Updating

Update
save() in the mongo shell
Modifier Operations

$inc
$set
$unset
$push
$pushAll
$pop
$pull
$pullAll

Upserts
Pushing a Unique Value

Checking the Outcome of an Update Request
A Note About Padding
See Also

Update

Shell syntax for update():

db.collection.update( criteria, objNew, upsert, multi )

Arguments:

criteria - query which selects the record to update;
objNew - updated object or $ operators (e.g., $inc) which manipulate the object
upsert - if this should be an "upsert"; that is, if the record does not exist, insert it
multi - if all documents matching criteria should be updated (the default is to only update the first document found)

save() in the mongo shell

The save() command in the mongo shell provides a shorthand syntax to perform a single object update with upsert:

// x is some JSON style object
db.mycollection.save(x); // updates if exists; inserts if new

save() does an upsert if x has no _id field and an insert if it does. Thus, normally, you will not need to explicitly request upserts, just use save().

Upsert means "update if present; insert is missing".

myColl.update( { name: "Joe" }, { name: "Joe", age: 20 }, true );

Modifier Operations

Modifier operations are performance enhancement techniques useful when updating an existing object in certain ways, such as incrementing a number. While a conventional implementation works:

var j=myColl.findOne( { name: "Joe" } );
j.n++;
myColl.save(j);

a modifier update has the advantage of no caller latency for the operation, atomicity , plus very little network data transfer. To perform such an operation, pass special modifier verbs in objNew that start with the dollar sign character ($):

db.people.update( { name:"Joe" }, { $inc: { n : 1 } } );

On a modifier operation, all members of objNew must be modifiers, which start with $.

$inc

{ $inc : { field : value } }

increments field by the number value if field is present in the object, otherwise sets field to the number value.

$set

{ $set : { field : value } }

sets field to value. All datatypes are supported with $set.

$unset

{ $unset : { field : 1} }

Deletes a given field. v1.3+

$push

{ $push : { field : value } }

appends value to field, if field is an existing array, otherwise sets field to the array [value] if field is not present. If field is present but is not an array, an error condition is raised.

$pushAll

{ $pushAll : { field : value_array } }

appends each value in value_array to field, if field is an existing array, otherwise sets field to the array value_array if field is not present. If field is present but is not an array, an error condition is raised.

$pop

{ $pop : { field : 1  } }

removes the last element in an array (ADDED in 1.1)

{ $pop : { field : -1  } }

removes the first element in an array (ADDED in 1.1) |

$pull

{ $pull : { field : _value } }

removes all occurrences of value from field, if field is an array. If field is present but is not an array, an error condition is raised.

$pullAll

{ $pullAll : { field : value_array } }

removes all occurrences of each value in value_array from field, if field is an array. If field is present but is not an array, an error condition is raised.

Upserts

You may use upsert with a modifier operation. In such a case, the modifiers will be applied to the update criteria member and the resulting object will be inserted. The following upsert example may insert the object {name:"Joe",x:1,y:1}.

db.people.update( { name:"Joe" }, { $inc: { x:1, y:1 } }, true );

There are some restrictions. A modifier may not reference the _id field, and two modifiers within an update may not reference the same field, for example the following is not allowed:

db.people.update( { name:"Joe" }, { $inc: { x: 1 }, $set: { x: 5 } } );

Pushing a Unique Value

To add a value to an array only if not already present, add $ne : <value> to your query expression:

update( {_id:'joe', tags: {"$ne": "baseball"}},
        {"$push": { tags : "baseball" } } );

Checking the Outcome of an Update Request

As described above, a non-upsert update may or may not modify an existing object. An upsert will either modify an existing object or insert a new object. The client may determine if its most recent message on a connection updated an existing object, by subsequently issuing a getlasterror command ( findOne( {getlasterror:1} ) ). If the result of the getlasterror command contains an updatedExisting field, the last message on the connection was an update request. If the updatedExisting field's value is true, that update request caused an existing object to be updated; if updatedExisting is false, no existing object was updated.

A Note About Padding

When you update an object in MongoDB, the update occurs in-place if the object has not grown in size. This is good for insert performance if the collection has many indexes.

Mongo also adaptively learns if objects in a collection tend to grow, and if they do, it adds some padding to prevent excessive movements.

Updating

Update

save() in the mongo shell

Modifier Operations

$inc

$set

$unset

$push

$pushAll

$pop

$pull

$pullAll

Upserts

Pushing a Unique Value

Checking the Outcome of an Update Request

A Note About Padding

See Also

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IF YOU HAVE A QUESTION, POST IT TO THE USER GROUP.

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