Is there a way to get the model name of a subdocument ?

I have this code :

        const BSchema = mongoose.Schema({
            name: {
                type: String
            },

        });
        BSchema.post('save', async function () {
            console.log(this.constructor.modelName, this.parent().constructor.modelName)
        })
        const BModel = mongoose.model('b', BSchema);

        const ASchema = mongoose.Schema({
            a: {
                type: BModel.schema,
            },
        });
        const AModel = mongoose.model('a', ASchema);

        const aModel = new AModel({ a: { name: 'hello' } })
        await aModel.save();

Expected output is : "b a" but I have "undefined a"