title: what creates the default ObjectID in mongodb?
body: <p>Quite new to MongoDB. Going through the documentation. </p>
  
  <p>From <a href="https://docs.mongodb.com/manual/reference/method/db.collection.insert/#id-field" rel="nofollow noreferrer">https://docs.mongodb.com/manual/reference/method/db.collection.insert/#id-field</a> <br/>
  <em>If the document does not specify an _id field, then MongoDB will add the _id field and assign a unique ObjectId for the document before inserting. Most drivers create an ObjectId and insert the _id field, but the mongod will create and populate the _id if the driver or application does not.</em></p>
  
  <p>Here it says MongoDB will create the _id. <br/></p>
  
  <p>From <a href="https://docs.mongodb.com/manual/reference/method/db.collection.save/?_ga=2.24854906.1181826191.1496665143-146886871.1493019056#save-a-new-document-without-specifying-an-id-field" rel="nofollow noreferrer">https://docs.mongodb.com/manual/reference/method/db.collection.save/?_ga=2.24854906.1181826191.1496665143-146886871.1493019056#save-a-new-document-without-specifying-an-id-field</a> <br/>
  <em>During the insert, the shell will create the _id field with a unique ObjectId value, as verified by the inserted document.</em></p>
  
  <p>And here, it says Mongo shell. <br/></p>
  
  <p>Is it the mongo shell that creates the default ObjectID or mongod? Hope someone can clarify!</p>