How to deal with "partial" dates (2010-00-00) from MySQL in Django?

First, thanks for all your answers. None of them, as is, was a good solution for my problem, but, for your defense, I should add that I didn't give all the requirements. But each one help me think about my problem and some of your ideas are part of my final solution.

So my final solution, on the DB side, is to use a varchar field (limited to 10 chars) and storing the date in it, as a string, in the ISO format (YYYY-MM-DD) with 00 for month and day when there's no month and/or day (like a date field in MySQL). This way, this field can work with any databases, the data can be read, understand and edited directly and easily by a human using a simple client (like mysql client, phpmyadmin, etc.). That was a requirement. It can also be exported to Excel/CSV without any conversion, etc. The disadvantage is that the format is not enforce (except in Django). Someone could write 'not a date' or do a mistake in the format and the DB will accept it (if you have an idea about this problem...).

This way it's also possible to do all of the special queries of a date field relatively easily. For queries with WHERE: <, >, <=, >= and = work directly. The IN and BETWEEN queries work directly also. For querying by day or month you just have to do it with EXTRACT (DAY|MONTH ...). Ordering work also directly. So I think it covers all the query needs and with mostly no complication.

On the Django side, I did 2 things. First, I have created a PartialDate object that look mostly like datetime.date but supporting date without month and/or day. Inside this object I use a datetime.datetime object to keep the date. I'm using the hours and minutes as flag that tell if the month and day are valid when they are set to 1. It's the same idea that steveha propose but with a different implementation (and only on the client side). Using a datetime.datetime object gives me a lot of nice features for working with dates (validation, comparaison, etc.).

Secondly, I have created a PartialDateField that mostly deal with the conversion between the PartialDate object and the database.

So far, it works pretty well (I have mostly finish my extensive unit tests).

You could store the partial date as an integer (preferably in a field named for the portion of the date you are storing, such as year, month or day) and do validation and conversion to a date object in the model.

EDIT

If you need real date functionality, you probably need real, not partial, dates. For instance, does "get everything after 2010-0-0" return dates inclusive of 2010 or only dates in 2011 and beyond? The same goes for your other example of May 2010. The ways in which different languages/clients deal with partial dates (if they support them at all) are likely to be highly idiosyncratic, and they are unlikely to match MySQL's implementation.

On the other hand, if you store a year integer such as 2010, it is easy to ask the database for "all records with year > 2010" and understand exactly what the result should be, from any client, on any platform. You can even combine this approach for more complicated dates/queries, such as "all records with year > 2010 AND month > 5".

SECOND EDIT

Your only other (and perhaps best) option is to store truly valid dates and come up with a convention in your application for what they mean. A DATETIME field named like date_month could have a value of 2010-05-01, but you would treat that as representing all dates in May, 2010. You would need to accommodate this when programming. If you had date_month in Python as a datetime object, you would need to call a function like date_month.end_of_month() to query dates following that month. (That is pseudocode, but could be easily implemented with something like the calendar module.)

It sounds like you want to store a date interval. In Python this would (to my still-somewhat-noob understanding) most readily be implemented by storing two datetime.datetime objects, one specifying the start of the date range and the other specifying the end. In a manner similar to that used to specify list slices, the endpoint would not itself be included in the date range.

For example, this code would implement a date range as a named tuple:

>>> from datetime import datetime>>> from collections import namedtuple>>> DateRange = namedtuple('DateRange', 'start end')>>> the_year_2010 = DateRange(datetime(2010, 1, 1), datetime(2011, 1, 1))>>> the_year_2010.start <= datetime(2010, 4, 20) < the_year_2010.endTrue>>> the_year_2010.start <= datetime(2009, 12, 31) < the_year_2010.endFalse>>> the_year_2010.start <= datetime(2011, 1, 1) < the_year_2010.endFalse

Or even add some magic:

>>> DateRange.__contains__ = lambda self, x: self.start <= x < self.end>>> datetime(2010, 4, 20) in the_year_2010True>>> datetime(2011, 4, 20) in the_year_2010False

This is such a useful concept that I'm pretty sure that somebody has already made an implementation available. For example, a quick glance suggests that the relativedate class from the dateutil package will do this, and more expressively, by allowing a 'years' keyword argument to be passed to the constructor.

However, mapping such an object into database fields is somewhat more complicated, so you might be better off implementing it simply by just pulling both fields separately and then combining them. I guess this depends on the DB framework; I'm not very familiar with that aspect of Python yet.

In any case, I think the key is to think of a "partial date" as a range rather than as a simple value.

edit

It's tempting, but I think inappropriate, to add more magic methods that will handle uses of the > and < operators. There's a bit of ambiguity there: does a date that's "greater than" a given range occur after the range's end, or after its beginning? It initially seems appropriate to use <= to indicate that the date on the right-hand side of the equation is after the start of the range, and < to indicate that it's after the end.

However, this implies equality between the range and a date within the range, which is incorrect, since it implies that the month of May, 2010 is equal to the year 2010, because May the 4th, 2010 equates to the both of them. IE you would end up with falsisms like 2010-04-20 == 2010 == 2010-05-04 being true.

So probably it would be better to implement a method like isafterstart to explicitly check if a date is after the beginning of the range. But again, somebody's probably already done it, so it's probably worth a look on pypi to see what's considered production-ready. This is indicated by the presence of "Development Status :: 5 - Production/Stable" in the "Categories" section of a given module's pypi page. Note that not all modules have been given a development status.

Or you could just keep it simple, and using the basic namedtuple implementation, explicitly check

>>> datetime(2012, 12, 21) >= the_year_2010.startTrue