fix(document): apply virtuals to subdocuments if parent schema has virtuals: true for backwards compatibility

[vkarpov15]

Fix #14771
Re: #14394
Re: #14623

Summary

Fixing #14771, which is a bug that was likely introduced by #14623. The issue is that older versions of Mongoose make parent virtuals option apply to subdocs presuming that the subdocs don't explicitly set virtuals.

const userLabSchema = new mongoose.Schema({
  capacityLevel: Number
});

userLabSchema.virtual('capacityLevelCeil').get(function() {
  return Math.ceil(this.capacityLevel);
});

const labPlotSchema = new mongoose.Schema({
  plotId: Number,
  lab: userLabSchema
});

const userSchema = new mongoose.Schema({
  username: String,
  labPlots: [labPlotSchema]
}, { toObject: { virtuals: true } });

const User = mongoose.model('User', userSchema);

const doc = new User({
  username: 'test',
  labPlots: [{
    plotId: 1,
    lab: { capacityLevel: 3.14 }
  }]
});
// In Mongoose 8.4.0, virtual is applied because top-level schema has `toObject: { virtuals: true }`
// In Mongoose 8.5.2, virtual is not applied because `userLabSchema` does not have `toObject: { virtuals: true }`
console.log(doc.toObject().labPlots[0].lab.capacityLevelCeil);

Examples