query-not-boolean

• Route: write-events
• Description: /preconditions/<index>: query does not return exactly one boolean: it returned <result>, with the place of the precondition in its list, counting from 0, and what the query returned instead: more than one result, a value of another type, or no result
• Details: precondition, the place of the precondition in its list, counting from 0.

Why It Happens#

An isEventQlQueryTrue precondition holds if its query returns exactly one row, and that row is true. The query runs, but returns something else: one row per event, since its projection uses no aggregation function, as PROJECT INTO e.type == 'io.eventsourcingdb.library.book-borrowed' does, no row at all, since such a query found no event, or a value that is not a boolean, as PROJECT INTO COUNT() without a comparison returns.

How to Fix It#

Write the query so that it always returns a single boolean, by comparing an aggregation function with a value, as PROJECT INTO COUNT() == 0 does. Sending the same request again does not help: whether a query without an aggregation function returns exactly one row depends on the events it finds.

Example#

For a write whose only precondition has the query FROM e IN events WHERE e.subject == '/books/42' PROJECT INTO e.type == 'io.eventsourcingdb.library.book-borrowed', while /books/42 has several events, EventSourcingDB answers with this error, as a line of the stream, and then ends the stream with its end item:

{
  "type": "error",
  "data": {
    "code": "https://docs.eventsourcingdb.io/errors/query-not-boolean",
    "description": "/preconditions/0: query does not return exactly one boolean: it returned more than one result",
    "details": {
      "precondition": 0
    }
  }
}