Creating, Updating, and Deleting Documents - MongoDB: The Definitive Guide - page 33

> db.papers.update({"authors cited" : {"$ne" : "Richie"}},
... {$push : {"authors cited" : "Richie"}})
This can also be done with "$addToSet" , which is useful for cases where "$ne" won't
work or where "$addToSet" describes what is happening better.
For instance, suppose you have a document that represents a user. You might have a
set of email addresses that they have added:
> db.users.findOne({"_id" : ObjectId("4b2d75476cc613d5ee930164")})
{
"_id" : ObjectId("4b2d75476cc613d5ee930164"),
"username" : "joe",
"emails" : [
"[e-mail]",
"[e-mail]",
"[e-mail]"
]
}
When adding another address, you can use "$addToSet" to prevent duplicates:
> db.users.update({"_id" : ObjectId("4b2d75476cc613d5ee930164")},
... {"$addToSet" : {"emails" : "[e-mail]"}})
> db.users.findOne({"_id" : ObjectId("4b2d75476cc613d5ee930164")})
{
"_id" : ObjectId("4b2d75476cc613d5ee930164"),
"username" : "joe",
"emails" : [
"[e-mail]",
"[e-mail]",
"[e-mail]",
]
}
> db.users.update({"_id" : ObjectId("4b2d75476cc613d5ee930164")},
... {"$addToSet" : {"emails" : "[e-mail]"}})
> db.users.findOne({"_id" : ObjectId("4b2d75476cc613d5ee930164")})
{
"_id" : ObjectId("4b2d75476cc613d5ee930164"),
"username" : "joe",
"emails" : [
"[e-mail]",
"[e-mail]",
"[e-mail]",
"[e-mail]"
]
}
You can also use "$addToSet" in conjunction with "$each" to add multiple unique val-
ues, which cannot be done with the "$ne" / "$push" combination. For instance, we could
use these modifiers if the user wanted to add more than one email address:
> db.users.update({"_id" : ObjectId("4b2d75476cc613d5ee930164")}, {"$addToSet" :
... {"emails" : {"$each" : ["[e-mail]", "[e-mail]", "[e-mail]"]}}})
> db.users.findOne({"_id" : ObjectId("4b2d75476cc613d5ee930164")})
{