Actual Question
How can i deal with different minimum date in Django DateField and Mysql DATE type. What can be done to deal with the issue?
TL:DR
datetime.date.min has lower value than minimum supported in Mysql for DATE type. Saving datetime.date.min to Mysql is possible but it is not guaranteed to work.
Context
I'm working with Django (v1.11) model:
class MyModel(model.Model):
begin_date = models.DateField(null=True, blank=True)
end_date = models.DateField(null=True, blank=True)
Currently the fields with NULL value designate the lower and higher date range limits respectively.
Example:
Date range from beginning of time to today will be:
mymodel.begin_date = None
mymodel.end_date = datetime.date(2019, 11, 20)
mymodel.save()
The usage of None/NULL creates a need to convert it to datetime.date.min/datetime.date.max on any sorting operations etc. Simply put: its inconvenient and adds unnecessary complexity.
My though was to do necessary migrations and start putting datetime.date.min/datetime.date.max instead of None/`NULL'
But there is a problem:
Mysql has different min/max date range
According to the mysql docs:
The DATE type is used for values with a date part but no time part.
MySQL retrieves and displays DATE values in 'YYYY-MM-DD' format. The
supported range is '1000-01-01' to '9999-12-31'.
According to python docs:
The smallest year number allowed in a date or datetime object. MINYEAR is 1.
The largest year number allowed in a date or datetime object. MAXYEAR is 9999.
The earliest representable date, date(MINYEAR, 1, 1).
The latest representable date, date(MAXYEAR, 12, 31).
It turns out that in Django you can still use datetime.date.min and it will be saved to the database. But it is not guranteed to work.
Because of:
For the DATE and DATETIME range descriptions, “supported” means that although earlier values might work, there is no guarantee.
My idea
I'm thinking about doing all necessary migrations, clean the code from None convertions and just use Field.default. Set it to default=datetime(1000, 1, 1) (minimum mysql supported date)
I think your idea will work fine.
Note that it's database-specific, though. The more ungainly approach of using None does have the virtue of working across databases.
Also note that apparently MySQL let's you store dates below that ostensible minimum value. Therefore, there could be dates in the database that will not be caught by your filter. So be sure to validate dates before you store them to make sure they are at or above the minimum.
class Test(Model):
time = DateTimeField()
# ...
row = Test.select()[0]
test.time
This returns a string that looks like this: 2017-01-23 01:01:39+01:00. How can I get it as a datetime object instead? Do I have to parse it manually?
Also I would be interested if there is any documentation on how to use the DateTimeField. The official documentation doesn't have anything on it.
Are you using SQLite? If so, SQLite doesn't have a dedicated datetime type, so datetimes are stored as strings in the DB. What peewee will do is recognize certain datetime formats coming out of the DB and convert them to datetime objects. What you need to do is ensure that either:
When you create/save your object, that you assign a datetime object to the field.
When reading back pre-existing data, that the data is in a recognized format.
The formats peewee supports out-of-the-box for datetime field are:
YYYY-mm-dd HH:MM:SS.ffffff
YYYY-mm-dd HH:MM:SS
YYYY-mm-dd
It looks like your has zone info. I'd suggest converting to UTC and dropping the zone info. That should fix it.
Have you tried adding a default like this?
time = DateTimeField(default=datetime.datetime.now())
Or when adding an entry add it as a datetime.datetime object directly:
test = Test(....., time=datetime.datetime.strptime("2018-3-15", '%Y-%m-%d'))
In the second case you don't need to specify anything in the class definition...
I want to save date in my django model into desired format.
post_date = str(request.POST['date'])
I am getting date from front end is 12/21/2016
i want to save this date in my django modal is like 21-Dec-2016.
How can i do it.
it will be good if you can provide some sample code.
Thanks in advance.
you want to store in DB string parameter? The easiest way convert input string to date and then convert from date to string. But of course it is more logical to store date format imho
I am writing/learning my first Django REST API. The database I am working on has all times are stored as an integers representing the seconds since Jan 01 1970 - aka Unix/Epoch time.
