M121 | Chapter 3 | Lab 3: $lookup stage

Incorrect Solution1

db.air_alliances.aggregate([
{
	$lookup:
	{
		"from": "air_routes",
		"foreignField": "airline.name",
		"localField": "airlines",
		"as": "route_docs"
	}
},
{
	$project:
	{
		_id: 0,
		name: 1,
		valid_routes_binary_array:
		{
			$map:
			{
				input: "$route_docs",
				as: "route",
				in:
				{
					//If either a Boeing 747 or an Airbus A380 --> 1
					//Else									   --> 0
					$cond: [{$in: ["$$route.airplane", ["747", "380"]]}, 1, 0]
				} 
			}
		}
	}
},
{
	$project:
	{
		name: 1,
		routes_count: {$sum: "$valid_routes_binary_array"}
	}
},
{
	$sort:
	{
		routes_count: -1
	}
}
])

Incorrect Solution2

db.air_alliances.aggregate([
{
	$lookup:
	{
		"from": "air_routes",
		"foreignField": "airline.name",
		"localField": "airlines",
		"as": "route_docs"
	}
},
{
	$project:
	{
		_id: 0,
		name: 1,
		valid_routes_array:
		{
			$filter:
			{
				input: "$route_docs",
				as: "route",
				cond:
				{
					$in: ["$$route.airplane", ['747', '380']]
				} 
			}
		}
	}
},
{
	$project:
	{
		name: 1,
		routes_count: {$size: "$valid_routes_array"}
	}
},
{
	$sort:
	{
		routes_count: -1
	}
}
])