Fix eager loading that non-select columns will be loaded

Related to #35210.

We sometimes use `select` to limit unused columns for performance.

For example, `GET /posts/1` (post detail) usually use (almost) all
columns, but `GET /posts` (post list) does not always use all columns
(e.g. use `id` and `title` for the list view, but `body` is not used).

If an association is eager loaded, the limited `select` doesn't works as
expected, eager loading will load all columns on the model, plus also
load the `select` columns additionally. It works differently with
natural load and preload. It means that changing natural load or preload
to eager load (or vice versa) is unsafe.

This fixes eager loading that always load all columns (plus extra
`select` columns), to respect the `select` columns like as others.

```ruby
post = Post.select("UPPER(title) AS title").first
post.title # => "WELCOME TO THE WEBLOG"
post.body  # => ActiveModel::MissingAttributeError

# Rails 6.0 (ignore the `select` values)
post = Post.select("UPPER(title) AS title").eager_load(:comments).first
post.title # => "Welcome to the weblog"
post.body  # => "Such a lovely day"

# Rails 6.1 (respect the `select` values)
post = Post.select("UPPER(title) AS title").eager_load(:comments).first
post.title # => "WELCOME TO THE WEBLOG"
post.body  # => ActiveModel::MissingAttributeError
```

activerecord/CHANGELOG.md

+*   Respect the `select` values for eager loading.
+
+    ```ruby
+    post = Post.select("UPPER(title) AS title").first
+    post.title # => "WELCOME TO THE WEBLOG"
+    post.body  # => ActiveModel::MissingAttributeError
+
+    # Rails 6.0 (ignore the `select` values)
+    post = Post.select("UPPER(title) AS title").eager_load(:comments).first
+    post.title # => "Welcome to the weblog"
+    post.body  # => "Such a lovely day"
+
+    # Rails 6.1 (respect the `select` values)
+    post = Post.select("UPPER(title) AS title").eager_load(:comments).first
+    post.title # => "WELCOME TO THE WEBLOG"
+    post.body  # => ActiveModel::MissingAttributeError
+    ```
+
+    *Ryuta Kamizono*
+
 *   Allow attribute's default to be configured but keeping its own type.
 
     ```ruby

activerecord/lib/active_record/associations/join_dependency.rb

@@ -34,7 +34,7 @@ def column_alias(node, column)
         Table = Struct.new(:node, :columns) do # :nodoc:
           def column_aliases
             t = node.table
-            columns.map { |column| t[column.name].as Arel.sql column.alias }
+            columns.map { |column| t[column.name].as(column.alias) }
           end
         end
         Column = Struct.new(:name, :alias)
@@ -105,13 +105,20 @@ def instantiate(result_set, strict_loading_value, &block)
         parents = model_cache[join_root]
 
         column_aliases = aliases.column_aliases(join_root)
-        column_names = explicit_selections(column_aliases, result_set)
+        column_names = []
+
+        result_set.columns.each do |name|
+          column_names << name unless /\At\d+_r\d+\z/.match?(name)
+        end
 
         if column_names.empty?
           column_types = {}
         else
           column_types = result_set.column_types
-          column_types = column_types.slice(*column_names) unless column_types.empty?
+          unless column_types.empty?
+            attribute_types = join_root.attribute_types
+            column_types = column_types.slice(*column_names).delete_if { |k, _| attribute_types.key?(k) }
+          end
           column_aliases += column_names.map! { |name| Aliases::Column.new(name, name) }
         end
 
@@ -134,6 +141,7 @@ def instantiate(result_set, strict_loading_value, &block)
       end
 
       def apply_column_aliases(relation)
+        @join_root_alias = relation.select_values.empty?
         relation._select!(-> { aliases.columns })
       end
 
@@ -145,18 +153,18 @@ def each(&block)
         attr_reader :join_root, :join_type
 
       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.each_with_object([]) do |name, result|
-            result << name unless /\At\d+_r\d+\z/.match?(name) || root_names.include?(name)
-          end
-        end
+        attr_reader :alias_tracker, :join_root_alias
 
