text: var mongoose = require("mongoose");
  var Schema = mongoose.Schema;
  var referenceSchema = new Schema({
    textbook: String,
    chapter: String,
    grammar: { type: Schema.Types.ObjectId, ref: "Grammar" }
  }, {
    timestamps: true
  });
  var Reference = mongoose.model("Reference", referenceSchema);
  
  module.exports = Reference;
  