MongoDB how to push multiple objects in a collection into an array

Question

If I have a multiple documents in a mongodb collection that look like this:

// document 1
{
    _id: '123',
    date: '5/10/15',

    charges: [{
        amount: 500,
        description: 'foo',
    },{
        amount: 400,
        description: 'bar',
    }],
}


// document 2    
{
    _id: '456',
    date: '5/11/15',

    charges: [{
        amount: 500,
        description: 'foo',
    },{
        amount: 300,
        description: 'foo',
    }],
}

I want to create and array of all charges that have an amount of 500. The result should look like this:

[{
    amount: 500,
    description: 'foo'
}, {
    amount: 500,
    description: 'foo'
}]

What is the most efficient way to accomplish this?

heinob · Accepted Answer · 2015-08-27 09:02:00Z

2

Try this:

db.collection.aggregate(
    [
        { 
            $unwind: "$charges"
        },
        {
            $match: {
                amount: 500
            }
        }
    ]
);

edited Aug 27, 2015 at 9:02

answered Aug 26, 2015 at 19:48

heinob

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Comments

Blakes Seven · Accepted Answer · 2015-08-26 19:58:02Z

Across documents you use the aggregation framework with $unwind and $group:

db.collection.aggregate([
    // Match documents with the required criteria
    { "$match": { "charges.amount": 500 } },

    // Unwind to de-normalize the content
    { "$unwind": "$charges" },

    // Filter the de-normalized documents
    { "$match": { "charges.amount": 500 } },

    // Group back the result
    { "$group": {
        "_id": null,
        "charges": { "$push": "$charges" }
    }}        
])

Or a bit more efficient in modern versions is to filter the array first:

db.collection.aggregate([
    // Match documents with the required criteria
    { "$match": { "charges.amount": 500 } },

    // Pre filter the array
    { "$redact": {
        "$cond": {
            "if": { "$eq": [{ "$ifNull": [ "$amount", 500 ] }, 500 ]},
            "then": "$$DESCEND",
            "else": "$$PRUNE"
        }
    }},

    // Unwind to de-normalize the content
    { "$unwind": "$charges" },

    // Group back the result
    { "$group": {
        "_id": null,
        "charges": { "$push": "$charges" }
    }}        
])

Future versions ( working in current development releases ) will have a more helpful $filter method:

db.collection.aggregate([
    // Match documents with the required criteria
    { "$match": { "charges.amount": 500 } },

    // Filter the array
    { "$project": {
        "charges": {
            "$filter": {
                "input": "$charges",
                "as": "charge",
                "cond": {
                    "$eq": [ "$$charge.amount", 500 ]
                }
            }
        }
    }},

    // Unwind to de-normalize the content
    { "$unwind": "$charges" },

    // Group back the result
    { "$group": {
        "_id": null,
        "charges": { "$push": "$charges" }
    }}        
])

All result in:

{
    "_id": null,
    "charges": [
        {
            amount: 500,
            description: 'foo'
        }, {
            amount: 500,
            description: 'foo'
        }
    ]
}

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