diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 90b5cb3b8f19d..2bc2811077f1d 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Merge joins from association scopes when using .includes() or .joins()
+
+    Fixes #12492.
+
+    *Mike Campbell*
+
 *   Prevent the inversed association from being reloaded on save.
 
     Fixes #9499.
diff --git a/activerecord/lib/active_record/associations/preloader/association.rb b/activerecord/lib/active_record/associations/preloader/association.rb
index 69b65982b3970..7bd3033d94285 100644
--- a/activerecord/lib/active_record/associations/preloader/association.rb
+++ b/activerecord/lib/active_record/associations/preloader/association.rb
@@ -118,6 +118,7 @@ def build_scope
 
           scope.select!   preload_values[:select] || values[:select] || table[Arel.star]
           scope.includes! preload_values[:includes] || values[:includes]
+          scope.joins! preload_values[:joins] || values[:joins]
 
           if preload_values.key? :order
             scope.order! preload_values[:order]
diff --git a/activerecord/lib/active_record/relation/query_methods.rb b/activerecord/lib/active_record/relation/query_methods.rb
index 9c9690215ab81..6bbfec588e7f7 100644
--- a/activerecord/lib/active_record/relation/query_methods.rb
+++ b/activerecord/lib/active_record/relation/query_methods.rb
@@ -400,6 +400,9 @@ def joins(*args)
     end
 
     def joins!(*args) # :nodoc:
+      args.reject!(&:blank?)
+      args.flatten!
+
       self.joins_values += args
       self
     end