Scheduling Filter on Database with Time Interval

Let's you have a science experiment called Weather and it records general i开发者_运维百科nformation about the weather, one of which is recording the humidity every minute. After say 10 hours, you'll have 600 database values stored.

Weather.objects.filter().exclude(time__lt = commence).exclude(time__gt = cease)

This would create a filter for the Weather table, between the commence and cease times.

Let's say the time criteria is equal to -4 hours from present, therefore it would return the last 4 hours worth of data which is equal to 300 values. How would I ask for an interval of 10? Meaning, from these 300 values, how would I receive the 10th one of each, so 10, 20, 30, 40 ... 280, 290, 300... which would mean 30 values are being returned from the filter.

EDIT:

Would it be possible to ask for an interval of say 10 seconds? As opposed to each individual 10th value..

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SQL doesn't have a way to ask for every tenth, so ORMs won't typically give you a way to express that need. You could do that work in Python:

samples = list(Weather.objects.filter().exclude(time__lt=commence).exclude(time__gt=cease))
for s10 in samples[::10]:
    # do something with every tenth...

This will pull all the samples from the database in one query, then only use 10% of them. You could also pull only the ones you need, one to a query:

sample_count = 600 # Somehow determine how many samples are in your time window.
samples = Weather.objects.filter().exclude(time__lt=commence).exclude(time__gt=cease)
for s_num in range(0, sample_count, 10):
    s = samples[s_num]
    # Now you have one sample in s.

To elaborate: range takes three arguments, start, stop, and step. For example:

>>> range(0, 100, 10)
[0, 10, 20, 30, 40, 50, 60, 70, 80, 90]

In the second example, the range expression is used to create the index values of the samples we want. Then samples[s_num] evaluates the QuerySet to get just that one value from the database.