A sample use of MongoDB $lookup to populate single $ref

mongodb_one_to_one_lookup.md

db.entitlements.aggregate(
  [
    {
      $lookup:{
        from:"customers", 
        localField:"customer",
        foreignField:"_id", 
        as:"customer"
      }
    },
    {
      $unwind:"$customer"
    }
  ]).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