Merge pull request #27860 from meinac/fix_left_joins_behaviour_with_merge

Fix relation merger issue with `left_outer_joins`.

activerecord/CHANGELOG.md

+*   Fix relation merger issue with `left_outer_joins`.
+
+    *Mehmet Emin İNAÇ*
+
 *   Don't allow destroyed object mutation after `save` or `save!` is called.
 
     *Ryuta Kamizono*

activerecord/lib/active_record/relation/merger.rb

@@ -52,7 +52,7 @@ def initialize(relation, other)
 
       NORMAL_VALUES = Relation::VALUE_METHODS -
                       Relation::CLAUSE_METHODS -
-                      [:includes, :preload, :joins, :order, :reverse_order, :lock, :create_with, :reordering] # :nodoc:
+                      [:includes, :preload, :joins, :left_outer_joins, :order, :reverse_order, :lock, :create_with, :reordering] # :nodoc:
 
       def normal_values
         NORMAL_VALUES
@@ -79,6 +79,7 @@ def merge
         merge_clauses
         merge_preloads
         merge_joins
+        merge_outer_joins
 
         relation
       end
@@ -129,6 +130,29 @@ def merge_joins
           end
         end
 
+        def merge_outer_joins
+          return if other.left_outer_joins_values.blank?
+
+          if other.klass == relation.klass
+            relation.left_outer_joins!(*other.left_outer_joins_values)
+          else
+            alias_tracker = nil
+            joins_dependency = other.left_outer_joins_values.map do |join|
+              case join
+              when Hash, Symbol, Array
+                alias_tracker ||= other.alias_tracker
+                ActiveRecord::Associations::JoinDependency.new(
+                  other.klass, other.table, join, alias_tracker
+                )
+              else
+                join
+              end
+            end
+
+            relation.left_outer_joins!(*joins_dependency)
+          end
+        end
+
         def merge_multi_values
           if other.reordering_value
             # override any order specified in the original relation

activerecord/lib/active_record/relation/query_methods.rb

@@ -979,6 +979,8 @@ def build_left_outer_joins(manager, outer_joins, aliases)
           case join
           when Hash, Symbol, Array
             :association_join
+          when ActiveRecord::Associations::JoinDependency
+            :stashed_join
           else
             raise ArgumentError, "only Hash, Symbol and Array are allowed"
           end

activerecord/test/cases/relation/merging_test.rb

@@ -72,6 +72,12 @@ def test_relation_merging_with_joins
     assert_equal 1, comments.count
   end
 
+  def test_relation_merging_with_left_outer_joins
+    comments = Comment.joins(:post).where(body: "Thank you for the welcome").merge(Post.left_outer_joins(:author).where(body: "Such a lovely day"))
+
+    assert_equal 1, comments.count
+  end
+
   def test_relation_merging_with_association
     assert_queries(2) do  # one for loading post, and another one merged query
       post = Post.where(body: "Such a lovely day").first