Mongodb and sorting sub array

Question

Not sure if this can be done, so thought I would ask.

I have the following mongodb/s

{
 "store":"abc",
 "offers":[{
    "spend":"100.00",
    "cashback":"10.00",
    "percentage":"0.10"
  },{
    "spend":"50.00",
    "cashback":"5.00",
    "percentage":"0.10"
  }]
}

and

 {
     "store":def",
     "offers":[{
        "spend":"50.00",
        "cashback":"2.50",
        "percentage":"0.05"
      },{
        "spend":"20.00",
        "cashback":"1.00",
        "percentage":"0.05"
      }]
    }

and

 {
         "store":ghi",
         "offers":[{
            "spend":"50.00",
            "cashback":"5.00",
            "percentage":"0.10"
          },{
            "spend":"20.00",
            "cashback":"2.00",
            "percentage":"0.10"
          }]
        }

the sort needs to be by percentage.

I am not sure if I would have to use usort of another PHP function to do it, or if Mongodb is smart enough to do what I want to do.

JohnnyHK · Accepted Answer · 2012-07-31 02:18:08Z

1

Amazingly, yes, mongodb can do this:

// Sort ascending, by minimum percentage value in the docs' offers array.
db.collection.find({}).sort({ 'offers.percentage': 1 });

// Sort descending, by maximum percentage value in the docs' offers array.
db.collection.find({}).sort({ 'offers.percentage': -1 });

answered Jul 31, 2012 at 2:18

JohnnyHK

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2 Comments

Stennie Over a year ago

That may sort the documents, but not sure how that helps with arrays within the docs .. or how Mongo would cope with unsorted arrays. For example, to predictably sort a doc that has %s of 0.05, 0.10 versus another with 0.10, 0.05.

RussellHarrower Over a year ago

would it be better if each offer was in its own collection row

Stennie · Accepted Answer · 2012-07-31 04:01:13Z

1

Given your data structure of arrays within documents, I don't think it makes sense to do this sort in MongoDB -- Mongo will be returning entire documents (not arrays).

If you are trying to compare offers it would probably make more sense to have a separate collection instead of an embedded array. For example, you could then find offers matching a cashback of at least $5 sorted by spend or percentage discount.

If you are just trying to order the offers within a single document, you could do this in PHP with a usort().

answered Jul 31, 2012 at 4:01

Stennie

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