Another glitch, and the solution.
On attempting to push the HW3 baseline to Heroku and creating the database there, Heroku complained that a gem related to postgresql was not installed.
It turned out the line in the gemfile that specified that a “pg” gem should be used in production (i.e. on Heroku) had been commented out.
After uncommenting it, it was also necessary to do:
bundle install
Perhaps I should have done
bundle install --without production
Not sure about that, as I’m pretty unclear how all these things work.
Notes to Web 