Move the `ActiveModel:Errors#full_message` method to the `Error` class:

- One regression introduced by the "AM errors as object" features is
  about the `full_messages` method.

  It's currently impossible to call that method if the `base` object
  passed in the constructor of `AM::Errors` doesn't respond to the
  `errors` method.
  That's because `full_messages` now makes a weird back and forth trip

  `AM::Errors#full_messages` -> `AM::Error#full_message` -> `AM::Errors#full_message`

  Since `full_message` (singular) isn't needed by AM::Errors, I moved
  it to the `AM::Error` (singular) class. This way we don't need to
  grab the `AM::Errors` object from the base.

4 files changed, 92 insertions, 83 deletions

diff --git a/activemodel/lib/active_model.rb b/activemodel/lib/active_model.rb
index c9140dc582..756473e38d 100644
--- a/activemodel/lib/active_model.rb
+++ b/activemodel/lib/active_model.rb
@@ -53,6 +53,7 @@ module ActiveModel
 
   eager_autoload do
     autoload :Errors
+    autoload :Error
     autoload :RangeError, "active_model/errors"
     autoload :StrictValidationFailed, "active_model/errors"
     autoload :UnknownAttributeError, "active_model/errors"
diff --git a/activemodel/lib/active_model/error.rb b/activemodel/lib/active_model/error.rb
index ea141fe107..aef28b5af0 100644
--- a/activemodel/lib/active_model/error.rb
+++ b/activemodel/lib/active_model/error.rb
@@ -8,6 +8,92 @@ module ActiveModel
     CALLBACKS_OPTIONS = [:if, :unless, :on, :allow_nil, :allow_blank, :strict]
     MESSAGE_OPTIONS = [:message]
 
+    class << self
+      attr_accessor :i18n_customize_full_message # :nodoc:
+    end
+    self.i18n_customize_full_message = false
+
+    def self.full_message(attribute, message, base_class) # :nodoc:
+      return message if attribute == :base
+      attribute = attribute.to_s
+
+      if i18n_customize_full_message && base_class.respond_to?(:i18n_scope)
+        attribute = attribute.remove(/\[\d\]/)
+        parts = attribute.split(".")
+        attribute_name = parts.pop
+        namespace = parts.join("/") unless parts.empty?
+        attributes_scope = "#{base_class.i18n_scope}.errors.models"
+
+        if namespace
+          defaults = base_class.lookup_ancestors.map do |klass|
+            [
+              :"#{attributes_scope}.#{klass.model_name.i18n_key}/#{namespace}.attributes.#{attribute_name}.format",
+              :"#{attributes_scope}.#{klass.model_name.i18n_key}/#{namespace}.format",
+            ]
+          end
+        else
+          defaults = base_class.lookup_ancestors.map do |klass|
+            [
+              :"#{attributes_scope}.#{klass.model_name.i18n_key}.attributes.#{attribute_name}.format",
+              :"#{attributes_scope}.#{klass.model_name.i18n_key}.format",
+            ]
+          end
+        end
+
+        defaults.flatten!
+      else
+        defaults = []
+      end
+
+      defaults << :"errors.format"
+      defaults << "%{attribute} %{message}"
+
+      attr_name = attribute.tr(".", "_").humanize
+      attr_name = base_class.human_attribute_name(attribute, default: attr_name)
+
+      I18n.t(defaults.shift,
+        default:  defaults,
+        attribute: attr_name,
+        message:   message)
+    end
+
+    def self.generate_message(attribute, type, base, options) # :nodoc:
+      type = options.delete(:message) if options[:message].is_a?(Symbol)
+      value = (attribute != :base ? base.send(:read_attribute_for_validation, attribute) : nil)
+
+      options = {
+        model: base.model_name.human,
+        attribute: base.class.human_attribute_name(attribute),
+        value: value,
+        object: base
+      }.merge!(options)
+
+      if base.class.respond_to?(:i18n_scope)
+        i18n_scope = base.class.i18n_scope.to_s
+        defaults = base.class.lookup_ancestors.flat_map do |klass|
+          [ :"#{i18n_scope}.errors.models.#{klass.model_name.i18n_key}.attributes.#{attribute}.#{type}",
+            :"#{i18n_scope}.errors.models.#{klass.model_name.i18n_key}.#{type}" ]
+        end
+        defaults << :"#{i18n_scope}.errors.messages.#{type}"
+
+        catch(:exception) do
+          translation = I18n.translate(defaults.first, options.merge(default: defaults.drop(1), throw: true))
+          return translation unless translation.nil?
+        end unless options[:message]
+      else
+        defaults = []
+      end
+
+      defaults << :"errors.attributes.#{attribute}.#{type}"
+      defaults << :"errors.messages.#{type}"
+
+      key = defaults.shift
+      defaults = options.delete(:message) if options[:message]
+      options[:default] = defaults
+
+      I18n.translate(key, options)
+    end
+
     def initialize(base, attribute, type = :invalid, **options)
       @base = base
       @attribute = attribute
@@ -28,7 +114,7 @@ module ActiveModel
     def message
       case raw_type
       when Symbol
-        base.errors.generate_message(attribute, raw_type, options.except(*CALLBACKS_OPTIONS))
+        self.class.generate_message(attribute, raw_type, @base, options.except(*CALLBACKS_OPTIONS))
       else
         raw_type
       end
@@ -39,7 +125,7 @@ module ActiveModel
     end
 
     def full_message
-      base.errors.full_message(attribute, message)
+      self.class.full_message(attribute, message, @base.class)
     end
 
