Updating embedded documents mongodb
We can use update modifiers to do this increment atomically.
Each URL and its number of page views is stored in a document that looks like this: Perl and PHP programmers are probably thinking that any character would have been a better choice than $.
Thus, conflicting updates can safely be sent in rapid-fire succession without any documents being corrupted: the last update will “win.”The simplest type of update fully replaces a matching document with a new one.
This can be useful to do a dramatic schema migration.
Update modifiers are special keys that can be used to specify complex update operations, such as altering, adding, or removing keys, and even manipulating arrays and embedded documents.
Suppose we were keeping website analytics in a collection and wanted to increment a counter each time someone visited a page.
The best way to avoid this situation is to make sure that your update always specifies a unique document, perhaps by matching on a key like “_id”Usually only certain portions of a document need to be updated.
Partial updates can be done extremely efficiently by using atomic update modifiers.
A common error people often make when starting out is to try to set the value of “foo” to “bar” by doing an update that looks like this: This will not function as intended.The update() method updates the values in the existing document.Once a document is stored in the database, it can be changed using the update method.The database will throw an error for this, and nothing will be changed.For example, suppose we create several documents with the same “name”, but we don’t realize it: E11001 duplicate key on update What happened?