Map class names back to integers in _write_attribute (Rails 7.1+ has_many reload)

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+### Fixed
+
+- On Rails 7.1+, reloading a `has_many` while holding records already built into it wrote the mapped class name into the integer type column (cast to `0`), so those in-memory records pointed at the wrong type. `_write_attribute` now maps the class name back to its integer ([#69](https://github.com/clio/polymorphic_integer_type/issues/69)).
+
 ## v3.5.0 (2026-04-30)
 
 ### Added

lib/polymorphic_integer_type/extensions.rb

@@ -153,6 +153,19 @@ def []=(attr_name, value)
       end
     end
 
+    # Rails 7.1+ copies reloaded has_many rows onto records the caller already
+    # holds with `_write_attribute(name, record[name])`
+    # (CollectionAssociation#merge_target_lists). `[]` above returns the mapped
+    # class name for these columns, so map it back to the integer here instead
+    # of letting the integer column cast "Drink" to 0.
+    def _write_attribute(attr_name, value)
+      name = attr_name.to_s
+      if value.is_a?(String) && _polymorphic_foreign_types.include?(name)
+        value = self.class.send("#{name}_mapping").key(value) || value
+      end
+      super(attr_name, value)
+    end
+
   end
 
 end

spec/polymorphic_integer_type_spec.rb

@@ -184,6 +184,30 @@
       end
     end
   end
+  context "When a has_many reload merges into records the caller still holds" do
+    # Rails 7.1+ copies the reloaded values onto the in-memory records with
+    # _write_attribute (CollectionAssociation#merge_target_lists), reading
+    # them with [] first, which returns the mapped class name (issue #69).
+    let(:source) { cat }
+
+    it "keeps the integer type on the in-memory records" do
+      first = source.source_links.new(target: kibble)
+      first.save!
+      second = source.source_links.new(target: milk)
+      second.save!
+
+      expect(source.source_links).not_to be_loaded
+      source.source_links.length
+
+      expect(source.source_links).to include(second)
+      expect(second._read_attribute("target_type")).to eql 2
+      expect(second.target_type).to eql "Drink"
+      expect(second.target).to eql milk
+      expect(first.target_type).to eql "Food"
+      expect(first._read_attribute("source_type")).to eql 2
+    end
+  end
+
   context "When a link is given polymorphic record" do
     let(:link) { Link.create(source: source) }
     let(:source) { cat }