ERROR: extension “btree_gist” must be installed in schema “heroku_ext”

I have developed a monkey-patch solution that is, well, definitely a hack, but better than pre-dating migrations, or deleting lines from schema.rb.

Put this in config/initializers. I called mine _enable_extension_hack.rb to make sure it gets loaded early.

module EnableExtensionHerokuMonkeypatch
  # Earl was here
  def self.apply_patch!
    adapter_const = begin
      Kernel.const_get('ActiveRecord::ConnectionAdapters::PostgreSQLAdapter')
    rescue NameError => ne
      puts "#{self.name} -- #{ne}"
    end

    if adapter_const
      patched_method = adapter_const.instance_method(:enable_extension)

      # Only patch this method if it's method signature matches what we're expecting
      if 1 == patched_method&.arity
        adapter_const.prepend InstanceMethods
      end
    end
  end

  module InstanceMethods
    def enable_extension(name)
      name_override = name

      if schema_exists?('heroku_ext')
        puts "enable_extension -- Adding SCHEMA heroku_ext"
        name_override = "#{name}\" SCHEMA heroku_ext -- Ignore trailing double quote"
      end

      super name_override
    end
  end
end

EnableExtensionHerokuMonkeypatch.apply_patch!

What does it do? It monkey-patches the enable_extension call that’s causing the problem in db/schema.rb. Specifically, if it finds that there is a schema called "heroku_ext", it will decorate the parameter given to enable_extension so that the SQL that gets executed specifies the "heroku_ext" schema. Like this:

CREATE EXTENSION IF NOT EXISTS "pg_stat_statements" SCHEMA heroku_ext -- Ignore trailing double quote"

Without this hack, the generated SQL looks like this:

CREATE EXTENSION IF NOT EXISTS "pg_stat_statements"