title: Mongoose-Array empty even with populate
body: <p>I have this two models and I want to populate the array historico but even though I used the method populate the array is still empty. The reason I thinks think this is happening is because i didn´t specify the join parameter. How can I do it? In the find method or in the populate?</p>
  
  <pre><code>var userSchema = new Schema({
    cartaoCidadao: {
      type: String,
      required: true,
      index: true,
      unique: true,
      match: /[0-9]{8}/,
    },
    password: { type: String, required: ["password is a required field"] },
    historico: [
      {
        type: Schema.Types.ObjectId,
        ref: "Request",
      },
    ],
    role: { type: String },
  
    estado: { type: String, enum: ["Infetado", "Suspeito", "Curado"] },
  });
  </code></pre>
  
  <pre><code>var requestSchema = new Schema({
    paciente: {
      type: mongoose.Schema.Types.ObjectId,
      ref: "User",
    },
    encaminhado: {
      type: String,
      required: [true, "encaminhado is a required field"],
      enum: ["Sim", "Não"],
    }, //vem do body
    pessoaRisco: {
      type: String,
      required: [true, "pessoaRisco is a required field"],
      enum: ["Sim", "Não"],
    }, //vem do body
    trabalhoRisco: {
      type: String,
      required: [true, "trabalhoRisco is a required field"],
      enum: ["Sim", "Não"],
    }, //vem do body
    estadoPedido: {
      type: String,
      enum: ["Pendente", "Agendado", "Concluído", "Aguarda Resultado"],
    },
    resultado: { type: String, enum: ["Positivo", "Negativo"] },
    dataExame: { type: Date }, //data tem de ser superior a data atual
  });
  </code></pre>
  
  <pre><code>userController.getUserById = function (req, res, next, id) {
    User.findOne({ _id: id })
    .populate("historico")
    .exec(function (err, user) {
      if (err) {
        next(err);
      } else {
        res.json(user);
      }
    });
  };
  
  userController.getOneUser = function (req, res) {
    res.json({
      cartaoCidadao: req.user.cartaoCidadao,
      historico: req.user.historico,
      estado: req.user.estado,
    });
  };
  </code></pre>