MongoDB get list of item.field in array of items

Question

I posted a question yesterday - that, because if not knowing "how" to ask, it was poorly presented - So if you recognize the structure/data, please re-read, I hope I am able to present it in a clearer manner.

I want to return an array of items from multiple nested arrays of objects.

My ultimate goal is something that look like this...

{person:1, cost: ['23,543','56,556','8,500']},
{person:2, cost: []}

This lets me know 2 pieces of information, 1- it allows me to total across two or more arrays of objects, and 2- it lets me see persons who have NOT filled in information.

The Scenario:

I have a couple array of objects, and I want to find out if people are filling in 'cost' on an object.

So a model that looks like this.

person : {
    my_cars: [
        {
            make:
            model:
            cost:
        }
    ],
    my_motorcycles: [
        {
            make:
            model:
            cost:
        }
    ]
}

Some data will look like this (and this data corresponds to my above desired result...)

person : {
    _id: 1
    ,my_cars: [
        {
            make: 'ford'
            model: 'taurus'
            cost: null
        },
        {
            make: 'ford'
            model: 'mustang'
            cost:'23,543'
        }
        ,{
            make: 'lincoln'
            model: 'navigator'
            cost: '56,556'
        }
    ]
    ,my_motorcycles: [
        {
            make: 'ducati'
            model: ''
            cost: '8,500'
        }
        ,{
            make: 'yamaha'
            model: 'v-star 650'
            cost: ''
        }
    ]
}
,person : {
    _id: 2
    ,my_cars: [
        {
            make: 'ford'
            model: 'taurus'
            cost: null
        },
        ,{
            make: 'chevy'
            model: 'chevette'
            cost: null
        }
    ]
    ,my_motorcycles: [
        {
            make: 'harley-davidson'
            model: 'softtail'
            cost: ''
        }
        ,{
            make: 'honda'
            model: 'x650'
            cost: ''
        }
    ]
}

So what I am trying to do is unwind but if I try

$unwind{
    path: "$my_cars.cost"
}
,$unwind{
    path: "$my_motorcycles.cost"
}

this is not working...

Any help on getting to an output close to knowing the Number of an instance of a field in multiple nested arrays is greatly appreciated

mickl · Accepted Answer · 2019-12-15 20:53:19Z

2

The list of non-empty values can be retrieved after $filter run on both arrays ($concatArrays). You can use $type to check if value is a string:

db.collection.aggregate([
    {
        $project: {
            _id: 0,
            person: "$_id",                
            cost: {
                $filter: {
                    input: { $concatArrays: [ "$my_cars.cost", "$my_motorcycles.cost" ] },
                    cond: {
                        $and: [
                            { $eq: [ { $type: "$$this" }, "string" ] },
                            { $ne: [ "$$this", "" ] }
                        ]
                    }                           
                }
            }                        
        }
    }
])

Mongo Playground

answered Dec 15, 2019 at 20:53

mickl

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2 Comments

j-p Over a year ago

Perfect - thx Mickl - I had tried this - but it did not work. $project: { email:1, cost: { $filter: { input: { $concatArrays: [ '$my_cars.cost', '$vidPS', '$my_motorcycles.cost' ] }, as: "x", cond: { '$$x': { $nin: [ null, "" ] } } } - your solution worked... can you explain "$$this"

mickl Over a year ago

@jpmyob $$this simply refers to currently processed array element so it will be subsequent costs in this case

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