remove documents with array field's size less than 3 in mongoDB

Question

i have a mongoDB collection named col that has documents that look like this

{
  {
    intField:123,
    strField:'hi',
    arrField:[1,2,3]
  },

  {
    intField:12,
    strField:'hello',
    arrField:[1,2,3,4]
  },

  {
    intField:125,
    strField:'hell',
    arrField:[1]
  }
}

Now i want to remove documents from collection col in which size of the array field is less than 2.

So i wrote a query that looks like this

db.col.remove({'arrField':{"$size":{"$lt":2}}})

Now this query doesnt do anything. i checked with db.col.find() and it returns all the documents. Whats wrong with this query?

Answer 1

With MongoDB 2.2+ you can use numeric indexes in condition object keys to do this:

db.col.remove({'arrField.2': {$exists: 0}})

This will remove any document that doesn't have at least 3 elements in arrField.

Answer 2

From the documentation for $size:

You cannot use $size to find a range of sizes (for example: arrays with more than 1 element).

The docs recommend maintaining a separate size field (so in this case, arrFieldSize) with the count of the items in the array if you want to try this sort of thing.