For enhancing the data processing workflows, it would be desirable to have a better control on the queries we to on readings.
Limitations:

• Created at is not necessarily the date of the first reading (could be after for devices not logging data, or before, for SD-card devices, or flash devices)
• There is no way to query only a certain amount of data points (regardless of the frequency of data-logging) to limit both the load on kairos and the later load on the data storer

Proposal:

• add first_reading_at to the device model: date of the first reading available
• add count to the list of possible parameters on the /readings query

For the count bit, possible requests would be:

• https://api.smartcitizen.me/v0/devices/17777/readings/?from=datetime Returns all readings from datetime
• https://api.smartcitizen.me/v0/devices/17777/readings/?from=datetime&count=500 Returns 500 readings from datetime
• https://api.smartcitizen.me/v0/devices/17777/readings/?count=500 Returns 500 readings from first_reading_at
• https://api.smartcitizen.me/v0/devices/17777/readings/?from=datetime&count=500&to=datetime Error, only choose from "count" or "to"