When preloading a belongs_to, the target should still be set (to nil) if there is no foreign key present. And the loaded flag should be set on the association proxy. This then allows us to remove the foreign_key_present? check from BelongsToAssociation#find_target. Also added a test for the same thing on polymorphic associations.

3 files changed, 16 insertions, 12 deletions

diff --git a/activerecord/lib/active_record/association_preload.rb b/activerecord/lib/active_record/association_preload.rb
index e3aee701a8..5aad2b4558 100644
--- a/activerecord/lib/active_record/association_preload.rb
+++ b/activerecord/lib/active_record/association_preload.rb
@@ -339,22 +339,27 @@ module ActiveRecord
             key = record.send(reflection.foreign_key)
             key && key.to_s
           end
-          id_map.delete nil
           klasses_and_ids[reflection.klass] = id_map unless id_map.empty?
         end
 
         klasses_and_ids.each do |klass, _id_map|
-          table       = klass.arel_table
           primary_key = (reflection.options[:primary_key] || klass.primary_key).to_s
-          method      = in_or_equal(_id_map.keys)
-          conditions  = table[primary_key].send(*method)
+          keys        = _id_map.keys.compact
 
-          custom_conditions = append_conditions(reflection, preload_options)
-          conditions = custom_conditions.inject(conditions) do |ast, cond|
-            ast.and cond
-          end
+          unless keys.empty?
+            table       = klass.arel_table
+            method      = in_or_equal(keys)
+            conditions  = table[primary_key].send(*method)
 
-          associated_records = klass.unscoped.where(conditions).apply_finder_options(options.slice(:include, :select, :joins, :order)).to_a
+            custom_conditions = append_conditions(reflection, preload_options)
+            conditions = custom_conditions.inject(conditions) do |ast, cond|
+              ast.and cond
+            end
+
+            associated_records = klass.unscoped.where(conditions).apply_finder_options(options.slice(:include, :select, :joins, :order)).to_a
+          else
+            associated_records = []
+          end
 
           set_association_single_records(_id_map, reflection.name, associated_records, primary_key)
         end
diff --git a/activerecord/lib/active_record/associations.rb b/activerecord/lib/active_record/associations.rb
index 35fd1395f7..e7d3e45da2 100644
--- a/activerecord/lib/active_record/associations.rb
+++ b/activerecord/lib/active_record/associations.rb
@@ -1501,6 +1501,7 @@ module ActiveRecord
           redefine_method("set_#{reflection.name}_target") do |target|
             association = association_proxy_class.new(self, reflection)
             association.target = target
+            association.loaded
             association_instance_set(reflection.name, association)
           end
         end
diff --git a/activerecord/lib/active_record/associations/belongs_to_association.rb b/activerecord/lib/active_record/associations/belongs_to_association.rb
index 4cde1c0960..b50ee689cd 100644
--- a/activerecord/lib/active_record/associations/belongs_to_association.rb
+++ b/activerecord/lib/active_record/associations/belongs_to_association.rb
@@ -55,9 +55,7 @@ module ActiveRecord
         end
 
         def find_target
-          if foreign_key_present?
-            scoped.first.tap { |record| set_inverse_instance(record) }
-          end
+          scoped.first.tap { |record| set_inverse_instance(record) }
         end
 
         def finder_options