Pymongo: Remove an element from array

Question

I'm trying to remove lowest price from the iPad's in my schema. I know how to find it using pymongo, but I don't how to remove it. Here's my schema:

{
    "_id": "sjobs",
    "items": [
        {
            "type": "iPod",
            "price": 20.00
        },
        {
            "type": "iPad",
            "price": 399.99
        },
        {
            "type": "iPad",
            "price": 199.99
        },
        {
            "type": "iPhone 5",
            "price": 300.45
        }
    ]
}
{
    "_id": "bgates",
    "items": [
        {
            "type": "MacBook",
            "price": 2900.99
        },
        {
            "type": "iPad",
            "price": 399.99
        },          
        {
            "type": "iPhone 4",
            "price": 100.00
        },
        {
            "type": "iPad",
            "price": 99.99
        }
    ]
}

I've got a python loop that finds the lowest sale price for iPad:

cursor = db.sales.find({'items.type': 'iPad'}).sort([('items', pymongo.DESCENDING)])
for doc in cursor:
    cntr = 0
    for item in doc['items']:
        if item['type'] == 'iPad' and resetCntr == 0:
            cntr = 1
            sales.update(doc, {'$pull': {'items': {item['type']}}})

That doesn't work. What do I need to do to remove lowest iPad price item?

As a generic comment: "doesn't work" is usually not enough information. — Erik Kaplun
– Erik Kaplun, Commented Sep 27, 2013 at 22:56
Sorry. I don't get an error msg. The program successfully runs, but nothing happens to my schema. Hence my stmt: "doesn't work". — inquisitive_one
– inquisitive_one, Commented Sep 27, 2013 at 23:55
is there a reason you're doing the searching and sorting in python (bringing all the documents over) instead of using an aggregation query to get the lowest price for each iPad item in each document _id and then issue an update for each? — Asya Kamsky
– Asya Kamsky, Commented Sep 29, 2013 at 6:49
also what is 'resetCntr' and where do you set it? also, you do realize you are not getting back the arrays item sorted, sort directive to find only applies to top level document order. — Asya Kamsky
– Asya Kamsky, Commented Sep 29, 2013 at 6:50

Asya Kamsky · Accepted Answer · 2013-09-29 07:15:19Z

Your Python code isn't doing what you think it's doing (unless there is a lot of it you didn't include). You don't need to do the sorting and iterating on the client side - you should make the server do the work. Run this aggregation pipeline (I'm giving shell syntax, you can call it from your Python, of course):

> r = db.sales.aggregate( {"$match"  : { "items.type":"iPad"} },
                          {"$unwind" :   "$items"}, 
                          {"$match"  : { "items.type":"iPad"} },
                          {"$group"  : { "_id" : "$_id", 
                                         "lowest" : {"$min":"$items.price"}, 
                                         "count":{$sum:1}
                                       }
                          }, 
                          {"$match"  : {count:{$gt:1}}}
);
{
    "result" : [
        {
            "_id" : "bgates",
            "lowest" : 99.99,
            "count" : 2
        },
        {
            "_id" : "sjobs",
            "lowest" : 199.99,
            "count" : 2
        }
    ],
    "ok" : 1
}

Now you can iterate over the "r.results" array and execute your update:

db.sales.update( { "_id" : r.results[0]._id }, 
                 { "$pull" : { "items" : { "type" : "iPad", "price" : r.result[0].lowest}}} );

Note that I only include records which have more than one iPad - since otherwise you may end up deleting the only iPad record in the array. If you want to delete all "non-highest" prices then you'd want to find the max and $pull all the elements $lt that price.

Draken · Accepted Answer · 2017-04-07 13:50:11Z

Disclaimer: The below code is not tested as I do not have mongo installed locally. However I did take my time writing it so im pretty confident its close to working

def remove_lowest_price(collection):
    cursor = collection.find({}, {'items': 1})
    for doc in cursor:
        items = doc['items']
        id = doc['_id']
        for item in items:
            lowest_price = 100000 # a huge number
            if item['type'] == 'i_pad' and item['price'] < lowest:
                lowest = item['price']
        # lowest now contains the price of the cheapest ipad
        collection.update(
            {'_id': id},
            {'$pull': {'items': {'price': lowest}}}
        )

Of course there will be a problem here if another item happens to have exactly the same price but I think it will be easy to improve from here

viktortnk · Accepted Answer · 2013-09-28 12:43:50Z

0

{'$pull': {'items': {item['type']}}}

This doesn't look like valid json, does it?

shouldn't be "sales.update(...)" be "db.sales.update(...)" in your example?

maybe it's better to have query in update operation:

db.sales.update({_id: doc[_id]}, ...)

rather than entire doc.

and finally the update body itself might be

{'$pull': {'items': {type: item['type']}}}

answered Sep 28, 2013 at 12:43

viktortnk

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1 Comment

Asya Kamsky Over a year ago

this is not js nor mongo shell, but Python, so sales is his collection here (equivalent to db.sales in the shell). The update in the question is the same as what you suggested.

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