When filtering arrays we often need to reduce to the first element matching a condition. This is currently achievable with the following:

{$project: {
  action: {$arrayElemAt: [{$filter: {
    input: '$actions',
    cond: {$eq: ['$$this.id', '$id']},
  }}, 0]},
}}

A cleaner and more performant option would be to have a dedicated `find` operator:

{$project: {
  action: {$find: {
    input: '$actions',
    cond: {$eq: ['$$this.id', '$id']},
  }},
}}

This idea is analagous to the ECMAScript Array.prototype.find, and would compliment the '$filter', '$map', and '$reduce' operators that are already implemented.