From an answer I wrote for StackOverflow:

Another possible cause:

In Rails 3.2.x, establish_connection has a default argument set from the environment:

From connection_specification.rb:

def self.establish_connection(spec = ENV["DATABASE_URL"])
  resolver = spec, configurations
  spec = resolver.spec

The way ConnectionSpecification::Resolver works depends on ENV['DATABASE_URL'] giving a nil if not set. (Normally, it would be something like postgres://...).

So, if you happen to have misconfigured DATABASE_URL such that ENV['DATABASE_URL'] == '', that will give you database configuration does not specify adapter.