Will set the property 'created_at' to the current date and time when an existing document is not found

ExampleMongoDB$setOnInsertStatement.ts

/** create connection pool to system DB in MongoDB
* assume we are using https://www.npmjs.com/package/mongodb
* assume we start with no document with property matching 'system_id: 123'
*/

system_states_collection.update(
  { system_id: 123 },
  {
      $set: { a: 4 },
      $setOnInsert: { created_at: new Date() }
  },
  { upsert: true }
)

/** 
Resulting document: 
{
  system_id: 123,
  a: 4,
  created_at: '2020-03-08T10:27:21.230Z'
}
*/

system_states_collection.update(
  { system_id: 123 },
  {
      $set: { b: 5 },
      $setOnInsert: { created_at: new Date() }
  },
  { upsert: true }
)

/** 
Resulting document: 
{
  system_id: 123,
  a: 4,
  b: 5
  created_at: '2020-03-08T10:27:21.230Z'
}
*/