Using Parse, how do I set a column unique or at least check if it exists first?

Question

I have a table of 'words', and what I want to do is, if a user uses that word, insert it into the table. But, if it already exists, update the count on that word.

I don't see a way to actually set columns as unique.. That said, how do I avoid the race condition:

t1 - user1 checks if 'Love' is used. NO t2 - user2 checks if 'Love' is used. NO t3 - user1 adds 'Love' t4 - user2 adds 'Love'

I do not want this to be in the database twice. Anyway to accomplish this in Parse?

score 5 · Answer 1 · answered Mar 20 '18 at 20:55

To avoid race conditions, you should make the column/field unique. Do that in the database that you use with with Parse (if you use mongoDB with your parse server, then here's a link for mongoDB for example - https://docs.mongodb.com/manual/core/index-unique/). Then in CloudCode - try to create a new word object and store it. If the word already exists, the store will fail and then you can use a query to find the existing word and increment instead.

score 1 · Accepted Answer · answered Oct 03 '13 at 07:10

You can use the 'exists' query to check for objects which have the key set:

var Word = Parse.Object.extend("words");
var query = new Parse.Query(Word);
query.exists("Love");
query.find({
  success: function(results) {
    if(results.length === 0){
      // Insert a new word object with the 'Love' key set to 1
      var newWord = new Word();
      newWord.set('Love', 1);
      newWord.save(null, {
        success: function(newWord) {
          alert('New object created with objectId: ' + newWord.id);
        },
        error: function(newWord, error) {
          alert('Failed to create new object, with error code: ' + error.description);
        }
      });
    } else {
      // Get the existing word object and increment its 'Love' key by 1
      var existingWord = results[0];
      var currCount = existingWord.get('Love');
      existingWord.set('Love', currCount + 1);
      existingWord.save(null, {
        success: function(existingWord) {
          alert('Existing object saved with objectId: ' + newWord.id);
        },
        error: function(existingWord, error) {
          alert('Failed to save existing object, with error code: ' + error.description);
        }
      });
    }
  },
  error: function(error) {
    alert("Error: " + error.code + " " + error.message);
  }
});

With regard to preventing race conditions, you could handle this with Parse Cloud Code. You could have a Cloud Function which handles the input of the data and it should handle the requests sequentially.

this answer looks right. why hasnt it been marked correct? Did it solve your problem @JP? — Zen, May 16 '14 at 15:43
in the code, there is an extra equal sign in `if(results.length === 0)` — Mahir, Aug 13 '14 at 09:05
Yes, in JavaScript === compares the value and type. See this SO question: http://stackoverflow.com/questions/359494/does-it-matter-which-equals-operator-vs-i-use-in-javascript-comparisons — Daniel Bank, Aug 13 '14 at 17:52
This code doesn't work if more than one requests executed concurrently. — Arsenius, Jul 03 '17 at 09:40
The check routine must be in the server side, not in the client side. I think the best solution is to use **CloudCode** or **Database UNIQUE KEY** to prevent saving duplicates. — Nadjib Rebahi, Nov 25 '21 at 07:59

Using Parse, how do I set a column unique or at least check if it exists first?

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