Retain selections with `includes` and `joins`

Applying `includes` and `joins` to a relation that selected additional
database fields would cause those additional fields not to be included
in the results even though they were queried from the database:

    posts = Post.select('1 as other').includes(:tbl).joins(:tbl)

    posts.to_sql.include?('1 as other')       #=> true
    posts.first.attributes.include?('other')  #=> false

This commit includes these additionally selected fields in the
instantiated results.

activerecord/CHANGELOG.md

+*   Retain explicit selections on the base model after applying `includes` and `joins`.
+
+    Resolves #34889.
+
+    *Patrick Rebsch*
+
 *   Allow attributes to be fetched from Arel node groupings.
 
     *Jeff Emminger*, *Gannon McGibbon*

activerecord/lib/active_record/associations/join_dependency.rb

@@ -105,7 +105,9 @@ def instantiate(result_set, &block)
 
         model_cache = Hash.new { |h, klass| h[klass] = {} }
         parents = model_cache[join_root]
+
         column_aliases = aliases.column_aliases join_root
+        column_aliases += explicit_selections(column_aliases, result_set)
 
         message_bus = ActiveSupport::Notifications.instrumenter
 
@@ -135,6 +137,13 @@ def apply_column_aliases(relation)
       private
         attr_reader :alias_tracker
 
+        def explicit_selections(root_column_aliases, result_set)
+          root_names = root_column_aliases.map(&:name).to_set
+          result_set.columns
+            .reject { |n| root_names.include?(n) || n =~ /\At\d+_r\d+\z/ }
+            .map { |n| Aliases::Column.new(n, n) }
+        end
+
         def aliases
           @aliases ||= Aliases.new join_root.each_with_index.map { |join_part, i|
             columns = join_part.column_names.each_with_index.map { |column_name, j|

activerecord/test/cases/relation/select_test.rb

@@ -2,10 +2,11 @@
 
 require "cases/helper"
 require "models/post"
+require "models/comment"
 
 module ActiveRecord
   class SelectTest < ActiveRecord::TestCase
-    fixtures :posts
+    fixtures :posts, :comments
 
     def test_select_with_nil_argument
       expected = Post.select(:title).to_sql
@@ -23,5 +24,20 @@ def test_reselect_with_default_scope_select
 
       assert_equal expected, actual
     end
+
+    def test_aliased_select_using_as_with_joins_and_includes
+      posts = Post.select("posts.id AS field_alias").joins(:comments).includes(:comments)
+      assert_includes posts.first.attributes, "field_alias"
+    end
+
+    def test_aliased_select_not_using_as_with_joins_and_includes
+      posts = Post.select("posts.id field_alias").joins(:comments).includes(:comments)
+      assert_includes posts.first.attributes, "field_alias"
+    end
+
+    def test_star_select_with_joins_and_includes
+      posts = Post.select("posts.*").joins(:comments).includes(:comments)
+      assert_not_includes posts.first.attributes, "posts.*"
+    end
   end
 end