Last day of 2021! After spending just over 5 hours scouring the internet I've resorted to posting for help.

Desired Behaviour:

• the list of documents returned by the route will contain populated documents instead of only the ref ids

Attempted Resolutions:

• added exec function after populate
• Stackoverflow lead me down a rabbit hole of believing the issue was related to mongoose connections, and so I revamped the backend so that my model files returned schemas instead of models, and then used mongoose.createConnection instead of .connect, and made a db.js module to hold a single instance of each model (connection.mode()) that route files could then access and share. I did not reach a point where I could test .populate so I git reset back to the below code

Error

MissingSchemaError: Schema hasn't been registered for model "progressions".
Use mongoose.model(name, schema)

Note that this error is thrown when trying to populate "lesson" as well

Progress Model

const Progress = mongoose.model('Progress', new Schema({
  lesson: {
    type: Schema.Types.ObjectId,
    ref: 'lesson'
  },
  progressions: [{
    type: Schema.Types.ObjectId,
    ref: 'progression'
  }],
  progress:{
    type: Number
  },
  user:{
    type: Schema.Types.ObjectId,
    ref: 'user'
  }
},{
  timestamps: true,
}));

module.exports = Progress;

Progress Routes File

router.get('/all', async (req, res) => {
  let user = await User.find({email:req.query.user})
  let data = await Progress.find({user:user}).populate('progressions')
  return res.status(201).json({
     data,
    message: 'data populated',
    success: true
  });
});

Progression Model

const Progression = mongoose.model('Progression', new Schema({

  type:{
    type:String
  },
  toBeReviewed:{
    type: Date,
    default: Date.now
  },
},{
  timestamps: true,
}));

module.exports = Progression;