MongoDB: Find the minimum element in array and delete it

Question

I have a documents in MongoDB, one of them looks like this:

{
"_id" : 100,
"name" : "Something",
"items" : [
    {
        "item" : 47,
        "color" : "red"
    },
    {
        "item" : 44,
        "color" : "green"
    },
    {
        "item" : 39,
        "color" : "blue"
    }
]
}

In every document I need to find the minimum item and delete it. So it should be like this:

{
"_id" : 100,
"name" : "Something",
"items" : [
    {
        "item" : 47,
        "color" : "red"
    },
    {
        "item" : 44,
        "color" : "green"
    }
]
}

It looks like findAndModify function should be used here but I can't go any further.

How to find the minimum element in array and delete it?

I'm using MongoDB and Pymongo driver.

haha, you doing the 10gen course? week3-2 ?

Mike Graf
– Mike Graf

2013-07-07 03:23:48 +00:00
Commented Jul 7, 2013 at 3:23 — Mike Graf
– Mike Graf, Commented Jul 7, 2013 at 3:23

megas · Accepted Answer · 2012-11-12 18:14:04Z

18

If you are not restricted to having the query be in one single step, you could try:

step 1) use the aggregate function with the $unwind and $group operators to find the minimum item for each document

myresults = db.megas.aggregate( [ { "$unwind": "$items" },  
    {"$group": { '_id':'$_id' , 'minitem': {'$min': "$items.item" } } } ] )

step 2) the loop through the results and $pull the element from the array

for result in myresults['result']:
    db.megas.update( { '_id': result['_id'] }, 
        { '$pull': { 'items': { 'item': result['minitem'] } } } )

edited Nov 12, 2012 at 18:14

megas

21.9k12 gold badges84 silver badges134 bronze badges

answered Nov 12, 2012 at 17:58

Kay

3,01221 silver badges13 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Mutant Over a year ago

If you want filter like where minimum of red, or green item, you can add {'$match': {"items.color": "red"} } in your aggregate syntax.

Alexander Revutsky Over a year ago

I added following code by mongo shell for(i=0; i<result.length; i++) { db.students.update({ '_id': result[i]['_id']}, { '$pull': { 'items': { 'item': result[i]['minitem'] } } } ) }

Prasanna Aarthi Over a year ago

Im able to runt he first query.. Im not able to perform update operation.. Im using for(i=0;i<result.length;i++)

flame3 Over a year ago

I had to remove the ['result'] to make it work. The code is like this: for result in myresults: db.megas.update( { '_id': result['_id'] }, { '$pull': { 'items': { 'item': result['minitem'] } } } )

juanreyesv · Accepted Answer · 2015-04-05 12:27:36Z

9

Please find my solution written in plain javascript. It should work straight through MongoDb shell

cursor = db.students.aggregate(
[{ "$unwind": "$items" }, 
 { "$match": { "items.color": "green"}},
 { "$group": {'_id': '$_id', 'minitem': {
                '$min': "$items.item"
            }
        }
    }

]);

cursor.forEach(
function(coll) {
    db.students.update({
        '_id': coll._id
    }, {
        '$pull': {
            'items': {
                'item': coll.minitem
            }
        }
    })
})

edited Apr 5, 2015 at 12:27

juanreyesv

8539 silver badges22 bronze badges

answered Jan 25, 2015 at 19:25

Faktor 10

1,8882 gold badges23 silver badges31 bronze badges

3 Comments

Faktor 10 Over a year ago

the match statement is optional. But useful to further narrow down a selection from array

KhoPhi Over a year ago

Just to add to the above answers, you could also look into the for... of of Javascript: developer.mozilla.org/en/docs/Web/JavaScript/Reference/…

Dev G Over a year ago

above will fetch at a time 20 records what to do to get all records and update them?

Xavier Guihot · Accepted Answer · 2022-04-20 20:42:10Z

Starting in Mongo 4.4, the $function aggregation operator allows applying a custom javascript function to implement behaviour not supported by the MongoDB Query Language.

And coupled with improvements made to db.collection.update() in Mongo 4.2 that can accept an aggregation pipeline, allowing the update of a field based on its own value,

We can manipulate and update an array in ways the language doesn't easily permit:

// {
//   "name" : "Something",
//   "items" : [
//     { "item" : 47, "color" : "red"   },
//     { "item" : 39, "color" : "blue"  },
//     { "item" : 44, "color" : "green" }
//   ]
// }
db.collection.update(
  {},
  [{ $set:
    { "items":
      { $function: {
          body: function(items) {
            var min = Math.min(...items.map(x => x.item));
            return items.filter(x => x.item != min);
          },
          args: ["$items"],
          lang: "js"
      }}
    }
  }],
  { multi: true }
)
// {
//   "name" : "Something",
//   "items" : [
//     { "item" : 47, "color" : "red"   },
//     { "item" : 44, "color" : "green" }
//   ]
// }

$function takes 3 parameters:

body, which is the function to apply, whose parameter is the array to modify. The function here simply consists in first finding the minimum item from the array in order to then filter it out.
args, which contains the fields from the record that the body function takes as parameter. In our case, "$items".
lang, which is the language in which the body function is written. Only js is currently available.

Collectives™ on Stack Overflow

MongoDB: Find the minimum element in array and delete it

3 Answers 3

4 Comments

3 Comments

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

4 Comments

3 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related