Node.js MongoDB HW#5.2 - Use aggregation framework calculate the average population of cities in California (abbreviation CA) and New York (NY) (taken together) with populations over 25,000. Note: To run this in Terminal, type in... mongo < [file name of this .js file]. This should run the command of the .js file you created with the mongo shell…

5.2

use hw5
db.zips.aggregate([
  /* First group via sum up of the values of the cities with the same name
	this is because a city will have multiple zips therefore, 
	multiple entries of cities with the same name */
	{
		$group: {
			_id: { state: "$state", city: "$city"},
			pop: { $sum: "$pop" }
		}
	}
	/* Match states "CT" and "NJ" */
	,{
		$match: {
			$or: [{ "_id.state": "NY" },{ "_id.state": "CA" }]
			// $or: [{ "_id.state": "CT" },{ "_id.state": "NJ" }]
		}
	}
	/* Match cities with population greater than 25,000 */
	,{
		$match: {
			pop: { $gt: 25000 }, 
		}
	}
	/* Average all the values of the previous match... Notice... grouping via 
	a non-existing value will group all of the values together */
	,{
		$group: {
			_id: 0,
			avgPop: { $avg: "$pop" }
		}
	}
])