Question

This page shows a update reaching into a previously retrieved (find) document and querying a subelement (array) to update it. I pretty much need to do the exact same thing. Code for the example:

> t.find()
{ "_id" : ObjectId("4b97e62bf1d8c7152c9ccb74"), "title" : "ABC",
  "comments" : [ { "by" : "joe", "votes" : 3 }, { "by" : "jane", "votes" : 7 } ] }

> t.update( {'comments.by':'joe'}, {$inc:{'comments.$.votes':1}}, false, true )

What are the rules governing find-followed-by-update, I haven’t noticed an explanation for this in the documentation. Does the same stuff apply to use of mongodb via drivers ? A link to the relevant semantics would be helpful. I am using the C++ driver.

edit: self answer

the 2 commands can be rolled into one (and this is one way of removing the ambiguity this question raises), the query part of an update can refer to a array sub-element, and the $ symbol will reference to it. I assume you can only reference one sub-element in the query part of an update operation. In my case the update operation looks as follows :

db.qrs.update ( { "_id" : ObjectId("4f1fa126adf93ab96cb6e848"), "urls.u_id" : 171 }, { "$inc" :  { "urls.$.CC": 1} })

The _id correctly “primes” the right unique row, and the second query element "urls.u_id" : 171 assures that the row in question has the right field. urls.$.CC then routes the $inc operation to the correct array entry.

recomendation to any mongodb dev or document writer

Do not show examples which have potential race conditions in them. Always avoid showing multiple operations that can be done atomically.

Answer 1

The rules are relatively straightforward. The results of the update may or may not be available to any subsequent reads depending on a number of things (slaveOk true/false in combination with repsets, update and find using different connections, write safety). You can guarantee it to be available if you do a safe write (w >= 1) and perform the find on the same connection. Most drivers offer functionality for this (typically “requestStart” and “requestDone”).

All that said, there’s a much better solution available to you for this, namely findAndModify. This operation finds a document, updates it and returns either the old version of the document or the newly updated version. This command is available in the C++ driver. For a reference look here : http://www.mongodb.org/display/DOCS/findAndModify+Command

EDIT : Please note that the “find” in the example is only there to show the reader of the documentation what the structure/schema of the documents inside the collection is to place the subsequent “update” in context. The “update” operation is in no way affected by the “find” before it.