Add new field to every document in a MongoDB collection

Same as the updating existing collection field, $set will add a new fields if the specified field does not exist.

Check out this example:

> db.foo.find()
> db.foo.insert({"test":"a"})
> db.foo.find()
{ "_id" : ObjectId("4e93037bbf6f1dd3a0a9541a"), "test" : "a" }
> item = db.foo.findOne()
{ "_id" : ObjectId("4e93037bbf6f1dd3a0a9541a"), "test" : "a" }
> db.foo.update({"_id" :ObjectId("4e93037bbf6f1dd3a0a9541a") },{$set : {"new_field":1}})
> db.foo.find()
{ "_id" : ObjectId("4e93037bbf6f1dd3a0a9541a"), "new_field" : 1, "test" : "a" }

EDIT:

In case you want to add a new_field to all your collection, you have to use empty selector, and set multi flag to true (last param) to update all the documents

db.your_collection.update(
  {},
  { $set: {"new_field": 1} },
  false,
  true
)

EDIT:

In the above example last 2 fields false, true specifies the upsert and multi flags.

Upsert: If set to true, creates a new document when no document matches the query criteria.

Multi: If set to true, updates multiple documents that meet the query criteria. If set to false, updates one document.

This is for Mongo versions prior to 2.2. For latest versions the query is changed a bit

db.your_collection.update({},
                          {$set : {"new_field":1}},
                          {upsert:false,
                          multi:true}) 

Since MongoDB version 3.2 you can use updateMany():

> db.yourCollection.updateMany({}, {$set:{"someField": "someValue"}})

To clarify, the syntax is as follows for MongoDB version 4.0.x:

db.collection.update({},{$set: {"new_field*":1}},false,true)

Here is a working example adding a published field to the articles collection and setting the field's value to true:

db.articles.update({},{$set: {"published":true}},false,true)

db.collection.updateMany({}, {$set: {"fieldName": ""}})

updateMany requires a matching condition for each document, since we are passing {} it is always true. And the second argument uses $set operator to add the required field in each document.