title: Populating array of collection which contains references to another collections returns empty array
body: <p>I have two models Vote and Link,I am trying to populate the votes array in link model,The votes array contains id's that references to the collection Vote,which only contains two fields link and User which also refs to same link model mentioned below and a user model respectively </p>
  
  <p>link Schema:-</p>
  
  <pre><code>const linkSchema = new mongoose.Schema(
    {
      description: {
        type: String,
        trim: true,
      },
      url: {
        type: String,
        trim: true,
      },
      postedBy: {
        type: mongoose.Types.ObjectId,
        ref: "User",
      },
      votes: [{ type: mongoose.Types.ObjectId, ref: "Vote" }],
    },
    {
      timestamps: true,
    }
  );
  
  linkSchema.index({ description: "text" });
  linkSchema.index({ createdAt: -1 });
  module.exports = mongoose.model("Link", linkSchema);
  </code></pre>
  
  <p>Vote schema:-</p>
  
  <pre><code>const mongoose = require("mongoose");
  
  const voteSchema = new mongoose.Schema({
    link: { type: mongoose.Types.ObjectId, ref: "Link" },
    user: { type: mongoose.Types.ObjectId, ref: "User" }, 
  });
  
  module.exports = mongoose.model("Vote", voteSchema);
  </code></pre>
  
  <p>but when i try to get the votes of a link,it always return an empty array ,My function:-</p>
  
  <pre><code>const votes = async ({ id }) =&gt; {
    const linkData = await Link.findById(id).populate("votes").exec();
    console.log(linkData);
  
  };
  
  </code></pre>
  
  <p>Output Data:-</p>
  
  <pre><code>{
    votes: [], //empty always
    _id: 5ecb21059a157117c03d4fac,
    url: 'https://www.apollographql.com/docs/react/',
    description: 'The best GraphQL client for React',
    postedBy: 5ec92a58bf38c32b38400705,
    createdAt: 2020-05-25T01:36:05.892Z,
    updatedAt: 2020-05-25T01:37:52.266Z,
    __v: 0
  }
  </code></pre>