How does mongoDB array lookup via Spring Data work?

Question

Java

There is such a couple of thousand objects like this:

{
      "_id" : ObjectId("5ffbfca73fb224209c6d27be"),
      "activeStates" : ["logicConnectionLost", ...],
       ...,
      "_class" : "Entity"
    }

I need to count the number of objects whose activeStates array contains at least one element from the input array?
I am using the
countByActiveStatesIn(Set states);
Does Mongo compare each against each until the end of all arrays, or until it finds the first match?

Answer 1

The answer depends on what kind of request the Spring Data will generate for the countByActiveStatesIn(Set states);.
According to table #16 in section 15.3 of the Spring Data MongoDB documentation , a query should be generated

db.collectionName.count({ activeStates: { $in: ["logicConnectionLost"] }})

which will count all documents whose field activeStatescontains at least one element from the passed list.
Which request was generated can be checked by setting the logging level to DEBUG:

logging.level.org.springframework.data.mongodb.core.MongoTemplate=DEBUG

#Если используете реактивный MongoTemplate:
logging.level.org.springframework.data.mongodb.core.ReactiveMongoTemplate=DEBUG