Ruby On Rails Retrieve Mysql Database Data 'a' Distinct 'b'

My database have a device reading list, columns are id ,device_id ,device_reading ,update_time How can I select all latest reading for each device? that means,I need sort data acco

Solution 1:

Supposing that model name is DeviceReading

DeviceReading.distinct(:device_id).order('update_time DESC').pluck(:device_reading)

You can use ASC in place of DESC if you want to sort by ascending order

Source: Distinct and pluck sort/order

EDIT: The distinct only works in rails 4 so if you are using rails 3 you should do something like this:

DeviceReading.select(:device_id).uniq.pluck(:device_reading)

However when I chain .order('update_time DESC') in above I get mysterious error. I am trying to find solution. meanwhile I Hope this helps you.

Source: distinct undefined error

Solution 2:

This questions looks quite similar to the one below:

Select first row in each GROUP BY group?

In the case when no elegant solution can be found the Rails way, you can first figure out the correct sql and use find_by_sql to retrieve the dataset.