1 files changed, 22 insertions, 19 deletions

diff --git a/activerecord/lib/active_record/relation/merger.rb b/activerecord/lib/active_record/relation/merger.rb
index 5dac00724a..b736b21525 100644
--- a/activerecord/lib/active_record/relation/merger.rb
+++ b/activerecord/lib/active_record/relation/merger.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "active_support/core_ext/hash/keys"
 
 module ActiveRecord
@@ -110,21 +112,20 @@ module ActiveRecord
           if other.klass == relation.klass
             relation.joins!(*other.joins_values)
           else
-            joins_dependency, rest = other.joins_values.partition do |join|
+            alias_tracker = nil
+            joins_dependency = other.joins_values.map do |join|
               case join
               when Hash, Symbol, Array
-                true
+                alias_tracker ||= other.alias_tracker
+                ActiveRecord::Associations::JoinDependency.new(
+                  other.klass, other.table, join, alias_tracker
+                )
               else
-                false
+                join
               end
             end
 
-            join_dependency = ActiveRecord::Associations::JoinDependency.new(other.klass,
-                                                                             joins_dependency,
-                                                                             [])
-            relation.joins! rest
-
-            @relation = relation.joins join_dependency
+            relation.joins!(*joins_dependency)
           end
         end
 
@@ -132,19 +133,17 @@ module ActiveRecord
           if other.reordering_value
             # override any order specified in the original relation
             relation.reorder! other.order_values
-          elsif other.order_values
+          elsif other.order_values.any?
             # merge in order_values from relation
             relation.order! other.order_values
           end
 
-          relation.extend(*other.extending_values) unless other.extending_values.blank?
+          extensions = other.extensions - relation.extensions
+          relation.extending!(*extensions) if extensions.any?
         end
 
         def merge_single_values
-          if relation.from_clause.empty?
-            relation.from_clause = other.from_clause
-          end
-          relation.lock_value ||= other.lock_value
+          relation.lock_value ||= other.lock_value if other.lock_value
 
           unless other.create_with_value.blank?
             relation.create_with_value = (relation.create_with_value || {}).merge(other.create_with_value)
@@ -152,11 +151,15 @@ module ActiveRecord
         end
 
         def merge_clauses
-          CLAUSE_METHODS.each do |method|
-            clause = relation.get_value(method)
-            other_clause = other.get_value(method)
-            relation.set_value(method, clause.merge(other_clause))
+          if relation.from_clause.empty? && !other.from_clause.empty?
+            relation.from_clause = other.from_clause
           end
+
+          where_clause = relation.where_clause.merge(other.where_clause)
+          relation.where_clause = where_clause unless where_clause.empty?
+
+          having_clause = relation.having_clause.merge(other.having_clause)
+          relation.having_clause = having_clause unless having_clause.empty?
         end
     end
   end