creating arrays of results based upon chunks of time - postgresql

Question

I've got an app that has lots of sensor_events being saved; I'd like to get results by a date then map those into chunks of 15 minute times... this is not the same as doing a group by in postgres as that would only return something averaged and I need the specific events...
What I'm thinking is given a day I get the beginning_of_day and split it up as 15 minute chunks as keys to a hash of arrays ie
def return_time_chunk_hash
t=Date.today
st = t.beginning_of_day
times = Hash.new
while st<t.end_of_day
times[st.to_formatted_s(:time)] = Array.new
st = st + 15.minutes
end
return times
end
And from that I would compare the sensor_events created_at date, find which bucket it belonged to and plop it in there. Once I've got it that way I know whether or not a chunk has any (.count) and if so can do all the data manipulation on the specific events.
Does this seem nutty? Is there a simpler way I'm not seeing?
Update:
I liked the way jgraft was thinking but thought it wouldn't work as I'd have to do multiple queries based upon the group column flag, but then I thought of the group_by of Enumerable so I tried something like this in the actual SensorEvent model:
def chunk
Time.at((self.created_at.to_f / 15.minutes).floor * 15.minutes).to_formatted_s(:time)
end
This allows me to get all the sensor events I need as usual (ie #se=SensorEvent.where(sensor_id: 10)) but then I could do #se.group_by(&:chunk) and I get those singular events grouped into a hash ie:
{"13:30"=>
[#<SensorEvent:0x007ffac0128438
id: 25006,
force: 0.0,
xaccel: 502.0,
yaccel: 495.0,
zaccel: 616.0,
battery: 0.0,
position: 25.0,
created_at: Thu, 18 Jun 2015 13:33:37 EDT -04:00,
updated_at: Thu, 18 Jun 2015 15:51:32 EDT -04:00,
deviceID: "D36330135FE3D36",
location: 3,
sensor_id: 10>,
#<SensorEvent:0x007ffac0128140
id: 25007,
force: 0.0,
xaccel: 502.0,
yaccel: 495.0,
zaccel: 616.0,
battery: 0.0,
position: 27.0,
created_at: Thu, 18 Jun 2015 13:39:46 EDT -04:00,
updated_at: Thu, 18 Jun 2015 15:51:32 EDT -04:00,
deviceID: "D36330135FE3D36",
location: 3,
sensor_id: 10>,
.........
The trouble is of course not every chunk of time might be created since there was no event to spawn it; also that being a hash it's not sorted in anyway:
res.keys
=> ["13:30",
"13:45",
"14:00",
"13:00",
"15:45",
"16:00",
"16:15",
"16:45",
"17:00",
"17:15",
"17:30",
"14:15",
"14:30",
I have to do calculations on the chunks of events; I might keep a master TIMECHUNKS array to compare / lookup in order...

Answer 1

Why not just add a column to the sensor_events table that specifies which block it belongs to? Would basically give you an array as you could do a query like:
SensorEvent.where(date: Date.today, block: 1)
and return a relation of the data in an array-esque format.
You could just add an after_create callback to the SensorEvents model that sets the block column.
class SensorEvent < ActiveRecord::Base
after_create :set_block
private
def set_block
value = ((4 * created_at.hour) + (created_at.min.to_f / 15).ceil).to_i
self.update_columns(block: value)
end