Preserve context for joins while merging relations

This is a backport of #10164, already merged into
master. The issue is described in lengthy detail
in issues #3002 and #5494.

1 files changed, 27 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/relation/spawn_methods.rb b/activerecord/lib/active_record/relation/spawn_methods.rb
index b8e384c2f3..90f2ac3cde 100644
--- a/activerecord/lib/active_record/relation/spawn_methods.rb
+++ b/activerecord/lib/active_record/relation/spawn_methods.rb
@@ -27,7 +27,7 @@ module ActiveRecord
         merge_relation_method(merged_relation, method, value) if value.present?
       end
 
-      merged_relation.joins_values += r.joins_values
+      merge_joins(merged_relation, r)
 
       merged_wheres = @where_values + r.where_values
 
@@ -146,6 +146,32 @@ module ActiveRecord
 
     private
 
+      def merge_joins(relation, other)
+        values = other.joins_values
+        return if values.blank?
+
+        if other.klass == relation.klass
+          relation.joins_values += values
+        else
+          joins_dependency, rest = values.partition do |join|
+            case join
+            when Hash, Symbol, Array
+              true
+            else
+              false
+            end
+          end
+
+          join_dependency = ActiveRecord::Associations::JoinDependency.new(
+            other.klass,
+            joins_dependency,
+            []
+          )
+
+          relation.joins_values += join_dependency.join_associations + rest
+        end
+      end
+
       def merge_relation_method(relation, method, value)
         relation.send(:"#{method}_values=", relation.send(:"#{method}_values") + value)
       end