Skip to content

SQL UPDATE for strings in MongoDB

MongoDB currently does not offer the equivalent of a SQL Update statement when you want to change the value of a string to something that is based on the current value. In my case I needed to find spaces and change them to underscores -- something done easily in php with str_replace(), and in MySQL with REPLACE.

If you had a table named foo with a column named "mystr" you could do this sort of update like this:

UPDATE foo SET mystr = REPLACE(mystr, ' ', '_');

MongoDB does have collection.update() that works with a series of "modifier" operations, but none of them are useful for doing modifications to the current value of a document field when that field is a string.

MongoDB does however support javascript, and the javascript replace() function implements a regular expression search and replace. I was able to accomplish the update using the forEach() method to process the resulting documents one at a time. While not as efficient as having mongo do this internally with a specially built update modifier, at least this method only needs to operate on documents that are known to require modifications, and could be run in the mongo interactive client easily.

Again, assuming the collection is named "foo" and the document field is named "mystr"{ mystr: /[ ]+/ }).forEach( function(u) { u.mystr = u.mystr.replace(/[ ]/g, "_");; } );

An existing patch promises to add a new update() modifier but until such a time as it exists, this approach can be used when you need to update strings in a collection, referencing the existing values in the transformation.
Defined tags for this entry: , , , , , ,

Mysql Update: Null + 1 is Null!

You can't add to Null

Here's something about mysql create table definitions that can easily catch you if you aren't careful. Consider this table definition:

mysql> CREATE TABLE screenshots (id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, votes INT);      
Query OK, 0 rows affected (0.09 sec)

What the user wanted was a simple table to keep track of user submitted screen shots. Of course the real table had a submission date, and name column but I've omitted those in order to focus on what can happen when you allow a numeric column to have NULL values.

In the application in question, when a user votes for the screen shot they like, the system should "count" the vote, by increasing the values in the "votes" column by one. Initially the developer working on this application was trying to read the value from the database, and in a PHP web script, they would increment this value and take the result and set "votes" to be equal to it in an UPDATE statement. I explained that this could cause lost votes, because if two or more users were voting at nearly the same time, each would overwrite the value of the "vote" column. In fact there are scenarios far worse than that --- a user with a cached page could vote and set the vote count back days or weeks. I didn't bother to mention the possibility that someone might recognize what was going on in the web form, and start tampering with it, since it was plainly evident that the form was passing the current number of votes.

One of the many benefits of using a relational database is built in concurrency. In an UPDATE statement, you can add to the value of the column without having to know what its original value is, just as computer languages allow assignment to a variable that references the variable's current value (ie. $a = $a + 1, $a++).

  1. UPDATE screenshots SET votes = votes + 1 WHERE id =  

All that's needed is to have the serverside language provide a value for a particular "id" and the votes will be tallied and updated correctly. Even more importantly, mysql will serialize the updates, insuring that no votes are lost.

However, given the original Mysql CREATE TABLE statement , what will happen if our code embeds the UPDATE statement provided? Continue reading "Mysql Update: Null + 1 is Null! "