Remove element from array in mongodb

问题:

I am new in mongodb and i want to remove the some element in array.

my document as below

{
  "_id" : ObjectId("4d525ab2924f0000000022ad"), 
  "name" : "hello", 
  "time" : [
      {
              "stamp" : "2010-07-01T12:01:03.75+02:00",
              "reason" : "new"
      },
      {
              "stamp" : "2010-07-02T16:03:48.187+03:00",
              "reason" : "update"
      },
      {
              "stamp" : "2010-07-02T16:03:48.187+04:00",
              "reason" : "update"
      },
      {
              "stamp" : "2010-07-02T16:03:48.187+05:00",
              "reason" : "update"
      },
      {
              "stamp" : "2010-07-02T16:03:48.187+06:00",
              "reason" : "update"
      }
  ]
}

in document, i want to remove first element(reason:new) and last element(06:00) .

and i want to do it using mongoquery, i am not using any java/php driver.

回答1:

If I'm understanding you correctly, you want to remove the first and last elements of the array if the size of the array is greater than 3. You can do this by using the findAndModify query. In mongo shell you would be using this command:

db.collection.findAndModify({
    query: { $where: "this.time.length > 3" },
    update: { $pop: {time: 1}, $pop: {time: -1} },
    new: true
});

This would find the document in your collection which matches the $where clause. The $where field allows you to specify any valid javascript method. Please note that it applies the update only to the first matched document.

You might want to look at the following docs also:

1. http://www.mongodb.org/display/DOCS/Advanced+Queries#AdvancedQueries-JavascriptExpressionsand%7B%7B%24where%7D%7D for more on the $where clause.
2. http://www.mongodb.org/display/DOCS/Updating#Updating-%24pop for more on $pop.
3. http://www.mongodb.org/display/DOCS/findAndModify+Command for more on findAndModify.

回答2:

You could update it with { $pop: { time: 1 } } to remove the last one, and { $pop: { time : -1 } } to remove the first one. There is probably a better way to handle it though.

回答3:

@javaamtho you cannot test for a size greater than 3 but only if it is exactly 3, for size greater than x number you should use the $inc operator and have a field you either 1 or -1 to in order to keep track when you remove or add items (use a separate field outside the array as below, time_count)

{
  "_id" : ObjectId("4d525ab2924f0000000022ad"), 
  "name" : "hello",
  "time_count" : 5,
  "time" : [
      {
              "stamp" : "2010-07-01T12:01:03.75+02:00",
              "reason" : "new"
      },
      {
              "stamp" : "2010-07-02T16:03:48.187+03:00",
              "reason" : "update"
      },
      {
              "stamp" : "2010-07-02T16:03:48.187+04:00",
              "reason" : "update"
      },
      {
              "stamp" : "2010-07-02T16:03:48.187+05:00",
              "reason" : "update"
      },
      {
              "stamp" : "2010-07-02T16:03:48.187+06:00",
              "reason" : "update"
      }
  ]
}

回答4:

If you would like to leave these time elements, you can use aggregate command from mongo 2.2+ to retrieve min and max time elements, unset all time elements, and push min and max versions (with some modifications it could do your job):

smax=db.collection.aggregate([{$unwind: "$time"},
{$project: {tstamp:"$time.stamp",treason:"$time.reason"}},
{$group: {_id:"$_id",max:{$max: "$tstamp"}}},
{$sort: {max:1}}])

smin=db.collection.aggregate([{$unwind: "$time"},
{$project: {tstamp:"$time.stamp",treason:"$time.reason"}},
{$group: {_id:"$_id",min:{$min: "$tstamp"}}},
{$sort: {min:1}}])

db.students.update({},{$unset: {"scores": 1}},false,true)

smax.result.forEach(function(o)  
{db.collection.update({_id:o._id},{$push:
{"time": {stamp: o.max ,reason: "new"}}},false,true)})

smin.result.forEach(function(o)
{db.collection.update({_id:o._id},{$push:
{"time": {stamp: o.min ,reason: "update"}}},false,true)})

回答5:

db.collection.findAndModify({ query: {$where: "this.time.length > 3"}, update: {$pop: {time: 1}, $pop{time: -1}}, new: true });

convert to PHP