Creates a stream that tracks changes to specified document fields in a Set.


changesOn(...fields: A => Any): Stream<A>


The changesOn() method creates a stream that tracks changes to specified document fields in a Set.

You can only call changesOn() on a Set from a supported source. The exact behavior of the method depends on this source. The calling Set isn’t changed.

Source Sets for streams support a limited number of transformations and filters.

A query that creates a stream returns a Stream.


Parameter Type Required Description




Comma-separated list of document field paths (using dot notation). The stream tracks changes to the values of these document field.

Return value

Type Description


Stream token used to subscribe to the stream.


Collection streams

You can use changesOn() to only track changes to specific fields for documents in a collection.


The query returns a stream token.


Index streams

Streams on indexes only send events for changes to the index’s terms or values fields.

For example, the following Product collection’s byCategory index has:

  • A term field of category

  • Value fields of name and price

collection Product {
  index byCategory {
    terms [.category]
    values [.name, .price]

When called on an index, changesOn() only accepts the index’s terms or values fields as arguments.

For example, in the following query, changesOn() only accepts .category, .name, or .price as arguments.

Product.byCategory('sports').changesOn(.category, .name)

Document streams

You can use changesOn() to track changes to a Set containing a single document. The following query only tracks changes to the name or price field of a single document.

let product = Product.byId(392174614626697728)!
Set.single(product).changesOn(.name, .price)

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