$filter "truthy" condition returns incorrect results

[gak10100]

When using $filter with a "truthy" condition it filters out all results.

const result = aggregate(
	[
		{
			urls: [
				'http://google.com',
				'http://msn.com',
				null
			]
		}
	],
	[
		{
			"$project": {
				"urls": {
					"$filter" : {
						"input" : "$urls",
						"as" : "url",
						"cond" : "$$url"
					}
				}
			}
		}
	]
);

console.log(result[0]);

Expected Result

{ urls: [ 'http://google.com', 'http://msn.com'] }

Actual Result

{ urls: [] }

[gak10100]

Please note that mongo filters out null and false but does NOT filter out empty string

[kofrasa]

Thanks for reporting this bug. Need to improve testing across a number of operators given the examples aren't always sufficient.

Feel free to send a PR.

[gak10100]

It appears $cond is inconsistent with mongo on empty string. I prefer mingo's implementation but for consistency.

{
	"$project": {
		"emptyString": { $cond: ['', true, false] },
		"null": { $cond: [null, true, false] },
		"undefined": { $cond: [undefined, true, false] },
		"-1": { $cond: [-1, true, false] },
		"0": { $cond: [0, true, false] },
		"1": { $cond: [1, true, false] }
	}
}

Mingo

{
  '0': false,
  '1': true,
  string: true,
  emptyString: false,
  null: false,
  undefined: false,
  '-1': true
}

Mongo

{
  '0': false,
  '1': true,
  string: true,
  emptyString: true,
  null: false,
  undefined: false,
  '-1': true
}

[kofrasa]

It appears $cond is inconsistent with mongo on empty string. I prefer mingo's implementation but for consistency.

{
	"$project": {
		"emptyString": { $cond: ['', true, false] },
		"null": { $cond: [null, true, false] },
		"undefined": { $cond: [undefined, true, false] },
		"-1": { $cond: [-1, true, false] },
		"0": { $cond: [0, true, false] },
		"1": { $cond: [1, true, false] }
	}
}

Mingo

{
  '0': false,
  '1': true,
  string: true,
  emptyString: false,
  null: false,
  undefined: false,
  '-1': true
}

Mongo

{
  '0': false,
  '1': true,
  string: true,
  emptyString: true,
  null: false,
  undefined: false,
  '-1': true
}

Will address all remaining cases in #321.