'mixedField': err ? err.toString() : null
}, function(updatedErr) {
});

(Example) Sample Code That Saved Error Using lodash.toPlainObject

const _ = require('lodash');
var err = new Error('good');

Model.updateOne({
  _id: targetId
}, {
  'mixedField': err ? _.toPlainObject(err) : null
}, function(updatedErr) {
});

Summary

I was puzzled because Error objects weren’t being saved to MongoDB, but after changing MongoDB’s LogLevel settings and checking, I found that no values were being passed to the update method in the first place.

I thought Schema.Types.Mixed fields could save anything, but it seems the Mongoose layer doesn’t cast Error objects and instead processes them as saving nothing.

I haven’t read through Mongoose’s source code yet, so I’d like to look into it later.

Reference Information

• Mongoose SchemaTypes
• Errors | Node.js Documentation
• Object.prototype.toString() - JavaScript | MDN
• Lodash Documentation

That’s all from the Gemba.