`preload` preserves readonly flag on associations. #15853

This is a partial fix for #15853. It only works when a `preload`
is issued and not an `eager_load`. I've added a skipped failing
test-case to keep in mind that we need to deal with `eager_load`.

activerecord/CHANGELOG.md

+*   `preload` preserves readonly flag for associations.
+
+    See #15853.
+
+    *Yves Senn*
+
 *   Assume numeric types have changed if they were assigned to a value that
     would fail numericality validation, regardless of the old value. Previously
     this would only occur if the old value was 0.

activerecord/lib/active_record/associations/preloader/association.rb

@@ -151,6 +151,10 @@ def build_scope
             end
           end
 
+          if preload_values[:readonly] || values[:readonly]
+            scope.readonly!
+          end
+
           if options[:as]
             scope.where!(klass.table_name => { reflection.type => model.base_class.sti_name })
           end

activerecord/test/cases/associations/eager_test.rb

@@ -1261,4 +1261,33 @@ def test_deep_including_through_habtm
       Author.eager_load(:posts_with_signature).to_a
     end
   end
+
+  test "preloading readonly association" do
+    # has-one
+    firm = Firm.where(id: "1").preload(:readonly_account).first!
+    assert firm.readonly_account.readonly?
+
+    # has_and_belongs_to_many
+    project = Project.where(id: "2").preload(:readonly_developers).first!
+    assert project.readonly_developers.first.readonly?
+
+    # has-many :through
+    david = Author.where(id: "1").preload(:readonly_comments).first!
+    assert david.readonly_comments.first.readonly?
+  end
+
+  test "eager-loading readonly association" do
+    skip "eager_load does not yet preserve readonly associations"
+    # has-one
+    firm = Firm.where(id: "1").eager_load(:readonly_account).first!
+    assert firm.readonly_account.readonly?
+
+    # has_and_belongs_to_many
+    project = Project.where(id: "2").eager_load(:readonly_developers).first!
+    assert project.readonly_developers.first.readonly?
+
+    # has-many :through
+    david = Author.where(id: "1").eager_load(:readonly_comments).first!
+    assert david.readonly_comments.first.readonly?
+  end
 end