title: How to search in Mongoose populate array [array containing ref]
body: <p>I'm new in MEAN stack development. Please anyone tell how to search in Mongoose populate array. that array containing ref.</p>
  <p><strong>Discussion Schema:</strong></p>
  <pre><code>const discussionSchema = new Schema({
    user_id: {
      type: mongoose.Schema.Types.ObjectId,
      ref: 'User',
      required: true
    },
    subject_title: {
      type: String,
      required: true
    },
  keywords: [
      {
        type: Schema.ObjectId,
        ref: 'Keyword',
        default: null
      }
  }, 
  {
    timestamps: true
  }
  )
  
  </code></pre>
  <p><strong>Keyword Schema:</strong></p>
  <pre><code>const keywordSchema = new Schema({
    keyword:{ 
      type: String,
      required: true,
      unique: true, 
    }
    
  }, {
    timestamps: true
  })
  </code></pre>
  <p><strong>How To Search Keyword String In keyword array containing ref ID of Keyword Model.</strong></p>