Pass through the `prepend` option to `AS::Callback`

I'm not sure what's the use case for this, but apparently it broke some apps.
Since it was not the intended result from #16210 I fixed it to not raise an
exception anymore. However, I didn't add documentation for it because I don't
know if this should be officially supported without knowing how it's meant to
be used.

In general, validations should be side-effect-free (other than adding to the
error message to `@errors`). Order-dependent validations seems like a bad idea.

Fixes #18002

2 files changed, 38 insertions, 2 deletions

diff --git a/activemodel/lib/active_model/validations.rb b/activemodel/lib/active_model/validations.rb
index c1e344b215..6a2668b8f7 100644
--- a/activemodel/lib/active_model/validations.rb
+++ b/activemodel/lib/active_model/validations.rb
@@ -87,7 +87,7 @@ module ActiveModel
         validates_with BlockValidator, _merge_attributes(attr_names), &block
       end
 
-      VALID_OPTIONS_FOR_VALIDATE = [:on, :if, :unless].freeze
+      VALID_OPTIONS_FOR_VALIDATE = [:on, :if, :unless, :prepend].freeze
 
       # Adds a validation method or block to the class. This is useful when
       # overriding the +validate+ instance method becomes too unwieldy and
diff --git a/activemodel/test/cases/validations_test.rb b/activemodel/test/cases/validations_test.rb
index 98e0266d7e..cef66f3c0d 100644
--- a/activemodel/test/cases/validations_test.rb
+++ b/activemodel/test/cases/validations_test.rb
@@ -172,7 +172,43 @@ class ValidationsTest < ActiveModel::TestCase
       Topic.validate :title, presence: true
     end
     message = 'Unknown key: :presence. Valid keys are: :on, :if, :unless. Perhaps you meant to call `validates` instead of `validate`?'
-    assert_equal message, error.message
+    assert_includes error.message, "Unknown key: :presence"
+    assert_includes error.message, "Perhaps you meant to call `validates` instead of `validate`?"
+  end
+
+  def test_callback_options_to_validate
+    klass = Class.new(Topic) do
+      attr_reader :call_sequence
+
+      def initialize(*)
+        super
+        @call_sequence = []
+      end
+
+      private
+        def validator_a
+          @call_sequence << :a
+        end
+
+        def validator_b
+          @call_sequence << :b
+        end
+
+        def validator_c
+          @call_sequence << :c
+        end
+    end
+
+    assert_nothing_raised do
+      klass.validate :validator_a, if: ->{ true }
+      klass.validate :validator_b, prepend: true
+      klass.validate :validator_c, unless: ->{ true }
+    end
+
+    t = klass.new
+
+    assert_predicate t, :valid?
+    assert_equal [:b, :a], t.call_sequence
   end
 
   def test_errors_conversions