[Answer]-Order repeating entries by sum of one field with distinct of another

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The MongoDB feature you are looking for is the Aggregation Framework.

Here is an example query in the mongo shell:

db.collection.aggregate(
    // Find matching documents (can take advantage of suitable index if present)
    { $match: {
        location: 1
    }},

    // Add up rates by user
    { $group: {
        _id: "$user",
        rates: { $sum: "$rate" }
    }},

    // Order by total rates (ascending)
    { $sort: { rates: 1 }}
)

Sample results given your data in the question:

[
    {
        "_id" : 6,
        "rates" : 3
    },
    {
        "_id" : 5,
        "rates" : 6
    },
    {
        "_id" : 4,
        "rates" : 7
    }
]

As an optional step in the aggregation, you might want to use $project to rename the grouped _id field to user.

👤Stennie

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