performance improvements to joins!

Before:

Calculating -------------------------------------
                  ar        87 i/100ms
-------------------------------------------------
                  ar      823.4 (±11.8%) i/s -       4089 in   5.070234s

After:

Calculating -------------------------------------
                  ar        88 i/100ms
-------------------------------------------------
                  ar      894.1 (±3.9%) i/s -       4488 in   5.028161s

Same test as 3a6dfca7f5f5bd45cea2f6ac348178e72423e1d5

2 files changed, 13 insertions, 5 deletions

diff --git a/activerecord/lib/active_record/relation/merger.rb b/activerecord/lib/active_record/relation/merger.rb
index 951eabe427..59226d316e 100644
--- a/activerecord/lib/active_record/relation/merger.rb
+++ b/activerecord/lib/active_record/relation/merger.rb
@@ -22,7 +22,17 @@ module ActiveRecord
       # the values.
       def other
         other = Relation.new(relation.klass, relation.table)
-        hash.each { |k, v| other.send("#{k}!", v) }
+        hash.each { |k, v|
+          if k == :joins
+            if Hash === v
+              other.joins!(v)
+            else
+              other.joins!(*v)
+            end
+          else
+            other.send("#{k}!", v)
+          end
+        }
         other
       end
     end
@@ -50,7 +60,7 @@ module ActiveRecord
       def merge
         normal_values.each do |name|
           value = values[name]
-          relation.send("#{name}!", value) unless value.blank?
+          relation.send("#{name}!", *value) unless value.blank?
         end
 
         merge_multi_values
diff --git a/activerecord/lib/active_record/relation/query_methods.rb b/activerecord/lib/active_record/relation/query_methods.rb
index ad10174adc..a9ace10093 100644
--- a/activerecord/lib/active_record/relation/query_methods.rb
+++ b/activerecord/lib/active_record/relation/query_methods.rb
@@ -258,13 +258,11 @@ module ActiveRecord
     #   User.joins(:posts)
     #   => SELECT "users".* FROM "users" INNER JOIN "posts" ON "posts"."user_id" = "users"."id"
     def joins(*args)
-      args.compact.blank? ? self : spawn.joins!(*args)
+      args.compact.blank? ? self : spawn.joins!(*args.flatten)
     end
 
     # Like #joins, but modifies relation in place.
     def joins!(*args)
-      args.flatten!
-
       self.joins_values += args
       self
     end