如何在MongoDB中计算实例

Question

If I want to sum up numerical values in MongoDB I do it like this:

totalOpenBalance: {
  $sum: "$openBalance"
} // sum up all "openBalance" values

But what I'm wondering is, what operator do I use when I want to sum up instances of something? Say if I have a property such as customer_id , and data that looks like this:

{
   "customer_id" : 445,
   "other_prop" : value
},
{
   "customer_id" : 446,
   "other_prop" : value
},

Note that I don't want to sum up the values assigned to "customer_id", but rather tally up how many instances of "customer_id" are in the collection of data. In other words, according to the data above, I should get "2" as my output. What operator do I use to do that?

To clarify, this is a step I need to add to an aggregation pipeline I'm using to generate a mongo view.

Answer 1

Any of the below should get you going:

Simple find :

db.collection.find({
    "customer_id": { $exists: true }
}).count()

Aggregate with $count :

db.collection.aggregate({
    $match: {
        "customer_id": { $exists: true }
    }
}, {
    $count: "numberOfInstances"
})

Aggregate with $group :

db.collection.aggregate({
    $match: {
        "customer_id": { $exists: true }
    }
}, {
    $group: {
        _id: null,
        "numberOfInstances": { $sum: 1 } // count instances
    }
})

Answer 2

You can simply use find and $exists and then count the returned rows

db.collection.find( { customer_id: { $exists: true } } ).count()

Or if you want to use aggregate (which I don't think you should do for such simple task) this is how you can do it.

 db.collection.aggregate({ $match: { "customer_id": { $exists: true } } }, { $group: { _id: null, "total": { $sum: 1 } } }) 

Here total property will give you the number of instances containing customer_id in them