flatten merged join_values before building the joins

fixes #10669

While joining_values special treatment is given to string values.
By flattening the array it ensures that string values are detected
as strings and not arrays.

activerecord/CHANGELOG.md

@@ -1,5 +1,15 @@
 ## unreleased ##
 
+*   Flatten merged join_values before building the joins.
+
+    While joining_values special treatment is given to string values.
+    By flattening the array it ensures that string values are detected
+    as strings and not arrays.
+
+    Fixes #10669.
+
+    *Neeraj Singh and iwiznia*
+
 *   Remove extra select and update queries on save/touch/destroy ActiveRecord model
     with belongs to reflection with option `touch: true`.
 

activerecord/lib/active_record/relation/query_methods.rb

@@ -799,7 +799,7 @@ def arel
     def build_arel
       arel = Arel::SelectManager.new(table.engine, table)
 
-      build_joins(arel, joins_values) unless joins_values.empty?
+      build_joins(arel, joins_values.flatten) unless joins_values.empty?
 
       collapse_wheres(arel, (where_values - ['']).uniq)
 

activerecord/test/cases/relation_test.rb

@@ -179,7 +179,7 @@ def test_references_values_dont_duplicate
       assert_equal ['foo = bar'], relation.where_values
     end
 
-    def test_relation_merging_with_merged_joins
+    def test_relation_merging_with_merged_joins_as_symbols
       special_comments_with_ratings = SpecialComment.joins(:ratings)
       posts_with_special_comments_with_ratings = Post.group("posts.id").joins(:special_comments).merge(special_comments_with_ratings)
       assert_equal 3, authors(:david).posts.merge(posts_with_special_comments_with_ratings).count.length
@@ -193,6 +193,13 @@ def test_respond_to_for_non_selected_element
       assert_equal false, post.respond_to?(:title), "post should not respond_to?(:body) since invoking it raises exception"
     end
 
+    def test_relation_merging_with_merged_joins_as_strings
+      join_string = "LEFT OUTER JOIN #{Rating.quoted_table_name} ON #{SpecialComment.quoted_table_name}.id = #{Rating.quoted_table_name}.comment_id"
+      special_comments_with_ratings = SpecialComment.joins join_string
+      posts_with_special_comments_with_ratings = Post.group("posts.id").joins(:special_comments).merge(special_comments_with_ratings)
+      assert_equal 3, authors(:david).posts.merge(posts_with_special_comments_with_ratings).count.length
+    end
+
   end
 
   class RelationMutationTest < ActiveSupport::TestCase