An idea to combat model pollution

Problem
-------

`has_paper_trail` adds too many methods to the ActiveRecord model.

> If I'm counting correctly, installing the paper_trail gem adds 36 instance
> methods and 10 class methods to all of your active record models. Of those
> 46, 13 have a prefix, either "pt_" or "paper_trail_". I don't know what the
> best way to deal with this is. Ideally, we'd add far fewer methods to
> people's models. If we were able to add fewer methods to models, then I
> wouldn't mind prefixing all of them.
> #703

Solution
--------

Add only two methods to the AR model.

1. An instance method `#paper_trail`
2. A class method `.paper_trail`

The instance method would return a `RecordTrail` and the class method would
return a `ClassTrail`. Those names are totally up for debate.

Advantages
----------

- Plain ruby, easy to understand
- Adding new methods to e.g. the `RecordTrail` does not add any methods to
  the AR model.
- Methods privacy is more strongly enforced.
- Enables isolated testing of e.g. `RecordTrail`; it can be constructed with a
  mock AR instance.

Disadvantages
-------------

- Two new classes, though they are simple.

.rubocop.yml

@@ -17,10 +17,6 @@ Metrics/AbcSize:
   Exclude:
     - 'test/dummy/db/migrate/*'
 
-Metrics/ClassLength:
-  Exclude:
-    - test/**/*
-
 # The Ruby Style Guide recommends to "Limit lines to 80 characters."
 # (/~https://github.com/bbatsov/ruby-style-guide#80-character-limits)
 # but 100 is also reasonable.

.rubocop_todo.yml

@@ -4,11 +4,16 @@
 Metrics/AbcSize:
   Max: 30 # Goal: 15
 
+Metrics/ClassLength:
+  Max: 151
+  Exclude:
+    - test/**/*
+
 Metrics/CyclomaticComplexity:
   Max: 13 # Goal: 6
 
 Metrics/ModuleLength:
-  Max: 313
+  Max: 317
 
 Metrics/PerceivedComplexity:
   Max: 16 # Goal: 7

lib/paper_trail/attribute_serializers/object_attribute.rb

@@ -32,7 +32,7 @@ def alter(attributes, serialization_method)
       end
 
       def object_col_is_json?
-        @model_class.paper_trail_version_class.object_col_is_json?
+        @model_class.paper_trail.version_class.object_col_is_json?
       end
     end
   end

lib/paper_trail/attribute_serializers/object_changes_attribute.rb

@@ -35,7 +35,7 @@ def alter(changes, serialization_method)
       end
 
       def object_changes_col_is_json?
-        @item_class.paper_trail_version_class.object_changes_col_is_json?
+        @item_class.paper_trail.version_class.object_changes_col_is_json?
       end
     end
   end