Replace with

Description

Replaces fields in the existing document with the fields in a given expression.

The expression can either be a literal map or an expression that evaluates to a map. If a given expression does not evaluate to a map, this stage returns an error.

There are 3 supported modes:

  • full_replace: Replaces the entire document with the result of the given expression, omitting fields that don't appear in it.
  • merge_overwrite_existing: Merges the expression with the existing document, overwriting field values in the document with values in the expression
  • merge_keep_existing: Merges the expression with the existing document, only adding field values when one does not exist in the document.

Examples

Create a cities collection with the following documents:

Node.js

await db.collection("cities").doc("SF").set({name: "San Francisco", population: 800000, location: {country: "USA", state: "California"}});
await db.collection("cities").doc("TO").set({name: "Toronto", population:  3000000, province: "ON", location: {country: "Canada", province: "Ontario"}});
await db.collection("cities").doc("NY").set({name: "New York", location: {country: "USA", state: "New York"}});
await db.collection("cities").cov("AT").set({name: "Atlantis", population: null});

Using the full_replace mode to get a mutated version of the document

Extract the nested location field, discarding all other data:

Node.js
  const names = await db.pipeline()
    .collection("/cities")
    .replace_with(Field.of("location"), "full_replace")
    .execute();
Java
Pipeline.Snapshot names =
    firestore.pipeline().collection("cities").replaceWith(field("location")).execute().get();
PipelineSnippets.java

Which produces the following documents:

{ country: "USA", state: "California" },
{ country: "Canada", province: "Ontario" },
{ country: "USA", state: "New York" },
{ }

Using the merge_overwrite_existing mode to set fields

Set the population field of all documents to 0, overwriting existing values:

Node.js
  const censoredResults = await db.pipeline()
    .collection("/cities")
    .replace_with(map("population", 0), "merge_overwrite_existing")
    .execute();

Which produces the following documents:

{name: "San Francisco", population: 0, location: {country: "USA", state: "California"}},
{name: "Toronto", population: 0, province: "ON", location: {country: "Canada", province: "Ontario"}},
{name: "New York", population: 0, location: {country: "USA", state: "New York"}},
{name: "Atlantis", population: 0}

Using the merge_keep_existing mode to add a default value

Setting a default value for location when it doesn't appear in a document:

Node.js
  const defaultedResults = await db.pipeline()
    .collection("/cities")
    .replace_with(map("location", "unknown"), "merge_keep_existing")
    .execute();

Which produces the following documents:

{ name: "San Francisco", population: 800000, location: { country: "USA", state: "California" } },
{ name: "Toronto", province: "ON", population: 3000000, location: { country: "Canada", province: "Ontario" } },
{ name: "New York", location: { country: "USA", state: "New York" } },
{ name: "Atlantis", population: null, location: "unknown" }