// Identifier
{
   _id: "12345xyz",
   client: "BKD2019",
   customer: {
          _id: "cus123", // Identifier[customer_id]
          customerName: "bezkoder",
          location: 29,
          country: "IN"
     }
}

// client
{
   _id: "12345xyz",
   client: "BKD2019",
   customer_id: "cus123",
}

// Customer
{
   _id: "cus123",  
   name: "bezkoder",
   location: 29,
   country: "male"
}