Right now I simply have two function that convert between this integer and a normal timestamp. So user input through parameters is converted to integers, and JSON responses are converted to timestamps.
I was wondering if there was a way to simply have Django "know" this conversion. So in my model:
class User(models.Model):
createDate = models.IntegerField(blank=True, null=True, db_column='createdate')
I could change to models.DateTimeField and createDate would be a time stamp. Correspondingly if I call:
data = Subgroup.objects.filter(createDate_gt='2016-30-04')
In my view Django knows to convert this to an integer and filter by that, rather than the string interpretation.
I know it's a pretty pedantic request but I've been impressed with Django so far and was curious :)
In one of my Django projects that use MySQL as the database, I need to have a date fields that accept also "partial" dates like only year (YYYY) and year and month (YYYY-MM) plus normal date (YYYY-MM-DD).
The date field in MySQL can deal with that by accepting 00 for the month and the day. So 2010-00-00 is valid in MySQL and it represent 2010. Same thing for 2010-05-00 that represent May 2010.
So I started to create a PartialDateField to support this feature. But I hit a wall because, by default, and Django use the default, MySQLdb, the python driver to MySQL, return a datetime.date object for a date field AND datetime.date() support only real date. So it's possible to modify the converter for the date field used by MySQLdb and return only a string in this format 'YYYY-MM-DD'. Unfortunately the converter use by MySQLdb is set at the connection level so it's use for all MySQL date fields. But Django DateField rely on the fact that the database return a datetime.date object, so if I change the converter to return a string, Django is not happy at all.
Someone have an idea or advice to solve this problem? How to create a PartialDateField in Django ?
EDIT
Also I should add that I already thought of 2 solutions, create 3 integer fields for year, month and day (as mention by Alison R.) or use a varchar field to keep date as string in this format YYYY-MM-DD.
But in both solutions, if I'm not wrong, I will loose the special properties of a date field like doing query of this kind on them: Get all entries after this date. I can probably re-implement this functionality on the client side but that will not be a valid solution in my case because the database can be query from other systems (mysql client, MS Access, etc.)
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.end
True
>>> the_year_2010.start <= datetime(2009, 12, 31) < the_year_2010.end
False
>>> the_year_2010.start <= datetime(2011, 1, 1) < the_year_2010.end
False
Or even add some magic:
>>> DateRange.__contains__ = lambda self, x: self.start <= x < self.end
>>> datetime(2010, 4, 20) in the_year_2010
True
>>> datetime(2011, 4, 20) in the_year_2010
False
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.start
True
Can you store the date together with a flag that tells how much of the date is valid?
Something like this:
YEAR_VALID = 0x04
MONTH_VALID = 0x02
DAY_VALID = 0x01
Y_VALID = YEAR_VALID
YM_VALID = YEAR_VALID | MONTH_VALID
YMD_VALID = YEAR_VALID | MONTH_VALID | DAY_VALID
Then, if you have a date like 2010-00-00, convert that to 2010-01-01 and set the flag to Y_VALID. If you have a date like 2010-06-00, convert that to 2010-06-01 and set the flag to YM_VALID.
So, then, PartialDateField would be a class that bundles together a date and the date-valid flag described above.
P.S. You don't actually need to use the flags the way I showed it; that's the old C programmer in me coming to the surface. You could use Y_VALID, YM_VALID, YMD_VALID = range(3) and it would work about as well. The key is to have some kind of flag that tells you how much of the date to trust.
Although not in Python - here's an example of how the same problem was solved in Ruby - using a single Integer value - and bitwise operators to store year, month and day - with month and day optional.
https://github.com/58bits/partial-date
Look at the source in lib for date.rb and bits.rb.
I'm sure a similar solution could be written in Python.
To persist the date (sortable) you just save the Integer to the database.