title: How could I get songs of a specific album filtered by Category using mongodb?
body: <p>Here is my Song model:</p>
  <pre><code>
  const Schema = mongoose.Schema;
  
  const songSchema = new Schema({
    name: {
      type: String,
      required: true,
    },
    singer: {
      type: String,
      required: true,
    },
  
    categoryId: {
      type: Schema.Types.ObjectId,
      ref: &quot;Category&quot;,
    },
  
    albumId: {
      type: Schema.Types.ObjectId,
      ref: &quot;Album&quot;,
    },
    __v: { type: Number, select: false },
  });
  
  module.exports = mongoose.model(&quot;Song&quot;, songSchema);
  </code></pre>
  <p>As the title said, I wish to get back all songs of a specific album filtered by category. I am still new to mongodb and would like to understand how could I use aggregate to solve my problem?</p>