Using MongoDB as a storage vessel for the D2 MobileWorldContentFile, I wanted to put together an aggregate of the activities...

index.rb

require_relative 'list_activities'

include Activities

puts Activities::list_all

list_activities.rb

require 'rubygems'
require 'bundler/setup'
require 'dotenv/load'
require 'mongo'

module Activities

    def list_all
        Mongo::Logger.level = Logger::FATAL
        client = Mongo::Client.new(ENV['DB_URL'])

        activity_defs = client[:DestinyActivityDefinition]

        activity_defs.aggregate([
        { "$match" => { 'isPvP' => false } },
        { 
        "$lookup" => {
            "localField" => "destinationHash",
            "foreignField" => "hash",
            "from" => "DestinyDestinationDefinition",
            "as" => "destination"
          }
        },
        { 
        "$lookup" => {
            "localField" => "placeHash",
            "foreignField" => "hash",
            "from" => "DestinyPlaceDefinition",
            "as" => "place"
          }
        },
        { 
        "$lookup" => {
            "localField" => "activityTypeHash",
            "foreignField" => "hash",
            "from" => "DestinyActivityTypeDefinition",
            "as" => "activityType"
          }
        },
        {
            "$project" => {
                "displayProperties" => 1,
                "destination.displayProperties" => 1,
                "place.displayProperties.name" => 1,
                "activityType.displayProperties.name" => 1
            }
        }
        ]).each do |activity|
            activity
        end
    end

end

output.bson

{
    "_id"=>BSON: :ObjectId('5d9e0fec5d0bdb2d2ccb0ed3'),
    "displayProperties"=>{
        "description"=>"Defeat enemies.",
        "name"=>"Defeat Enemies",
        "icon"=>"/common/destiny2_content/icons/c60303e278aa5fc566a04e98c3d8024c.png", 
        "hasIcon"=>true
    }, 
    "destination"=>[
        {
            "displayProperties"=>{
                "description"=>"Mars", 
                "name"=>"Hellas Basin", 
                "hasIcon"=>false
            }
        },
        {
            "displayProperties"=>{
                "description"=>"Mars",
                "name"=>"Hellas Basin",
                "hasIcon"=>false
            }
        }
    ],
    "place"=>[
        {
            "displayProperties"=>{
                "name"=>"Mars"
            }
        }
    ], 
    "activityType"=>[
        {
            "displayProperties"=>{
                "name"=>"Explore"
            }
        }
    ]
}