Removes item_subtype requirement for model-specific limits.

* When enforcing limits for a model, it was previously required to
  have the item_subtype column on the versions table to obtain the
  configured limit for a given model type, especially for models
  subclassed via STI.

* This change instead references the item on the version record,
  which is already available via the association cache, to determine
  the properly configured limit. This change drops the requirement of
  having item_subtype to configure model-specific limits.

CHANGELOG.md

@@ -4,7 +4,8 @@ This project follows [semver 2.0.0](http://semver.org/spec/v2.0.0.html) and the
 recommendations of [keepachangelog.com](http://keepachangelog.com/).
 
 ## Unreleased
-
+- [#1309](https://github.com/paper-trail-gem/paper_trail/pull/1309) -
+  Removes `item_subtype` requirement when specifying model-specific limits.
 ### Breaking Changes
 
 - None

README.md

@@ -613,10 +613,6 @@ has_paper_trail limit: 2
 has_paper_trail limit: nil
 ```
 
-To use a per-model limit, your `versions` table must have an
-`item_subtype` column. See [Section
-4.b.1](https://github.com/paper-trail-gem/paper_trail#4b1-the-optional-item_subtype-column).
-
 ## 3. Working With Versions
 
 ### 3.a. Reverting And Undeleting A Model

lib/paper_trail/model_config.rb

@@ -18,11 +18,6 @@ module PaperTrail
       `abstract_class`. This is fine, but all application models must be
       configured to use concrete (not abstract) version models.
     STR
-    E_MODEL_LIMIT_REQUIRES_ITEM_SUBTYPE = <<~STR.squish.freeze
-      To use PaperTrail's per-model limit in your %s model, you must have an
-      item_subtype column in your versions table. See documentation sections
-      2.e.1 Per-model limit, and 4.b.1 The optional item_subtype column.
-    STR
     DPR_PASSING_ASSOC_NAME_DIRECTLY_TO_VERSIONS_OPTION = <<~STR.squish
       Passing versions association name as `has_paper_trail versions: %{versions_name}`
       is deprecated. Use `has_paper_trail versions: {name: %{versions_name}}` instead.
@@ -124,7 +119,6 @@ module PaperTrail
       @model_class.send :include, ::PaperTrail::Model::InstanceMethods
       setup_options(options)
       setup_associations(options)
-      check_presence_of_item_subtype_column(options)
       @model_class.after_rollback { paper_trail.clear_rolled_back_versions }
       setup_callbacks_from_options options[:on]
     end
@@ -151,16 +145,6 @@ module PaperTrail
       ::ActiveRecord::Base.belongs_to_required_by_default
     end
 
-    # Some options require the presence of the `item_subtype` column. Currently
-    # only `limit`, but in the future there may be others.
-    #
-    # @api private
-    def check_presence_of_item_subtype_column(options)
-      return unless options.key?(:limit)
-      return if version_class.item_subtype_column_present?
-      raise Error, format(E_MODEL_LIMIT_REQUIRES_ITEM_SUBTYPE, @model_class.name)
-    end
-
     def check_version_class_name(options)
       # @api private - `version_class_name`
       @model_class.class_attribute :version_class_name

lib/paper_trail/reifier.rb

@@ -117,8 +117,6 @@ module PaperTrail
       # this method returns the constant `Dog`. If `attrs["species"]` is blank,
       # this method returns the constant `Animal`. You can see this particular
       # example in action in `spec/models/animal_spec.rb`.
-      #
-      # TODO: Duplication: similar `constantize` in VersionConcern#version_limit
       def version_reification_class(version, attrs)
         inheritance_column_name = version.item_type.constantize.inheritance_column
         inher_col_value = attrs[inheritance_column_name]

lib/paper_trail/version_concern.rb

@@ -379,16 +379,22 @@ module PaperTrail
     # The version limit can be global or per-model.
     #
     # @api private
-    #
-    # TODO: Duplication: similar `constantize` in Reifier#version_reification_class
     def version_limit
-      if self.class.item_subtype_column_present?
-        klass = (item_subtype || item_type).constantize
-        if klass&.paper_trail_options&.key?(:limit)
-          return klass.paper_trail_options[:limit]
-        end
+      if limit_option?(item.class)
+        item.class.paper_trail_options[:limit]
+      elsif base_class_limit_option?(item.class)
+        item.class.base_class.paper_trail_options[:limit]
+      else
+        PaperTrail.config.version_limit
       end
-      PaperTrail.config.version_limit
+    end
+
+    def limit_option?(klass)
+      klass.respond_to?(:paper_trail_options) && klass.paper_trail_options.key?(:limit)
+    end
+
+    def base_class_limit_option?(klass)
+      klass.respond_to?(:base_class) && limit_option?(klass.base_class)
     end
   end
 end

spec/paper_trail/config_spec.rb

@@ -52,17 +52,6 @@ module PaperTrail
         limited_bike.save
         assert_equal 2, limited_bike.versions.length
       end
-
-      context "when item_subtype column is absent" do
-        it "uses global version_limit" do
-          PaperTrail.config.version_limit = 6
-          names = PaperTrail::Version.column_names - ["item_subtype"]
-          allow(PaperTrail::Version).to receive(:column_names).and_return(names)
-          bike = LimitedBicycle.create!(name: "My Bike") # has_paper_trail limit: 3
-          10.times { bike.update(name: SecureRandom.hex(8)) }
-          assert_equal 7, bike.versions.length
-        end
-      end
     end
   end
 end