MONGOID-5547 create index needs to load models (#5526)

* make the load_models task actually load models

required decoupling the model loading code from Rails, which required
some creative rearchitecting of some brittle tests.

* fix other tests that relied on juggling the Rails constant

* tidy up the FeatureSandbox

* fix typo

* parenthesize arguments to `defined?`

* extract array to constant for better documentation

* update docs

* don't try to parse uri string if it hasn't been set

Author: jamis

source/reference/indexes.txt

@@ -143,13 +143,42 @@ in Rails console:
     # Remove indexes for Model
     Model.remove_indexes
 
+Telling Mongoid Where to Look For Models
+----------------------------------------
+
+For non-Rails applications, Mongoid's rake tasks will look for models in
+``./app/models`` and ``./lib/models``. For Rails, Mongoid will look in
+``./app/models`` (or wherever you've configured Rails to look for models). If
+your models are in another location, you will need to tell Mongoid where to
+look for them with ``Mongoid.model_paths=``. You can do this by setting it
+in your application's Rakefile:
+
+.. code-block:: ruby
+
+    # Rakefile
+
+    # keep the defaults, but add more paths to look for models
+    Mongoid.model_paths += [ "./src/models", "./lib/documents" ]
+
+    # or, override the defaults entirely
+    Mongoid.model_paths = [ "./src/models", "./lib/documents" ]
+
+Make sure that these paths are in your application's load path, as well. For
+example:
+
+.. code-block:: ruby
+
+    # Rakefile
+
+    $LOAD_PATHS.concat [ "./src/models", "./lib/documents" ]
+
+
 Using Rake Tasks With Non-Rails Applications
 --------------------------------------------
 
 Mongoid's Rake tasks are automatically loaded in Rails applications using
 Mongoid. When using Mongoid with a non-Rails application, these tasks must
-be loaded manually. This can be achieved by loading them in the Rakefile and
-providing an ``:environment`` task to load your application's models:
+be loaded manually:
 
 .. code-block:: ruby
 
@@ -158,10 +187,6 @@ providing an ``:environment`` task to load your application's models:
     require 'mongoid'
     load 'mongoid/tasks/database.rake'
 
-    task :environment do
-      # Require/load your application's models here
-    end
-
 If your application uses Bundler, you can require ``bundler/setup`` instead of
 explicitly requiring ``mongoid``:
 
@@ -171,7 +196,3 @@ explicitly requiring ``mongoid``:
 
     require 'bundler/setup'
     load 'mongoid/tasks/database.rake'
-
-    task :environment do
-      # Require/load your application's models here
-    end