text: Q:
  
  How to make firestore create nested object as sub collections on write?
  
  Consider my below code. I want this code to create a sub-collection called items under the collection test, but right the items added an array field in test collection. Note, this is for firestore not firebase realtime db.
  data = {
  "first": "new",
  "born": 1815,
  "items": [
    {
      "field": 2
    }
   ]
  };
  db.collection("test").add(data)
  
  A:
  
  Your data object is a single object, and when you store a single object as a document using the code you gave, that object will occupy that single document.
  If you want to write data to a subcollection, you'll have to write it separately, being very clear to the API that you want a subcollection underneath it:
  db.collection("test")
      .document("doc_id")
      .collection("items")
  
  or:
  db.collection("test/doc_id/items")
  
  To put it another way, if you want to write any two documents, you'll have to perform to two add/update/create calls at different locations.  You can't create two documents with one call.
  
  