Come eseguire la ricerca nidificata $ lookup in MongoDB?

$lookup 3.6 la sintassi ti consente di unire tabelle nidificate e $unwind decostruisce un campo array dai documenti di input per produrre un documento per ogni elemento. Qualcosa del genere

position.aggregate([
  { "$lookup": {
    "from": "companies",
    "let": { "companyId": "$company_id" },
    "pipeline": [
      { "$match": { "$expr": { "$eq": [ "$_id", "$$companyId" ] } } },
      { "$lookup": {
        "from": "industries",
        "let": { "industry_id": "$industry_id" },
        "pipeline": [
          { "$match": { "$expr": { "$eq": [ "$_id", "$$industry_id" ] } } }
        ],
        "as": "industry"
      }},
      { "$unwind": "$industry" }
    ],
    "as": "company"
  }},
  { "$unwind": "$company" }
])

Con la versione 3.4

position.aggregate([
  { "$lookup": {
    "from": "companies",
    "localField": "company_id",
    "foreignField": "_id",
    "as": "companies"
  }},
  { "$unwind": "$companies" },
  { "$lookup": {
    "from": "industries",
    "localField": "companies.industry_id",
    "foreignField": "_id",
    "as": "companies.industry"
  }},
  { "$unwind": "$companies.industry" },
  { "$group": {
    "_id": "$_id",
    "companies": { "$push": "$companies" }
  }}
])