Caching keys lack sub second precision

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• Problem/bug report.
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Description

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Bug reports:

The mechanism for building a cache key by default uses the rails updated_at field, which when stringified truncates the millisecond precision, which means that if two requests arrive and mutate a record in less than a second, the first write will win - which is the exact opposite of what you would want.

This is the monkey patched fix we've deployed

    keys = cache_ids.map do |(id, cache_key)|
      # If the cache key is a time object make it a high precision time object
      if cache_key.is_a? Time
        cache_key = cache_key.iso8601(4)
      end
      [type, id, cache_key, serializer_config_key, context_key]
    end

Start by locating the default cache-key builder and the handling of Rails updated_at values. Reproduce two record mutations less than a second apart, then verify that the resulting cache keys preserve sub-second precision and that the regression is covered by a test.