Skip to main content

Revisiting Retrieving Documents Between Two Dates From CouchDB

In a previous post I outlined how I was retrieving documents from CouchDB with a start date property less than the current date, and and end date property greater than the current date. To summarize, in my CouchDB view I created some date/time strings in JavaScript and only emitted documents in the view that met the date criteria.

My previous post got referenced in the CouchBase newsletter, and I'm really glad it did because while I came up with what I thought was a clever solution it was also wrong. (D'OH!)

The issue I didn't consider that some kind commenters on the previous post pointed out is that my approach creates side effects because I'm emitting documents in the view based on information that isn't in the document itself. Specifically since I'm using the current system date/time when the view is created, the documents included in the view will be ones for which the criteria is valid when the view is created.

What this means is that although views get updated with current data as data within documents changes, since the entire view isn't generated each time the criteria used to determine whether or not documents are included in the view is a fixed point in time. To put it another way, my current system date/time that was current when the view was first created essentially becomes hard-coded once the view is created, which isn't at all what I needed. This causes issues if the start and end date properties in the documents change after they've been added to the view because the view only checked to see if the date criteria was met at the time the document was added to the view.

There are some great suggestions in the comments on my previous post for including data in the document itself that would allow only valid documents to be pulled right from Couch, and you'll certainly want to check those out if you're dealing with a ton of data. The solution I'm using will not be ideal for massive datasets but since that isn't the situation I'm in with this data, I wanted to share the solution I came up with in case this works for other people.

To describe my documents again, I have documents that need to be displayed on a web page if their start date/time property is less than the current date/time and if their end date/time property is greater than the current date/time.

Since the valid ranges go in opposite directions for those fields, I didn't see a way to do something like have an array key that included both the start and end dates that would allow me to get only the documents I want back from Couch. But what I can do is use a single document property as a key in Couch and get close to what I want, and then I can pare the documents down further in the application code.

In my case the end date is a more strong limiting criteria since over time there will be a large number of documents with both start and end dates in the past, but documents with end dates >= the current date will be much fewer in number (only a handful in the case of this specific data).

The first step to fix my issue was to rewrite my view to eliminate the date/time check in JavaScript since that's the cause of the unwanted side effect, and emit documents using the end date/time property as the key. I have some other criteria as well (checking type and a couple of other fields to pull valid documents for this particular display), but the basic view is now very simple:

function(doc) {
  emit(doc.dtEnd, doc);

With the end date/time as the key, on the application side I can simply use the current date/time as my start key when I call this view, and that gives me all documents with a valid end date/time (>= current date/time).

At this point I may still have documents that shouldn't be displayed based on the start date/time, however, since when people enter data into this application they can schedule things for future display (i.e. both start and end date/time are in the future). But, again since I'm not dealing with a huge amount of data once I limit by the end date/time, it's simply a matter of looping over the documents I get back from Couch and checking for a valid start date/time (<= current date/time) and only displaying those documents.

The issue my original view code created makes total sense now, so thanks to the commenters on my previous post who pointed out the fatal flaw in my approach. Nothing like doing something wrong as a means of learning.


Popular posts from this blog

Running a Django Application on Windows Server 2012 with IIS

This is a first for me since under normal circumstances we run all our Django applications on Linux with Nginx, but we're in the process of developing an application for another department and due to the requirements around this project, we'll be handing the code off to them to deploy. They don't have any experience with Linux or web servers other than IIS, so I recently took up the challenge of figuring out how to run Django applications on Windows Server 2012 with IIS.

Based on the dated or complete lack of information around this I'm assuming it's not something that's very common in the wild, so I thought I'd share what I came up with in case others need to do this.

This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Assumptions and CaveatsThe operating system is Windows Server 2012 R2, 64-bit. If another variant of the operating system is being used, these instructions may not work properly.All of the soft…

Installing and Configuring NextPVR as a Replacement for Windows Media Center

If you follow me on Google+ you'll know I had a recent rant about Windows Media Center, which after running fine for about a year suddenly decided as of January 29 it was done downloading the program guide and by extension was therefore done recording any TV shows.

I'll spare you more ranting and simply say that none of the suggestions I got (which I appreciate!) worked, and rather than spending more time figuring out why, I decided to try something different.

NextPVR is an awesome free (as in beer, not as in freedom unfortunately ...) PVR application for Windows that with a little bit of tweaking handily replaced Windows Media Center. It can even download guide data, which is apparently something WMC no longer feels like doing.

Background I wound up going down this road in a rather circuitous way. My initial goal for the weekend project was to get Raspbmc running on one of my Raspberry Pis. The latest version of XBMC has PVR functionality so I was anxious to try that out as a …

Fixing DPI Scaling Issues in Skype for Business on Windows 10

My setup for my day job these days is a Surface Pro 4 and either an LG 34UC87M-B or a Dell P2715Q monitor, depending on where I'm working. This is a fantastic setup, but some applications have trouble dealing with the high pixel density and don't scale appropriately.
One case in point is Skype for Business. For some reason it scales correctly as I move between the Surface screen and the external monitor when I use the Dell, but on the LG monitor Skype is either massive on the external monitor, or tiny on the Surface screen.
After a big of digging around I came across a solution that worked for me, which is to change a setting in Skype's manifest file (who knew there was one?). On my machine the file is here: C:\Program Files\Microsoft Office\Office16\LYNC.EXE.MANIFEST
And the setting in question is this:
Which I changed to this: <dpiAware>False/PM</dpiAware>
Note that you'll probably have to edit the file as administr…