Why is $ne: null causing the return of empty arrays?

Seeking help on filtering arrays in MongoDB where contents are not null or exist but still seeing empty arrays being returned...

const userSchema = new mongoose.Schema({
email: String, 
password: String, 
googleId: String, 
facebookId: String, 
secret: Array
});

app.get("/secrets", function(req, res) {
User.find({secret: {$ne: null} }, function(err, secrets) {
if (err) {
  console.log(err);
} else {
  if (secrets) {
   console.log(secrets); 
    res.render("secrets", {
      secrets: secrets
    });
   };
  };
 }); 
});

After researching online, I understand that $ne: null only filters out documents where the secret array doesn't exist. But what to do when it's an empty array? Any tips on resolving this issue for a newbie like me? Your kindness is appreciated! :)

Answer №1

In programming, it is important to note that null is not equal to an empty array []. Therefore, when writing conditions in your code, you must explicitly handle both cases.

The MongoDB query operator $nin can be used to find users whose secret value is not included in a specified array containing multiple values.

User.find({secret: {$nin: [null, [] ] } },...

Additionally, utilizing the $exists operator will include documents where the field is null, with the exception of those where the secret field does not exist at all.

Answer №2

When examining the data in the documents for the presence of values in the secret field that are missing, undefined, null, empty array, or populated array, there are several approaches you can take to specifically target only the populated arrays:

{$eval:{$gt:[0,{$size:"$secret"}]}}- Determine the size of thesecrets` array and select arrays that are not empty.

  • {"secret.0":{$exists:true}}
    - This will only match the document if there is a first element present in the secret array (implicitly excluding non-array fields)

  • If all elements in the secret array are of the same type, a type-specific query may be utilized. For example, if the elements in the secret array are strings,

    {"secret.0":{$gte:""}}
    will match any array with a string as its first element. An index on {"secret.0":1} can also help optimize this process.

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