         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|
+            column_names = if join_part == join_root && !join_root_alias
+              primary_key = join_root.primary_key
+              primary_key ? [primary_key] : []
+            else
+              join_part.column_names
+            end
+
+            columns = column_names.each_with_index.map { |column_name, j|
               Aliases::Column.new column_name, "t#{i}_r#{j}"
             }
             Aliases::Table.new(join_part, columns)

activerecord/lib/active_record/associations/join_dependency/join_part.rb

@@ -17,7 +17,7 @@ class JoinPart # :nodoc:
         # association.
         attr_reader :base_klass, :children
 
-        delegate :table_name, :column_names, :primary_key, to: :base_klass
+        delegate :table_name, :column_names, :primary_key, :attribute_types, to: :base_klass
 
         def initialize(base_klass, children)
           @base_klass = base_klass

activerecord/test/cases/finder_test.rb

@@ -1375,7 +1375,14 @@ def test_with_limiting_with_custom_select
       limit: 3, order: "posts.id"
     ).to_a
     assert_equal 3, posts.size
-    assert_equal [0, 1, 1], posts.map(&:author_id).sort
+    assert_equal [1, 1, nil], posts.map(&:author_id)
+  end
+
+  def test_custom_select_takes_precedence_over_original_value
+    posts = Post.select("UPPER(title) AS title")
+    assert_equal "WELCOME TO THE WEBLOG", posts.first.title
+    assert_equal "WELCOME TO THE WEBLOG", posts.preload(:comments).first.title
+    assert_equal "WELCOME TO THE WEBLOG", posts.eager_load(:comments).first.title
   end
 
   def test_eager_load_for_no_has_many_with_limit_and_joins_for_has_many

activerecord/test/cases/relation/select_test.rb

@@ -25,6 +25,22 @@ def test_reselect_with_default_scope_select
       assert_equal expected, actual
     end
 
+    def test_non_select_columns_wont_be_loaded
+      posts = Post.select("UPPER(title) AS title")
+
+      assert_non_select_columns_wont_be_loaded(posts.first)
+      assert_non_select_columns_wont_be_loaded(posts.preload(:comments).first)
+      assert_non_select_columns_wont_be_loaded(posts.eager_load(:comments).first)
+    end
+
+    def assert_non_select_columns_wont_be_loaded(post)
+      assert_equal "WELCOME TO THE WEBLOG", post.title
+      assert_raise(ActiveModel::MissingAttributeError) do
+        post.body
+      end
+    end
+    private :assert_non_select_columns_wont_be_loaded
+
     def test_type_casted_extra_select_with_eager_loading
       posts = Post.select("posts.id * 1.1 AS foo").eager_load(:comments)
       assert_equal 1.1, posts.first.foo
@@ -32,18 +48,12 @@ def test_type_casted_extra_select_with_eager_loading
 
     def test_aliased_select_using_as_with_joins_and_includes
       posts = Post.select("posts.id AS field_alias").joins(:comments).includes(:comments)
-      assert_equal %w(
-        id author_id title body type legacy_comments_count taggings_with_delete_all_count taggings_with_destroy_count
-        tags_count indestructible_tags_count tags_with_destroy_count tags_with_nullify_count field_alias
-      ), posts.first.attributes.keys
+      assert_equal %w(id field_alias), posts.first.attributes.keys
     end
 
     def test_aliased_select_not_using_as_with_joins_and_includes
       posts = Post.select("posts.id field_alias").joins(:comments).includes(:comments)
-      assert_equal %w(
-        id author_id title body type legacy_comments_count taggings_with_delete_all_count taggings_with_destroy_count
-        tags_count indestructible_tags_count tags_with_destroy_count tags_with_nullify_count field_alias
-      ), posts.first.attributes.keys
+      assert_equal %w(id field_alias), posts.first.attributes.keys
     end
 
     def test_star_select_with_joins_and_includes