In MongoDB search in an array and sort by number of matches

Question

The question is the next one:

Get documents with tags in list, ordered by total number of matches

But they say that is possible using Aggregation Framework,

Answer 1

Yes, it's possible using Aggregation Framework.

Assumptions

• The dataset used here is the same used in Get documents with tags in list, ordered by total number of matches
• tags attribute is a set (no repeated elements)

Query

This approach forces you to unwind the results and reevaluate the match predicate with unwinded results, so its really inefficient.

db.test_col.aggregate(
    {$match: {tags: {$in: ["shirt","cotton","black"]}}}, 
    {$unwind: "$tags"}, 
    {$match: {tags: {$in: ["shirt","cotton","black"]}}}, 
    {$group: {
        _id:{"_id":1}, 
        matches:{$sum:1}
    }}, 
    {$sort:{matches:-1}}
);

Expected Results

{
    "result" : [
        {
            "_id" : {
                "_id" : ObjectId("5051f1786a64bd2c54918b26")
            },
            "matches" : 3
        },
        {
            "_id" : {
                "_id" : ObjectId("5051f1726a64bd2c54918b24")
            },
            "matches" : 2
        },
        {
            "_id" : {
                "_id" : ObjectId("5051f1756a64bd2c54918b25")
            },
            "matches" : 1
        }
    ],
    "ok" : 1
}

Answer 2

Using $size and $setIntersection will solve this efficiently, without causing memory multiplication.

tagList = ['shirt', 'cotton', 'black']

db.test_col.aggregate(
    {$match: {tags: {$in: ["shirt","cotton","black"]}}}, 
    {$project: 
        {"title":1,"tags":1}, 
        {$order: 
            {"$size": 
                {"$setIntersection": [ tagList, "$tags" ]}}, 
    {$sort:{order:-1}}
    );

1. First we match the documents which have at least one element matching.

2. Then we project Keys/Columns we need along with a new order key/column. Order is generated by taking the count of intersected elements between 'tags in db' and 'tags from query'.

3. Then we do a simple sort in descending order. This worked for me. Similar Question answered here