MONGOID-5566 fix breaking app_spec.rb when run locally (#5550)

* app_spec fails if MONGODB_URI is not set; let's relax that a bit

* when possible, specify the Rails gem version when invoking 'rails new'

* don't insert a blank string into the argument list

Author: jamis

spec/integration/app_spec.rb

@@ -10,6 +10,15 @@ def check_call(cmd, **opts)
   Mrss::ChildProcessHelper.check_call(cmd, **opts)
 end
 
+def gem_version_argument(version)
+  "_#{version}_" if version
+end
+
+def insert_rails_gem_version(cmd)
+  gem_version = gem_version_argument(SpecConfig.instance.installed_rails_version)
+  cmd.tap { cmd[1,0] = gem_version if gem_version }
+end
+
 describe 'Mongoid application tests' do
   before(:all) do
     unless SpecConfig.instance.app_tests?
@@ -91,7 +100,7 @@ def prepare_new_rails_app(name)
 
     Dir.chdir(TMP_BASE) do
       FileUtils.rm_rf(name)
-      check_call(%W(rails new #{name} --skip-spring --skip-active-record), env: clean_env)
+      check_call(insert_rails_gem_version(%W(rails new #{name} --skip-spring --skip-active-record)), env: clean_env)
 
       Dir.chdir(name) do
         adjust_rails_defaults
@@ -265,7 +274,7 @@ def build_mongodb_uri(parts)
   def write_mongoid_yml
     # HACK: the driver does not provide a MongoDB URI parser and assembler,
     # and the Ruby standard library URI module doesn't handle multiple hosts.
-    parts = parse_mongodb_uri(SpecConfig.instance.safe_uri)
+    parts = parse_mongodb_uri(SpecConfig.instance.uri_str)
     parts[:database] = 'mongoid_test'
     uri = build_mongodb_uri(parts)
     p uri

spec/support/spec_config.rb

@@ -5,31 +5,26 @@
 class SpecConfig
   include Singleton
 
+  DEFAULT_MONGODB_URI = "mongodb://127.0.0.1:27017"
+
   def initialize
     if ENV['MONGODB_URI']
       @uri_str = ENV['MONGODB_URI']
-      @uri = Mongo::URI.new(@uri_str)
-    end
-  end
-
-  attr_reader :uri_str
-  attr_reader :uri
-
-  def addresses
-    if @uri
-      @uri.servers
     else
       STDERR.puts "Environment variable 'MONGODB_URI' is not set, so the default url will be used."
       STDERR.puts "This may lead to unexpected test failures because service discovery will raise unexpected warnings."
       STDERR.puts "Please consider providing the correct uri via MONGODB_URI environment variable."
-      ['127.0.0.1:27017']
+      @uri_str = DEFAULT_MONGODB_URI
     end
+    
+    @uri = Mongo::URI.new(@uri_str)
   end
 
-  # returns the URI string, or constructs one from the defaults if no URI
-  # string was given.
-  def safe_uri
-    @uri_str || "mongodb://#{addresses.first}"
+  attr_reader :uri_str
+  attr_reader :uri
+
+  def addresses
+    @uri.servers
   end
 
   def mri?
@@ -67,4 +62,19 @@ def rails_version
     end
     v || '6.1'
   end
+
+  # Scrapes the output of `gem list` to find which versions of Rails are
+  # installed, and looks for first one that best matches whatever rails version
+  # was requested (see `#rails_version`).
+  #
+  # @return [ String | nil ] the version of the requested Rails install, or
+  #    nil if nothing matches.
+  def installed_rails_version
+    output = `gem list --exact rails`
+    if output =~ /^rails \((.*)\)/
+      versions = $1.split(/,\s*/)
+      rails_version_re = /^#{rails_version}(?:\..*)?$/
+      versions.detect { |v| v =~ rails_version_re }
+    end
+  end
 end