A Sample MongoDB $lookup to populate many $ref

mongodb_one_to_many_lookup.md

db.parties.aggregate(
  [
    {
      $match:{
        "roles":{
            $ne:null // []
          }
        }
    },
    {
      $unwind:"$roles"
    },
    {
      $lookup:{
        from:"roles", 
        localField:"roles",
        foreignField:"_id", 
        as:"roles"
      }
    }]).pretty();

References

• https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/
• https://www.compose.io/articles/deeper-into-3-2-lookup/
• http://www.codeproject.com/Articles/1077839/Working-with-MongoDBs-lookup-Aggregator
• https://www.mongodb.com/blog/post/joins-and-other-aggregation-enhancements-coming-in-mongodb-3-2-part-1-of-3-introduction