How to print the count of array elements along with another variable in MongoDB

Question

I have a data collection which contains records in the following format.

{
    "_id": 22,
    "title": "Hibernate in Action",
    "isbn": "193239415X",
    "pageCount": 400,
    "publishedDate": ISODate("2004-08-01T07:00:00Z"),
    "thumbnailUrl": "https://s3.amazonaws.com/AKIAJC5RLADLUMVRPFDQ.book-thumb-images/bauer.jpg",
    "shortDescription": "\"2005 Best Java Book!\" -- Java Developer's Journal",
    "longDescription": "Hibernate practically exploded on the Java scene. Why is this open-source tool so popular  Because it automates a tedious task: persisting your Java objects to a relational database. The inevitable mismatch between your object-oriented code and the relational database requires you to write code that maps one to the other. This code is often complex, tedious and costly to develop. Hibernate does the mapping for you.    Not only that, Hibernate makes it easy. Positioned as a layer between your application and your database, Hibernate takes care of loading and saving of objects. Hibernate applications are cheaper, more portable, and more resilient to change. And they perform better than anything you are likely to develop yourself.    Hibernate in Action carefully explains the concepts you need, then gets you going. It builds on a single example to show you how to use Hibernate in practice, how to deal with concurrency and transactions, how to efficiently retrieve objects and use caching.    The authors created Hibernate and they field questions from the Hibernate community every day - they know how to make Hibernate sing. Knowledge and insight seep out of every pore of this book.",
    "status": "PUBLISH",
    "authors": ["Christian Bauer", "Gavin King"],
    "categories": ["Java"]
}

I want to print title, and authors count where the number of authors is greater than 4. I used the following command to extract records which has more than 4 authors.

db.books.find({authors:{$exists:true},$where:'this.authors.length>4'},{_id:0,title:1});

But unable to print the number of authors along with the title. I tried to use the following command too. But it gave only the title list.

db.books.find({authors:{$exists:true},$where:'this.authors.length>4'},{_id:0,title:1,'this.authors.length':1});

Could you please help me to print the number of authors here along with the title?

mickl · Accepted Answer · 2019-05-07 12:17:21Z

2

You can use aggregation framework's $project with $size to reshape your data and then $match to apply filtering condition:

db.collection.aggregate([
    {
        $project: {
            title: 1,
            authorsCount: { $size: "$authors" }
        }
    },
    {
        $match: {
            authorsCount: { $gt: 4 }
        }
    }
])

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answered May 7, 2019 at 12:17

mickl

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1 Comment

Nipuna Dilhara Over a year ago

Could you please have a look at this too. stackoverflow.com/q/56027966/7234570

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