how to sort array of objects by arbitrary list in mongo

Question

I was looking for a way to sort array of object by an arbitrary list. Lets assume I have this array of objects.

[
  {
   "_id": "4JEEuhNIae",
   "category": "grocery"
  },
  {
   "_id": "4JW7miNITl",
   "category": "food"
  },
  {
   "_id": "4Je4kmrrbZ",
   "category": "coffee"
  },
  {
   "_id": "4JgAh3N86x",
   "category": "coffee"
  }
]

This is the array that I would like to use as sorting criteria. Records with foodshould come first, then coffeeand grocery.

['food','coffee','grocery']

Result should be:

[
  {
   "_id": "4JW7miNITl",
   "category": "food"
  },
  {
   "_id": "4Je4kmrrbZ",
   "category": "coffee"
  },
  {
   "_id": "4JgAh3N86x",
   "category": "coffee"
  },
  {
   "_id": "4JEEuhNIae",
   "category": "grocery"
  },
]

How can I do this type of sorting on mongodb by using mongoose? I really don't want to make any operations on the code after fetching data.

Your code is a list field inside a document ? Or it is a list of documents ? — sergiuz
– sergiuz, Commented Nov 1, 2016 at 9:01

chridam · Accepted Answer · 2016-11-01 10:46:48Z

2

You could try running a custom comparator function with the native JavaScript sort() method on the array returned from the cursor.toArray() method:

var order = ["food", "coffee", "grocery"];
var docs = db.collection.find().toArray().sort(function(a, b) { 
    return order.indexOf(a.category) - order.indexOf(b.category);
});
printjson(docs);

Sample Output

[
    {
        "_id" : "4JW7miNITl",
        "category" : "food"
    },
    {
        "_id" : "4Je4kmrrbZ",
        "category" : "coffee"
    },
    {
        "_id" : "4JgAh3N86x",
        "category" : "coffee"
    },
    {
        "_id" : "4JEEuhNIae",
        "category" : "grocery"
    }
]

With the new MongoDB 3.4 version, you should be able to leverage the use of the native MongoDB operators $addFields and $indexOfArray in the aggregation framework.

The $addFields pipeline step allows you to $project new fields to existing documents without knowing all the other existing fields.
The $indexOfArray expression returns position of particular element in a given array.

So putting that altogether you could try the following aggregate operation (with MongoDB 3.4):

var order = ["food", "coffee", "grocery"],
    projection = { 
        "$addFields" : { 
            "__order" : { "$indexOfArray" : [ order, "$category" ] } 
        } 
    },
    sort = { "$sort" : { "__order" : 1 } };
db.collection.aggregate([ projection, sort]);

answered Nov 1, 2016 at 10:46

chridam

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1 Comment

Ahmet Tanakol Over a year ago

It seems i can't directly write custom sort function for mongoose, github.com/Automattic/mongoose/issues/798 I need to use db.eval well thank you

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