4 files changed, 21 insertions, 5 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 929f241b1b..a1a4d2646f 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,8 @@
+*   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.
 
diff --git a/activerecord/lib/active_record/aggregations.rb b/activerecord/lib/active_record/aggregations.rb
index 3ff41ed81b..8bed5bca28 100644
--- a/activerecord/lib/active_record/aggregations.rb
+++ b/activerecord/lib/active_record/aggregations.rb
@@ -256,15 +256,16 @@ module ActiveRecord
         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
diff --git a/activerecord/test/cases/aggregations_test.rb b/activerecord/test/cases/aggregations_test.rb
index 5536702f58..8a728902a8 100644
--- a/activerecord/test/cases/aggregations_test.rb
+++ b/activerecord/test/cases/aggregations_test.rb
@@ -138,6 +138,11 @@ class AggregationsTest < ActiveRecord::TestCase
     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
diff --git a/activerecord/test/models/customer.rb b/activerecord/test/models/customer.rb
index afe4b3d707..3338aaf7e1 100644
--- a/activerecord/test/models/customer.rb
+++ b/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)