Raise error when has_many through is defined before through association

https://github.com/rails/rails/issues/26834

This change raises an error if a has_many through association
is defined before the through association.

2 files changed, 18 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/associations.rb b/activerecord/lib/active_record/associations.rb
index 1db5fc0dd1..c05a6c87df 100644
--- a/activerecord/lib/active_record/associations.rb
+++ b/activerecord/lib/active_record/associations.rb
@@ -97,6 +97,16 @@ module ActiveRecord
     end
   end
 
+  class HasManyThroughOrderError < ActiveRecordError #:nodoc:
+    def initialize(owner_class_name = nil, reflection = nil, through_reflection = nil)
+      if owner_class_name && reflection && through_reflection
+        super("Cannot have a has_many :through association '#{owner_class_name}##{reflection.name}' which goes through '#{owner_class_name}##{through_reflection.name}' before the through association is defined.")
+      else
+        super("Cannot have a has_many :through association before the through association is defined.")
+      end
+    end
+  end
+
   class ThroughCantAssociateThroughHasOneOrManyReflection < ActiveRecordError #:nodoc:
     def initialize(owner = nil, reflection = nil)
       if owner && reflection
diff --git a/activerecord/lib/active_record/reflection.rb b/activerecord/lib/active_record/reflection.rb
index f3e81ee1e2..a0016f0735 100644
--- a/activerecord/lib/active_record/reflection.rb
+++ b/activerecord/lib/active_record/reflection.rb
@@ -931,6 +931,14 @@ module ActiveRecord
           raise HasOneThroughCantAssociateThroughCollection.new(active_record.name, self, through_reflection)
         end
 
+        if parent_reflection.nil?
+          reflections = active_record.reflections.keys.map(&:to_sym)
+
+          if reflections.index(through_reflection.name) > reflections.index(name)
+            raise HasManyThroughOrderError.new(active_record.name, self, through_reflection)
+          end
+        end
+
         check_validity_of_inverse!
       end