Mongodb - create ranking of values from field array - mongodb

Question

I am trying to sort a mongodb aggregate I don't what it is happening. I was searching some solution in stack overflow but they didn't work and I don't know why...
My idea is return a ranking of values from the field array (tags). I could achieve the list the sum of values but I can not sort it...
This is the query that I could do and it seems that it works:
db.getCollection("metadata").aggregate(
{$unwind: '$tags'},
{$group: {_id:'$tags.name', total: {$sum: 1}}}
);
Because I receive this result that It has sense:
{
"_id" : "kite",
"total" : 1.0
}
{
"_id" : "piggy bank",
"total" : 1.0
}
{
"_id" : "sorrel",
"total" : 1.0
}
{
"_id" : "eggnog"
"total" : 4.0
}
{
"_id" : "Weimaraner",
"total" : 1.0
}
{
"_id" : "bassinet",
"total" : 15.0
}
{
"_id" : "squirrel monkey",
"total" : 1.0
}
{
"_id" : "bath towel",
"total" : 6.0
}
TRIES
When I tried something like this:
db.getCollection("metadata").aggregate(
{$unwind: '$tags'},
{$group: {_id:'$tags.name', total: {$sum: 1}}},
{$sort: {total: -1}}
);
RESULT TRY:
{
"_id" : "baboon",
"total" : 12.0
}
{
"_id" : "snow leopard",
"total" : 4.0
}
{
"_id" : "green lizard",
"total" : 5.0
}
{
"_id" : "Dandie Dinmont",
"total" : 7.0
}
{
"_id" : "echidna",
"total" : 8.0
}
{
"_id" : "bee eater",
"total" : 6.0
}
or like this:
db.getCollection("metadata").aggregate(
{$unwind: '$tags'},
{$group: {_id: { name:'$tags.name', total: {$sum: 1}}}},
{$sort: {total: -1}}
);
The result doesn't sort or directly not sum the values...
EXTRA
This is the query if I want to list all the entries with the array:
db.getCollection('metadata').find({tags: {$exists: true}})
And the result is:
/* 2 */
{
"_id" : ObjectId("5900af3ff6844d2f7519fe13"),
"user_id" : 23,
"company_id" : 1,
"created" : ISODate("2017-04-26T14:31:27.000Z"),
"md5file" : "fdd30b1ca52e1c15f330f46c0079498c",
"path" : "/storage/emulated/0/DCIM/Camera/IMG_20160605_133703.jpg",
"image_width" : 3456,
"image_height" : 4608,
"originalTags" : [
{
"name" : "sleeping bag",
"percentage" : 0.7529412
},
{
"name" : "diaper",
"percentage" : 0.05490196
},
{
"name" : "bib",
"percentage" : 0.039215688
}
],
"tags" : [
{
"name" : "sleeping bag",
"percentage" : 0.7529412
}
]
}
/* 3 */
{
"_id" : ObjectId("5900af3ff6844d2f7519fe14"),
"user_id" : 23,
"company_id" : 1,
"created" : ISODate("2017-04-26T14:31:27.000Z"),
"md5file" : "22612c8bc99d1031146f7c9918555572",
"path" : "/storage/emulated/0/DCIM/Camera/IMG_20160605_164243.jpg",
"image_width" : 4608,
"image_height" : 3456,
"originalTags" : [
{
"name" : "bath towel",
"percentage" : 0.62352943
},
{
"name" : "quilt",
"percentage" : 0.101960786
},
{
"name" : "cradle",
"percentage" : 0.043137256
}
],
"tags" : [
{
"name" : "bath towel",
"percentage" : 0.62352943
}
]
}

Answer 1

Aggregation pipeline is an array. It should be wrapped in square brackets []:
db.getCollection("metadata").aggregate(
[
{$unwind: '$tags'},
{$group: {_id:'$tags.name', total: {$sum: 1}}},
{$sort: {total: -1}}
]
);