Dollar-Prefixed Field Names

Insert Operations

Dollar ($) prefixed fields are permitted as top level and nested field names for inserts.

db.sales.insertOne( {
   "$price": 50.00,
   "quantity": 30
} )

Dollar ($) prefixed fields are permitted on inserts using otherwise reserved words. Operator names like $inc can be used as field names as well as words like id, db, and ref.

db.books.insertOne( {
   "$id": "h1961-01",
   "location": {
      "$db": "novels",
      "$ref": "2007042768",
      "$inc": true
} } )

An update which creates a new document during an upsert is treated as an insert rather than an update for field name validation. Upserts can accept dollar ($) prefixed fields. However, upserts are a special case and similar update operations may cause an error if the match portion of the update selects an existing document.

This code sample has upsert: true so it will insert a new document if the collection doesn't already contain a document that matches the query term, { "date": "2021-07-07" }. If this sample code matches an existing document, the update will fail since $hotel is dollar ($) prefixed.

db.expenses.updateOne(
   { "date": "2021-07-07" },
   { $set: {
      "phone": 25.17,
      "$hotel": 320.10
   } },
   { upsert: true }
)

Document Replacing Updates

Update operators either replace existing fields with new documents or else modify those fields. In cases where the update performs a replacement, dollar ($) prefixed fields are not permitted as top level field names.

Consider a document like

{
   "_id": "E123",
   "address": {
      "$number": 123,
      "$street": "Elm Road"
   },
   "$rooms": {
      "br": 2,
      "bath": 1
   }
}

You could use an update operator that replaces an existing document to modify the address.$street field but you could not update the $rooms field that way.

db.housing.updateOne(
   { "_id": "E123" },
   { $set: { "address.$street": "Elm Ave" } }
)

Use $setField as part of an aggregation pipeline to update top level dollar ($) prefixed fields like $rooms.

Document Modifying Updates

When an update modifies, rather than replaces, existing document fields, dollar ($) prefixed fields can be top level field names. Subfields can be accessed directly, but you need a helper method to access the top level fields.

Consider a collection with documents like this inventory record:

{
   _id: ObjectId("610023ad7d58ecda39b8d161"),
   "part": "AB305",
   "$bin": 200,
   "quantity": 100,
   "pricing": { sale: true, "$discount": 60 }
}

The pricing.$discount subfield can be queried directly.

db.inventory.findAndModify( {
    query: { "part": { $eq: "AB305" } },
    update: { $inc: { "pricing.$discount": 10 } }
} )

Use $getField and $literal to access the value of the top level $bin field.

db.inventory.findAndModify( {
   query: { $expr: {
      $eq: [ { $getField: { $literal: "$bin" } }, 200 ]
   } },
   update: { $inc: { "quantity": 10 } }
} )

Updates Using Aggregation Pipelines

Use $setField, $getField, and $literal in the $replaceWith stage to modify dollar ($) prefixed fields in an aggregation pipeline.

Consider a collection of school records like:

db.school.insertOne (
{
   "_id": 100001,
   "$term": "fall",
   "registered": true,
   "grade": 4
} )

Create a new collection for the spring semester using a pipeline to update the dollar ($) prefixed $term field.

db.school.aggregate( [
   { $match: { "registered": true } },
   { $replaceWith: {
      $setField: {
         field: { $literal: "$term" },
         input: "$$ROOT",
         value: "spring"
   } } },
   { $out: "spring2022" }
] )