Raise helpful error when CVC is abstract

Instead of crashing when misconfigured Custom Version Classes are
used, an error will be raised earlier, with a much more helpful message.

[Fixes #961]

CHANGELOG.md

@@ -26,7 +26,9 @@ recommendations of [keepachangelog.com](http://keepachangelog.com/).
 
 ### Added
 
-- None
+- [#961](https://github.com/airblade/paper_trail/issues/961) - Instead of
+  crashing when misconfigured Custom Version Classes are used, an error will be
+  raised earlier, with a much more helpful message.
 
 ### Fixed
 

lib/paper_trail/model_config.rb

@@ -12,6 +12,16 @@ module PaperTrail
       or disable belongs_to_required_by_default.
     STR
 
+    E_HPT_ABSTRACT_CLASS = <<~STR.squish.freeze
+      An application model (%s) has been configured to use PaperTrail (via
+      `has_paper_trail`), but the version model it has been told to use (%s) is
+      an `abstract_class`. This could happen when an advanced feature called
+      Custom Version Classes (http://bit.ly/2G4ch0G) is misconfigured. When all
+      version classes are custom, PaperTrail::Version is configured to be an
+      `abstract_class`. This is fine, but all application models must be
+      configured to use concrete (not abstract) version models.
+    STR
+
     def initialize(model_class)
       @model_class = model_class
     end
@@ -98,6 +108,14 @@ module PaperTrail
       Gem::Version.new(ActiveRecord::VERSION::STRING)
     end
 
+    # Raises an error if the provided class is an `abstract_class`.
+    # @api private
+    def assert_concrete_activerecord_class(class_name)
+      if class_name.constantize.abstract_class?
+        raise format(E_HPT_ABSTRACT_CLASS, @model_class, class_name)
+      end
+    end
+
     def cannot_record_after_destroy?
       Gem::Version.new(ActiveRecord::VERSION::STRING).release >= Gem::Version.new("5") &&
         ::ActiveRecord::Base.belongs_to_required_by_default
@@ -123,6 +141,8 @@ module PaperTrail
 
       @model_class.send :attr_accessor, :paper_trail_event
 
+      assert_concrete_activerecord_class(@model_class.version_class_name)
+
       @model_class.has_many(
         @model_class.versions_association_name,
         -> { order(model.timestamp_sort_order) },

spec/dummy_app/app/versions/abstract_version.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+class AbstractVersion < ActiveRecord::Base
+  include PaperTrail::VersionConcern
+  self.abstract_class = true
+end

spec/paper_trail/model_config_spec.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+module PaperTrail
+  ::RSpec.describe ModelConfig do
+    describe "when has_paper_trail is called" do
+      it "raises an error" do
+        expect {
+          class MisconfiguredCVC < ActiveRecord::Base
+            has_paper_trail class_name: "AbstractVersion"
+          end
+        }.to raise_error(
+          /use concrete \(not abstract\) version models/
+        )
+      end
+    end
+  end
+end