Using mongoose to get the distinct values of an object in array for all documents

Question

I'm using Mongoose in nodejs. So here is a sample Publisher model:

name: String,
books: [{
    title: String,
    author: {
        name: String,
        age: Number
    }
}]

I want to get all the distinct author names that match a regex. I'm new to noSql so maybe I'm just doing something dumb. I figure I want to do something like:

Publisher.aggregate([
    {$unwind: '$books'},
    {$match: {
        'books.author.name': new RegExp(req.params.regex)
    }},
    {$group: {
        _id: '$books.author.name'
    }},
    {$group: {
        distinctValues: {$addToSet: '$_id'}
    }}
], function(err, result) {
    if(err) throw err;
    res.json(result);
})

This seems like it should work according to: https://dba.stackexchange.com/questions/59661/mongodb-distinct-in-an-array

Any and all input you guys have to help me with this would be much appreciated.

Alexander Suvorov · Accepted Answer · 2016-06-06 08:20:43Z

2

You didnt provide an error you get when performing this query, but it seems you get something like: 'a group specification must include an _id'. The issue with your query is here:

{$group: {
    distinctValues: {$addToSet: '$_id'}
}}

$group operator MUST include an _id field

So far $group operator provides already distinct group this last pipeline seems redundant, next works for me:

Publisher.aggregate([
    {$unwind: '$books'},
    {$match: {
        'books.author.name': new RegExp(req.params.regex)
    }},
    {$group: {
        _id: '$books.author.name'
    }}
], function(err, result) {
    if(err) throw err;
    res.json(result);
})

answered Jun 6, 2016 at 8:20

Alexander Suvorov

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2 Comments

Mike V. Over a year ago

That works. Thanks. I'm confused though. The values returned seem to be distinct but your change does not specify any requirement on uniqueness. Why is that?

Alexander Suvorov Over a year ago

Thats how $group operator works, you can check the documentation, they have general examples up there (docs.mongodb.com/manual/reference/operator/aggregation/group). From the docs: 'Groups documents by some specified expression and outputs to the next stage a document for each distinct grouping'

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