Ensure empty has_many :through association preloaded via joins is mar…

…ked as loaded. Fixes #2054.

activerecord/lib/active_record/associations/join_dependency.rb

@@ -188,13 +188,12 @@ def construct_association(record, join_part, row)
           association = join_part.instantiate(row) unless row[join_part.aliased_primary_key].nil?
           set_target_and_inverse(join_part, association, record)
         else
-          return if row[join_part.aliased_primary_key].nil?
-          association = join_part.instantiate(row)
+          association = join_part.instantiate(row) unless row[join_part.aliased_primary_key].nil?
           case macro
           when :has_many, :has_and_belongs_to_many
             other = record.association(join_part.reflection.name)
             other.loaded!
-            other.target.push(association)
+            other.target.push(association) if association
             other.set_inverse_instance(association)
           when :belongs_to
             set_target_and_inverse(join_part, association, record)

activerecord/test/cases/associations/has_many_through_associations_test.rb

@@ -808,4 +808,12 @@ def test_create_bang_returns_falsy_when_join_record_has_errors
       assert !c.save
     end
   end
+
+  def test_preloading_empty_through_association_via_joins
+    person = Person.create!(:first_name => "Gaga")
+    person = Person.where(:id => person.id).where('readers.id = 1 or 1=1').includes(:posts).to_a.first
+
+    assert person.posts.loaded?, 'person.posts should be loaded'
+    assert_equal [], person.posts
+  end
 end