Use this function for commands that read such as "count" or "distinct".

Read preferences, read concern, and collation can be overridden by various sources. In a transaction, read concern and write concern are prohibited in opts and the read preference must be primary or NULL. The highest-priority sources for these options are listed first in the following table. No write concern is applied.

================== ============== ========= Read Preferences Read Concern Collation ================== ============== ========= read_prefs opts opts Transaction Transaction database, client, collection ================== ============== =========

See the example for transactions and for the "distinct" command with opts.