How to get data from multiple collections using C# mongodb

{ "_id" : 1, "item" : "abc", "price" : 12, "quantity" : 2 }
{ "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1 }
{ "_id" : 3  }

{ "_id" : 1, "sku" : "abc", description: "product 1", "instock" : 120 }
{ "_id" : 2, "sku" : "def", description: "product 2", "instock" : 80 }
{ "_id" : 3, "sku" : "ijk", description: "product 3", "instock" : 60 }
{ "_id" : 4, "sku" : "jkl", description: "product 4", "instock" : 70 }

<pre>
        public IEnumerable<Product> GetProducts()
        {
            return _db.GetCollection<Product>("Products").FindAll();
        }

public class Product
   {
       public ObjectId Id { get; set; }
       [BsonElement("ProductId")]
       public int ProductId { get; set; }
       [BsonElement("ProductName")]
       public string ProductName { get; set; }
       [BsonElement("Price")]
       public int Price { get; set; }
       [BsonElement("Category")]
       public string Category { get; set; }
   }