     # See if error matches provided +attribute+, +type+ and +options+.
diff --git a/activemodel/lib/active_model/errors.rb b/activemodel/lib/active_model/errors.rb
index e9f92dba82..22839bd9fb 100644
--- a/activemodel/lib/active_model/errors.rb
+++ b/activemodel/lib/active_model/errors.rb
@@ -69,11 +69,6 @@ module ActiveModel
 
     LEGACY_ATTRIBUTES = [:messages, :details].freeze
 
-    class << self
-      attr_accessor :i18n_customize_full_message # :nodoc:
-    end
-    self.i18n_customize_full_message = false
-
     attr_reader :errors
     alias :objects :errors
 
@@ -467,47 +462,7 @@ module ActiveModel
     #
     #   person.errors.full_message(:name, 'is invalid') # => "Name is invalid"
     def full_message(attribute, message)
-      return message if attribute == :base
-      attribute = attribute.to_s
-
-      if self.class.i18n_customize_full_message && @base.class.respond_to?(:i18n_scope)
-        attribute = attribute.remove(/\[\d\]/)
-        parts = attribute.split(".")
-        attribute_name = parts.pop
-        namespace = parts.join("/") unless parts.empty?
-        attributes_scope = "#{@base.class.i18n_scope}.errors.models"
-
-        if namespace
-          defaults = @base.class.lookup_ancestors.map do |klass|
-            [
-              :"#{attributes_scope}.#{klass.model_name.i18n_key}/#{namespace}.attributes.#{attribute_name}.format",
-              :"#{attributes_scope}.#{klass.model_name.i18n_key}/#{namespace}.format",
-            ]
-          end
-        else
-          defaults = @base.class.lookup_ancestors.map do |klass|
-            [
-              :"#{attributes_scope}.#{klass.model_name.i18n_key}.attributes.#{attribute_name}.format",
-              :"#{attributes_scope}.#{klass.model_name.i18n_key}.format",
-            ]
-          end
-        end
-
-        defaults.flatten!
-      else
-        defaults = []
-      end
-
-      defaults << :"errors.format"
-      defaults << "%{attribute} %{message}"
-
-      attr_name = attribute.tr(".", "_").humanize
-      attr_name = @base.class.human_attribute_name(attribute, default: attr_name)
-
-      I18n.t(defaults.shift,
-        default:  defaults,
-        attribute: attr_name,
-        message:   message)
+      Error.full_message(attribute, message, @base.class)
     end
 
     # Translates an error message in its default scope
@@ -535,40 +490,7 @@ module ActiveModel
     # * <tt>errors.attributes.title.blank</tt>
     # * <tt>errors.messages.blank</tt>
     def generate_message(attribute, type = :invalid, options = {})
-      type = options.delete(:message) if options[:message].is_a?(Symbol)
-      value = (attribute != :base ? @base.send(:read_attribute_for_validation, attribute) : nil)
-
-      options = {
-        model: @base.model_name.human,
-        attribute: @base.class.human_attribute_name(attribute),
-        value: value,
-        object: @base
-      }.merge!(options)
-
-      if @base.class.respond_to?(:i18n_scope)
-        i18n_scope = @base.class.i18n_scope.to_s
-        defaults = @base.class.lookup_ancestors.flat_map do |klass|
-          [ :"#{i18n_scope}.errors.models.#{klass.model_name.i18n_key}.attributes.#{attribute}.#{type}",
-            :"#{i18n_scope}.errors.models.#{klass.model_name.i18n_key}.#{type}" ]
-        end
-        defaults << :"#{i18n_scope}.errors.messages.#{type}"
-
-        catch(:exception) do
-          translation = I18n.translate(defaults.first, options.merge(default: defaults.drop(1), throw: true))
-          return translation unless translation.nil?
-        end unless options[:message]
-      else
-        defaults = []
-      end
-
-      defaults << :"errors.attributes.#{attribute}.#{type}"
-      defaults << :"errors.messages.#{type}"
-
-      key = defaults.shift
-      defaults = options.delete(:message) if options[:message]
-      options[:default] = defaults
-
-      I18n.translate(key, options)
+      Error.generate_message(attribute, type, @base, options)
     end
 
     def marshal_load(array) # :nodoc:
diff --git a/activemodel/lib/active_model/railtie.rb b/activemodel/lib/active_model/railtie.rb
index eb7901c7e9..65e20b9791 100644
--- a/activemodel/lib/active_model/railtie.rb
+++ b/activemodel/lib/active_model/railtie.rb
@@ -14,7 +14,7 @@ module ActiveModel
     end
 
     initializer "active_model.i18n_customize_full_message" do
-      ActiveModel::Errors.i18n_customize_full_message = config.active_model.delete(:i18n_customize_full_message) || false
+      ActiveModel::Error.i18n_customize_full_message = config.active_model.delete(:i18n_customize_full_message) || false
     end
   end
 end