Why do firebase collections seem to begin with a null row?

Question

I've created a simple firebase data set to test some REST calls with (see the image below). I'm wondering why, when I query the collections in the database, firebase always return a null row first and then the actual rows. Here is what I get when I export the data. Notice the null rows under systems and system_types:

{
  "systems" : [ null, {
    "system_type_id" : 2,
    "name" : "Commodore 128",
    "id" : 1
  }, {
    "system_type_id" : 1,
    "name" : "Difference Engine",
    "id" : 2
  }, {
    "system_type_id" : 2,
    "name" : "Osborne",
    "id" : 3
  } ],
  "system_types" : [ null, {
    "name" : "Babbage",
    "id" : 1
  }, {
    "name" : "Von Neumann",
    "id" : 2
  } ]
}

Answer 1

Firebase automatically detects when an object 'looks like' an array, and converts it accordingly. Since 'systems' and 'system_types' have children of 1, 2, and 3, Firebase is automatically converting these to arrays at output time. However, since you didn't specify a value for index 0, Firebase is just inserting an empty element.

I'd suggest either using zero-based ID's, or perhaps starting your id's with something non-numeric so we know it's not an array (ie: item1, item2, item3).

You could also just ignore the null element -- In Firebase null and non-existent are the same thing.