Mongoose find term in populate array

Question

I have two collections: Consultas and Pacientes. I try to get consultations for the patient's name by a term, but was unsuccessful.

var consultaSchema = new Schema({

    //detalles de la consulta
    paciente_c: { type: Schema.Types.ObjectId, ref: 'Paciente', required: true },
    medio_c: { type: String, required: false },
    /* ... */
}, { timestamps: true } );

module.exports = mongoose.model('Consulta', consultaSchema);

var pacienteSchema = new Schema({

    nombre: { type: String, required: [true, 'El nombre es necesario']},
    apellido: { type: String, required: false },
    /* ... */
}, { timestamps: true } );

module.exports = mongoose.model('Paciente', pacienteSchema);

And the query:

Consulta.find({})
    .populate('paciente_c', 'nombre apellido')
    //.or( { paciente_c: { nombre: regex } } )
    .or( { 'paciente_c.nombre': regex } )
    .exec( (err, consultas )=> {
        if (err) {
            reject('Error al cargar consultas !!!', err);
        } else {
            resolve(consultas)
        }
    });

EDIT: Adding the following line

.populate( 'paciente_c', null, { nombre: { $in: regex } } )

The query brings all the consultations and only populates 'patient_c' in which it finds the term, but it is not what I need. I just want to get the queries in which the term is found and not all.

EDIT 2: The following line work fine, but how it compares between two values?

return new Promise((resolve, reject)=> {

    Consulta.find({})
        .populate({
            path: 'paciente_c',
            match: { nombre: { $in: regex } }
        })
        .exec( (err, consultas )=> {
            if (err) {
                reject('Error al cargar consultas !!!', err);
            } else {
                consultas = consultas.filter(c => c.paciente_c != null);
                resolve(consultas)
            }
        });
});