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Distinct

The distinct operation returns all unique values for a specified field across a collection, equivalent to SQL's SELECT DISTINCT. It is useful for building autocomplete lists, enumerating categories, or checking what values actually exist in production data.

Basic usage​

import cats.effect.IO

// Collect all distinct string values
val statuses: IO[Iterable[String]] = collection.distinct[String]("status").all

// Stream distinct values one at a time
val stream: fs2.Stream[IO, String] = collection.distinct[String]("status").stream

With a filter​

Pass a Filter to restrict the documents considered before computing distinct values:

import mongo4cats.operations.Filter

// Only look at active documents
val activeCategories: IO[Iterable[String]] =
  collection.distinct[String]("category", Filter.eq("active", true)).all

Complex value types​

If the field holds a document or a type other than a primitive, use the appropriate type parameter:

import mongo4cats.bson.Document

// Field holds an embedded document
val distinctDocs: IO[Iterable[Document]] = collection.distinct[Document]("address").all

For a custom case class, provide a MongoCodecProvider[MyClass] in the implicit scope (e.g. via Circe derivation) and use distinctWithCodec:

import io.circe.generic.auto._
import mongo4cats.circe._

final case class Tag(name: String, weight: Double)

val distinctTags: IO[Iterable[Tag]] =
  collection.distinctWithCodec[Tag]("tags").all

Alternatively, add codec support to the collection on the fly:

val distinctTags: IO[Iterable[Tag]] =
  collection.withAddedCodec(myTagCodecRegistry).distinct[Tag]("tags").all

Using with a client session (transactions)​

client.startSession.use { session =>
  collection.distinct[String](session, "status", Filter.empty).all
}