[Rails] Saving values to the db as metric but displaying/editing in imperial.

I have a requirement to store values in a database as unitless decimals(12,9) which will be presumed to be metric values. For display/editing purposes I need the user to be able to choose their input method -- no mixing and matching on a form, just a simple User#current_units call to determine the expected units.

For example.

item:

  name: item1

  diameter: 150

  width: 6

When User#current_units is "mm" all of my form fields are straight forward... 

f.label "Diameter"

f.text_field :diameter

My issue is when a current_units is "in", what's a good method for doing all the translation? The text_field for diameter should read "5.9055". If a user altered it to read "6" then it should save back to the db as "152.400000000"

I've tried everything from the ruby-units gem to composed_of (which is practically useless with rails 4 getting rid of it). storing the units in the db is a no go only because I need to be able to search for "items where diameter > x and diameter < y"

If anyone has any theories on a good starting point I'd love to elaborate further on what I've tried and failed with.

Thanks,

- FJM

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