Looks like ibm_db isn’t maintained for a long time (2 years!)… And also they require you to have the old interpreter Ruby 2.2.6 which is way different than what we have today (Ruby 2.7).
There is an opened issue regarding the crash while using Ruby 2.4.4+
I have no idea about IBM_DB so I can suggest you to do one of the 3 things (they might not work for sure):
Installing Ruby 2.2.6 and Rails 5.0.7 using RVM.
Using any other alternatives to IBM_DB.
Report a bug on Github and wait until they fix the issue.
I am trying to fix it. I have fixed many things and now it works for ruby 2.6.5 and rails 6.01 but the only thing is when I use server info api it crashes.
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