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// MongoDB's update() and save() methods are used to update document into a collection. 
// The update() method updates the values in the existing document while the save() 
// method replaces the existing document with the document passed in save() method.

//Syntax - Update
db.COLLECTION_NAME.update(SELECTION_CRITERIA, UPDATED_DATA)
db.collection.updateOne(<filter>, <update>, <options>)
db.collection.updateMany(<filter>, <update>, <options>)
db.collection.replaceOne(<filter>, <update>, <options>)

//e.g.
db.mycol.update({'title':'MongoDB Overview'},{$set:{'title':'New MongoDB Tutorial'}})

//By default, MongoDB will update only a single document. 
// To update multiple documents, you need to set a parameter 'multi' to true.
db.mycol.update({'title':'MongoDB Overview'},{$set:{'title':'New MongoDB Tutorial'}},{multi:true})


//Syntax - Save
//The save() method replaces the existing document with the new document passed in the save() method.
db.COLLECTION_NAME.save({_id:ObjectId(),NEW_DATA})

//e.g
db.mycol.save(
   {
      "_id" : ObjectId(5983548781331adf45ec5), "title":"Tutorials Point New Topic",
      "by":"Tutorials Point"
   }
)

// MongoDB does not support multi-document atomic transactions. However, it does provide atomic operations on a single document. So if a document has hundred fields the update statement will either update all the fields or none, hence maintaining atomicity at the document-level.
// The recommended approach to maintain atomicity would be to keep all the related information, which is frequently updated together in a single document using embedded documents. This would make sure that all the updates for a single document are atomic.

{
   "_id":1,
   "product_name": "Samsung S3",
   "category": "mobiles",
   "product_total": 5,
   "product_available": 3,
   "product_bought_by": [
      {
         "customer": "john",
         "date": "7-Jan-2014"
      },
      {
         "customer": "mark",
         "date": "8-Jan-2014"
      }
   ]
}

//In this document, we have embedded the information of the customer who buys the product in the product_bought_by field. Now, whenever a new customer buys the product, we will first check if the product is still available using product_available field. If available, we will reduce the value of product_available field as well as insert the new customer's embedded document in the product_bought_by field. We will use findAndModify command for this functionality because it searches and updates the document in the same go.

db.products.findAndModify({ 
   query:{_id:2,product_available:{$gt:0}}, 
   update:{ 
      $inc:{product_available:-1}, 
      $push:{product_bought_by:{customer:"rob",date:"9-Jan-2014"}} 
   }    
})