Properly include the from clause when merging relations

The code that set the from clause was removed in
bdc5141652. I did not give any reason for
doing so. My assumption was that I intended to change it to use the
clause objects, but forgot. We appeared to not have test coverage for
this case.

Fixes #22996

activerecord/lib/active_record/relation/merger.rb

@@ -141,6 +141,9 @@ module ActiveRecord
       end
 
       def merge_single_values
+        if relation.from_clause.empty?
+          relation.from_clause = other.from_clause
+        end
         relation.lock_value ||= other.lock_value
 
         unless other.create_with_value.blank?

activerecord/test/cases/relation/merging_test.rb

@@ -104,6 +104,13 @@ class RelationMergingTest < ActiveRecord::TestCase
     post = PostThatLoadsCommentsInAnAfterSaveHook.create!(title: "First Post", body: "Blah blah blah.")
     assert_equal "First comment!", post.comments.where(body: "First comment!").first_or_create.body
   end
+
+  def test_merging_with_from_clause
+    relation = Post.all
+    assert relation.from_clause.empty?
+    relation = relation.merge(Post.from("posts"))
+    refute relation.from_clause.empty?
+  end
 end
 
 class MergingDifferentRelationsTest < ActiveRecord::TestCase