Properly create through records when called with where

Various behaviors needed by associations (such as creating the through
record) are lost when `where` is called, since we stop having a
`CollectionProxy` and start having an `AssociationRelation` which does
not contain this behavior. I *think* we should be able to rm
`AssociationRelation`, but we have tests saying the changes required to
do that would be bad (Without saying why. Of course. >_>)

Fixes #19073.

activerecord/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   Correctly create through records when created on a has many through
+    association when using `where`.
+
+    Fixes #19073.
+
+    *Sean Griffin*
+
 *   Add `SchemaMigration.create_table` support any unicode charsets for MySQL.
 
     *Ryuta Kamizono*

activerecord/lib/active_record/association_relation.rb

@@ -13,6 +13,19 @@ module ActiveRecord
       other == to_a
     end
 
+    def build(*args, &block)
+      scoping { @association.build(*args, &block) }
+    end
+    alias new build
+
+    def create(*args, &block)
+      scoping { @association.create(*args, &block) }
+    end
+
+    def create!(*args, &block)
+      scoping { @association.create!(*args, &block) }
+    end
+
     private
 
     def exec_queries

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -595,6 +595,12 @@ class HasManyThroughAssociationsTest < ActiveRecord::TestCase
     assert posts(:thinking).reload.people(true).collect(&:first_name).include?("Jeb")
   end
 
+  def test_through_record_is_built_when_created_with_where
+    assert_difference("posts(:thinking).readers.count", 1) do
+      posts(:thinking).people.where(first_name: "Jeb").create
+    end
+  end
+
   def test_associate_with_create_and_no_options
     peeps = posts(:thinking).people.count
     posts(:thinking).people.create(:first_name => 'foo')