title: Product.find({category: req.params.category}) doesn't work
body: <p>Now in product schema, I have a category attached to it like that </p>
  
  <pre><code>        "_id" : ObjectId("5e6dfde62764a11b34ccc9a3"),
          "title" : "t-shirt",
          "price" : 12,
          "category" : "T-shirts",
          "description" : "&lt;p&gt;tshirts description goes here&lt;/p&gt;\r\n",
          "images" : [ ],
          "__v" : 0
  </code></pre>
  
  <p>and my route is like that:</p>
  
  <pre><code>
  router.get('/category/:category', function(req, res){
      var categorySlug = req.params.category;
      Category.findOne({slug: categorySlug}, function(err, category){
          if(err){
              console.log(err);
          } else {
              Product.find({category: categorySlug}, function(err, products){
                  if(err){
                      console.log(err);
                  } else {
                      console.log(category)
                      console.log(products)
                      res.render('client/cat_product', {products, category, title: products.title});
                  }
              })
          }
  
      })
  })
  </code></pre>
  
  <p>The problem is: 
  when I console.log(category) it displays the category normally but when I console.log(products) it gives me a blank array with no products in it </p>
  
  <p>these are the schema models for the category and the product: </p>
  
  <pre><code>const mongoose = require('mongoose');
  const Schema = mongoose.Schema;
  
  const CategorySchema = new Schema({
      title: {
          type: String,
          required: true
      },
      slug: String,
      products: [
          {
              type: Schema.Types.ObjectId,
              ref: 'Product'
          }
      ]
  
  });
  
  const Category = mongoose.model('Category', CategorySchema);
  module.exports = Category;
  </code></pre>
  
  <hr>
  
  <pre><code>const mongoose = require('mongoose');
  const Schema = mongoose.Schema;
  
  const ProductSchema = new Schema({
      title: {
          type: String,
          required: true
      },
      description: {
          type: String,
          required: true
      },
      category: {
          type: String,
          required: true
      },
      price: {
          type: Number,
          required: true
      },
      images: [
          {
              url: String,
              public_id: String
          }
      ]
  });
  
  
  module.exports = mongoose.model('Product', ProductSchema);
  </code></pre>