[FIXED] Using ReactiveMongoTemplate with Kotlin coroutines

Issue

I am developing a reactive application using Spring Boot 2.3.0, MongoDB, and Kotlin. Unlikely, I need to develop a custom repository using the ReactiveMongoTemplate object. Since all the application uses Kotlin coroutines, I will be very happy if also this repository could use them.

For example, I developed a simple update on a Mongo collection:

override suspend fun addQuantityToStockInAPortfolio(
    portfolio: String, 
    stock: String,
    quantity: Long): UpdateResult? {
  return mongo.updateFirst(
      Query.query(Criteria.where("_id").isEqualTo(portfolio)),
      Update().inc("stocks.$stock", quantity),
      MongoPortfolio::class.java
   ).block()
}

The only way I found to translate a Mono<T> into the return type of a suspend fun is to call the block() method on it.

Is there a more natural way to proceed?

Solution

Yes, there is. block() is not idiomatic and can lead to problems by blocking a thread which is not supposed to be blocked.

You should use one of the await extension functions like awaitSingle() which will not block the thread, just suspend instead.

import kotlinx.coroutines.reactive.awaitSingle

// other code

override suspend fun addQuantityToStockInAPortfolio(
    portfolio: String, 
    stock: String,
    quantity: Long): UpdateResult {
  return mongo.updateFirst(
      Query.query(Criteria.where("_id").isEqualTo(portfolio)),
      Update().inc("stocks.$stock", quantity),
      MongoPortfolio::class.java
   ).awaitSingle()
}

Answered By - Martin Tarjányi
Answer Checked By - Mary Flores (JavaFixing Volunteer)