Fix `reflection.association_primary_key` for `has_many` associations

It is incorrect to treat `options[:primary_key]` as
`association_primary_key` if `has_many` associations because the
`:primary_key` means the column on the owner record, not on the
association record. It will break `ids_reader` and `ids_writer`.

```ruby
  people(:david).essay_ids
  # => ActiveRecord::StatementInvalid: Mysql2::Error: Unknown column 'essays.first_name' in 'field list': SELECT `essays`.first_name FROM `essays` WHERE `essays`.`writer_id` = 'David'
```

Fixes #14439.

2 files changed, 11 insertions, 4 deletions

diff --git a/activerecord/lib/active_record/associations/collection_association.rb b/activerecord/lib/active_record/associations/collection_association.rb
index 1a424896fe..ea9ab297d1 100644
--- a/activerecord/lib/active_record/associations/collection_association.rb
+++ b/activerecord/lib/active_record/associations/collection_association.rb
@@ -55,13 +55,16 @@ module ActiveRecord
         pk_type = reflection.association_primary_key_type
         ids = Array(ids).reject(&:blank?)
         ids.map! { |i| pk_type.cast(i) }
-        records = klass.where(reflection.association_primary_key => ids).index_by do |r|
-          r.send(reflection.association_primary_key)
+
+        primary_key = reflection.association_primary_key
+        records = klass.where(primary_key => ids).index_by do |r|
+          r.public_send(primary_key)
         end.values_at(*ids).compact
+
         if records.size != ids.size
-          found_ids = records.map { |record| record.send(reflection.association_primary_key) }
+          found_ids = records.map { |record| record.public_send(primary_key) }
           not_found_ids = ids - found_ids
-          klass.all.raise_record_not_found_exception!(ids, records.size, ids.size, reflection.association_primary_key, not_found_ids)
+          klass.all.raise_record_not_found_exception!(ids, records.size, ids.size, primary_key, not_found_ids)
         else
           replace(records)
         end
diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index d2b85e168b..8e1ef840fa 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -715,6 +715,10 @@ module ActiveRecord
           Associations::HasManyAssociation
         end
       end
+
+      def association_primary_key(klass = nil)
+        primary_key(klass || self.klass)
+      end
     end
 
     class HasOneReflection < AssociationReflection # :nodoc: