How to programmatically create a new subdocument from outside document?

[niftylettuce]

For example, the only way right now that is documented to create a subdocument is:

const Model = mongoose.model('MyModel', schema);
const obj = new Model();
const subDocument = obj.someSubSchema.create(); // <--

I think it might be nice to document, if possible, a way to create the subDocument without having to create an instance of the model?

[vkarpov15]

Document arrays have a create() method:

'use strict';
  
const mongoose = require('mongoose');

mongoose.set('useFindAndModify', false);

const { Schema } = mongoose;

run().catch(err => console.log(err));

async function run() {
  await mongoose.connect('mongodb://localhost:27017/test', {
    useNewUrlParser: true,
    useUnifiedTopology: true
  });

  await mongoose.connection.dropDatabase();

  const nestedSchema = new Schema({ name: String });
  nestedSchema.methods.getAnswer = () => 42;

  const schema = new Schema({
    arr: [nestedSchema]
  });
  const Model = mongoose.model('Test', schema);

  const doc = new Model({ arr: [] });
  const subdoc = doc.arr.create({ name: 'foo' });

  console.log(subdoc instanceof mongoose.Document); // true
  console.log(subdoc.getAnswer()); // 42
}

But I've struggled to find a case where this is useful. Can you elaborate on your use case for this?

[niftylettuce]

Yes, see my comment/solution here: mblarsen/mongoose-hidden#58 (comment)

[vkarpov15]

@niftylettuce I took a closer look at this, it looks like your issue won't be solved by .create() since you want to convert a POJO to a subdoc without creating a top-level document. In that case, you can use SchemaType#cast() to make Mongoose cast the POJO as shown below:

'use strict';
  
const mongoose = require('mongoose');

mongoose.set('useFindAndModify', false);

const { Schema } = mongoose;

run().catch(err => console.log(err));

async function run() {
  await mongoose.connect('mongodb://localhost:27017/test', {
    useNewUrlParser: true,
    useUnifiedTopology: true
  });

  await mongoose.connection.dropDatabase();

  const nestedSchema = new Schema({ name: String });
  nestedSchema.methods.getAnswer = () => 42;

  const schema = new Schema({
    arr: [nestedSchema],
    singleNested: nestedSchema
  });
  const Model = mongoose.model('Test', schema);

  // Cast to doc array
  let subdoc = Model.schema.path('arr').cast([{ name: 'foo' }])[0];
  console.log(subdoc instanceof mongoose.Document); // true
  console.log(subdoc.getAnswer()); // 42

  // Cast to single nested subdoc
  subdoc = Model.schema.path('singleNested').cast({ name: 'bar' });
  console.log(subdoc instanceof mongoose.Document); // true
  console.log(subdoc.getAnswer()); // 42
}

More on schematypes: https://mongoosejs.com/docs/schematypes.html