Question

I am trying to search a MongoDB database filtering on a specific data item. Specifically, I want USA = PG-13 for my filter.

How to make a request: to get movies, where MPAA for USA = PG-13?

collection country:

{
   "_id": ObjectId("4fd79ec34c9fda9d05000080"),
   "en": "USA" 
}
{
   "_id": ObjectId("4fd79ec34c9fda9d0500007f"),
   "en": "Hong Kong" 
}

collection movie:

{
   "_id": ObjectId("4fd79ec34c9fda9d05000081"),
   "movieId": {
     "imdb": "0848228" 
  },
   "movieTitle": "The Avengers",
   "movieYearSpan": {
     "start": "2012",
     "end": "2012" 
  },
   "movieType": "Movie",
   "movieMpaa": {
     "verdict": "Rated PG-13",
     "0": {
       "mpaa": "IIA",
       "country": ObjectId("4fd79ec34c9fda9d0500007f") 
    },
     "1": {
       "mpaa": "PG-13",
       "country": ObjectId("4fd79ec34c9fda9d05000080") 
    } 
  } 
}

I tried to first get the ID for the USA.

$cursorCountry = $collectionCountry->find(array("en" => "USA"));
$idCountry = $cursorCountry->getNext();
$_id = $idCountry["_id"];
$cursorMovie = $collectionMovie->find(array("movieMpaa.country" => $_id, "movieMpaa.mpaa" => "PG-13"));

Does not work!
How then to make a request?
To get movies, where MPAA for USA = PG-13?

Answer 1

As your data is laid out, there is no good query here.

If you can change your movies collection to the following, then your query will work:

   "movieMpaa": {
     "verdict": "Rated PG-13",
     "countries": [
         {
           "mpaa": "IIA",
           "country": ObjectId("4fd79ec34c9fda9d0500007f") 
       },
         {
           "mpaa": "PG-13",
           "country": ObjectId("4fd79ec34c9fda9d05000080") 
        } 
     ]

When movieMpaa.countries is an array of objects, then you can query into that array for movieMpaa.countries.country. MongoDB will recognize the array and “drill into” the objects.

However, there is another way structure this that probably much easier longer term:

   "movieMpaa": {
     "verdict": "Rated PG-13",
     "countries": {
         ObjectId("4fd79ec34c9fda9d0500007f") : { "mpaa": "IIA" },
         ObjectId("4fd79ec34c9fda9d05000080") : { "mpaa": "PG-13" }
     }

If a country can only have one rating, then technically that countries value is a dictionary of Countries => ratings. The structure above stores them this way.

However, the use of ObjectId is a little ugly there. Note that you can override the ID in the country collection. Consider using the 2 or 3-character ISO codes instead. These are easier to read.

   "movieMpaa": {
     "verdict": "Rated PG-13",
     "countries": {
         "UK" : { "mpaa": "IIA" },
         "US" : { "mpaa": "PG-13" }
     }

db.movies.find({'movieMpaa.countries.US': 'PG-13'})