Ensure hashes can be passed to attributes using `composed_of`

This behavior was broken by 36e9be85. When the value is assigned
directly, either through mass assignment or directly assigning a hash,
the hash gets passed through to this writer method directly. While this
is intended to handle certain cases, when an explicit converter has been
provided, we should continue to use that instead. The positioning of the
added guard caused the new behavior to override that case.

Fixes #25210

activerecord/CHANGELOG.md

+*   Ensure hashes can be assigned to attributes created using `composed_of`.
+    Fixes #25210.
+
+    *Sean Griffin*
+
 *   Fix logging edge case where if an attribute was of the binary type and
     was provided as a Hash.
 

activerecord/lib/active_record/aggregations.rb

@@ -256,15 +256,16 @@ def reader_method(name, class_name, mapping, allow_nil, constructor)
         def writer_method(name, class_name, mapping, allow_nil, converter)
           define_method("#{name}=") do |part|
             klass = class_name.constantize
-            if part.is_a?(Hash)
-              raise ArgumentError unless part.size == part.keys.max
-              part = klass.new(*part.sort.map(&:last))
-            end
 
             unless part.is_a?(klass) || converter.nil? || part.nil?
               part = converter.respond_to?(:call) ? converter.call(part) : klass.send(converter, part)
             end
 
+            if part.is_a?(Hash)
+              raise ArgumentError unless part.size == part.keys.max
+              part = klass.new(*part.sort.map(&:last))
+            end
+
             if part.nil? && allow_nil
               mapping.each { |key, _| self[key] = nil }
               @aggregation_cache[name] = nil

activerecord/test/cases/aggregations_test.rb

@@ -138,6 +138,11 @@ def test_custom_converter
     assert_equal 'Barnoit GUMBLEAU', customers(:barney).fullname.to_s
     assert_kind_of Fullname, customers(:barney).fullname
   end
+
+  def test_assigning_hash_to_custom_converter
+    customers(:barney).fullname = { first: "Barney", last: "Stinson" }
+    assert_equal "Barney STINSON", customers(:barney).name
+  end
 end
 
 class OverridingAggregationsTest < ActiveRecord::TestCase

activerecord/test/models/customer.rb

@@ -64,7 +64,12 @@ class Fullname
 
   def self.parse(str)
     return nil unless str
-    new(*str.to_s.split)
+
+    if str.is_a?(Hash)
+      new(str[:first], str[:last])
+    else
+      new(*str.to_s.split)
+    end
   end
 
   def initialize(first, last = nil)