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

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