relationship in mongodb code example

Example 1: include relation in mongo query

db.user.aggregate([
    { $project: { "userId": 1, "userKeyEncrypted": 1, "uuid":1, "roles": 1, "passwordHash": 1, "disabled": 1, companyRefs: { $let: { vars: { refParts: { $objectToArray: "$$ROOT.company" }}, in: "$$refParts.v" } } } },
    { $match: { "companyRefs": { $exists: true } } },
    { $project: { "userId": 1, "userKeyEncrypted": 1, "uuid":1, "roles": 1, "passwordHash": 1, "disabled": 1, "companyRef": { $arrayElemAt: [ "$companyRefs", 1 ] } } },
    { $lookup: { from: "company", localField: "companyRef", foreignField: "_id", as: "company_data" } },
    { $project: { "userId": 1, "userKeyEncrypted": 1, "uuid":1, "roles": 1, "passwordHash": 1, "disabled": 1,  "companyUuid": { $arrayElemAt: [ "$company_data.uuid", 0 ] } } }
])

Example 2: one to many mapping in mongodb

// patron document
{
   _id: "joe",
   name: "Joe Bookreader"
}

// address documents
{
   patron_id: "joe", // reference to patron document
   street: "123 Fake Street",
   city: "Faketon",
   state: "MA",
   zip: "12345"
}

{
   patron_id: "joe",
   street: "1 Some Other Street",
   city: "Boston",
   state: "MA",
   zip: "12345"
}