Rails application using Postgres adapter can’t activate pg

The pg gem recently released version 1.0.0 which is not yet compatible with Rails.

If you’re on Rails 5, change your Gemfile’s pg requirement to the following¹:

gem 'pg', '~> 0.18'

or on Rails < 5, this:

gem 'pg', '~> 0.11'

And then run

bundle update pg

¹ Bundler will effectively do the exact same thing with either line, but this way you’ll match the Rails source code’s runtime check exactly, as well as the version emitted by rails new‘s Gemfile generator.