Coroutine reads and cardinality

Use Flow for multiple documents

findAsFlow<T>(query) and findAllAsFlow<T>() call asFlow() on Spring Data publishers. Calling the extension does not immediately load a list. Subscription begins when a consumer collects the Flow.

fun findAdults(city: String): Flow<User> {
    val query = queryOf(
        "age".criteria() gte 20,
        "city".criteria() eq city,
    ).sortAscBy("name")

    return mongoOperations.findAsFlow(query)
}

Zero documents produces an empty Flow. Collecting the same Flow again can create another subscription and query.

Optional and required reads

Expected result	API	Zero documents
Absence is valid	findOneOrNullSuspending<T>, findByIdOrNullSuspending<T>	null
A document is required	findOneSuspending<T>, findByIdSuspending<T>	NoSuchElementException

The two families use awaitSingleOrNull() and awaitSingle(), respectively. Neither turns a connection or mapping failure into null.

suspend fun requireUser(id: String): User =
    mongoOperations.findByIdSuspending(id)

suspend fun findUser(id: String): User? =
    mongoOperations.findByIdOrNullSuspending(id)

Attach domain meaning such as HTTP 404 outside the repository. Mapping every driver failure to 404 would hide an outage as missing data.

Count and exists

countSuspending<T>(query) returns Long; existsSuspending<T>(query) returns Boolean. Use exists instead of reading a document list when only presence matters.

suspend fun emailExists(email: String): Boolean =
    mongoOperations.existsSuspending<User>(
        queryOf("email".criteria() eq email)
    )

No match returns 0L or false, not a failure.

Cancellation and flow control

Cancelling Flow collection is propagated to the reactive subscription. Do not catch it and emit normal data. For large results, add an explicit order and limit, and observe consumer throughput together with pool usage.

Source and tests

• ReactiveMongoOperationsCoroutines.kt
• ReactiveMongoOperationsCoroutinesTest.kt
• QueryExtensionsTest.kt

Next chapter

Writes and atomic operations separates write results and atomicity boundaries.