Move flattening records added to an association farther out

There are many ways that things end up getting passed to `concat`. Not
all of those entry points called `flatten` on their input. It seems that
just about every method that is meant to take a single record, or that
splats its input, is meant to also take an array. `concat` is the
earliest point that is common to all of the methods which add records to
the association. Partially fixes #18689

activerecord/lib/active_record/associations/collection_association.rb

@@ -151,6 +151,7 @@ def create!(attributes = {}, &block)
       # be chained. Since << flattens its argument list and inserts each record,
       # +push+ and +concat+ behave identically.
       def concat(*records)
+        records = records.flatten
         if owner.new_record?
           load_target
           concat_records(records)
@@ -549,7 +550,7 @@ def replace_common_records_in_memory(new_target, original_target)
         def concat_records(records, should_raise = false)
           result = true
 
-          records.flatten.each do |record|
+          records.each do |record|
             raise_on_type_mismatch!(record)
             add_to_target(record) do |rec|
               result &&= insert_record(rec, true, should_raise) unless owner.new_record?

activerecord/test/cases/counter_cache_test.rb

@@ -180,4 +180,13 @@ class ::SpecialReply < ::Reply
       SpecialTopic.reset_counters(special.id, :lightweight_special_replies)
     end
   end
+
+  test "counters are updated both in memory and in the database on create" do
+    car = Car.new(engines_count: 0)
+    car.engines = [Engine.new, Engine.new]
+    car.save!
+
+    assert_equal 2, car.engines_count
+    assert_equal 2, car.reload.engines_count
+  end
 end