Find

Find operation can be used for retrieving a subset of the existing data from a MongoDB collection with the option for specifying what data to return, the number of documents to return and in what order.

The result of an operation can be returned in the following forms:

  • The first document that matches a query - F[Option[T]]
  • All documents bundled in a single collection - F[Iterable[T]]
  • A Stream of documents where each item is emitted as soon as it is obtained - fs2.Stream[F, T]

Find operation can be executed by calling find method on a MongoCollection[F, T] instance:

import mongo4cats.bson.Document

val data: fs2.Stream[IO, Document] = collection.find.stream

To specify what data to return, additional filters can be passed in:

import mongo4cats.collection.operations.Filter

val filter1 = Filter.eq("field1", "foo")
val filter2 = Filter.eq("field2", "bar")
val filter3 = Filter.exists("field3")
val data: IO[Option[Document]] = collection.find((filter1 || filter2) && filter2).first

As can be noted from the example above, filters are composable and can be chained together using logical operators || and &&. The full list of available filters can be found either by exploring API of the mongo4cats.database.operations.Filter companion object or by vising the official MongoDB documentation.

To reduce the number of returned document, skip and limit methods can be applied:

val data = IO[Iterable[Document]] = collection.find
  .skip(10) // skip the first 10 
  .limit(100) // take the next 100
  .all

The ordering of the data can be enforced by calling either sortBy or sort method:

import mongo4cats.collection.operations.Sort

// sort in ascending order by field1, then in descending order by field2
val data = IO[Iterable[Document]] = collection.find.sort(Sort.asc("field1").desc("field2")).all

// same as the above but without Sort specification
val data = IO[Iterable[Document]] = collection.find.sortBy("field1").sortByDesc("field2").all