eventSource.where()

Learn: Event Feeds and Event Streams

Create an event source that emits events for a subset of another event source’s tracked Set.

Signature

where(predicate: (A => Boolean | Null)) => EventSource<A>

Description

The where() method returns an event source. The event source emits events for a subset of another event source’s tracked Set.

The subset’s elements must match a provided predicate function. The predicate must return a Boolean or Null.

Parameters

Parameter Type Required Description

predicate

Function

Yes

Anonymous predicate function that’s compared to an existing event source's Set elements. The function returns true for matches, false for mismatches, or Null.

Return value

Type Description

EventSource

String-encoded token for an event source. The event source emits events for Set elements that match the provided predicate function.

Examples

// Only emit events for `Customer` documents
// with an `.address.state` of `DC`.
Customer.all()
  .eventSource()
  .where(.address.state == "DC")
// String-encoded token for the new event source
"g9WD1YPG..."

When consumed as an Event Feed or Event Stream, the event source emits events for the subset matching the provided predicate function:

// The new event source only emits events
// for `Customer` documents with an
// an `.address.state` of `DC`.
{
  "type": "update",
  "data": {
    "@doc": {
      "id": "111",
      "coll": {
        "@mod": "Customer"
      },
      "ts": {
        "@time": "2099-10-30T20:18:18.390Z"
      },
      "cart": {
        "@ref": {
          "id": "413111684333305922",
          "coll": {
            "@mod": "Order"
          }
        }
      },
      "orders": {
        "@set": "hdW..."
      },
      "name": "Alice Appleseed",
      "email": "alice.appleseed@example.com",
      "address": {
        "street": "87857 Mendota Court",
        "city": "Washington",
        "state": "DC",
        "postalCode": "20220",
        "country": "US"
      }
    }
  },
  "txn_ts": 1730319498390000,
  "cursor": "gsG...",
  "stats": {
    "read_ops": 3,
    "storage_bytes_read": 1049,
    "compute_ops": 1,
    "processing_time_ms": 50,
    "rate_limits_hit": []
  }
}

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