Merge pull request #33611 from willianveiga/feature/reselect-method

Add reselect method

activerecord/CHANGELOG.md

@@ -382,6 +382,12 @@
 
     *Federico Martinez*
 
+*   Add `reselect` method. This is a short-hand for `unscope(:select).select(fields)`.
+
+    Fixes #27340.
+
+    *Willian Gustavo Veiga*
+
 *   Add basic API for connection switching to support multiple databases.
 
     1) Adds a `connects_to` method for models to connect to multiple databases. Example:

activerecord/lib/active_record/relation/query_methods.rb

@@ -240,6 +240,27 @@ module ActiveRecord
       self
     end
 
+    # Allows you to change a previously set select statement.
+    #
+    #   Post.select(:title, :body)
+    #   # SELECT `posts.title`, `posts.body` FROM `posts`
+    #
+    #   Post.select(:title, :body).reselect(:created_at)
+    #   # SELECT `posts.created_at` FROM `posts`
+    #
+    # This is short-hand for <tt>unscope(:select).select(fields)</tt>.
+    # Note that we're unscoping the entire select statement.
+    def reselect(*args)
+      check_if_method_has_arguments!(:reselect, args)
+      spawn.reselect!(*args)
+    end
+
+    # Same as #reselect but operates on relation in-place instead of copying.
+    def reselect!(*args) # :nodoc:
+      self.select_values = args
+      self
+    end
+
     # Allows to specify a group attribute:
     #
     #   User.group(:name)

activerecord/test/cases/relation/select_test.rb

@@ -11,5 +11,10 @@ module ActiveRecord
       expected = Post.select(:title).to_sql
       assert_equal expected, Post.select(nil).select(:title).to_sql
     end
+
+    def test_reselect
+      expected = Post.select(:title).to_sql
+      assert_equal expected, Post.select(:title, :body).reselect(:title).to_sql
+    end
   end
 end

guides/source/active_record_querying.md

@@ -807,6 +807,32 @@ SELECT * FROM articles WHERE id > 10 ORDER BY id DESC LIMIT 20
 
 ```
 
+### `reselect`
+
+The `reselect` method overrides an existing select statement. For example:
+
+```ruby
+Post.select(:title, :body).reselect(:created_at)
+```
+
+The SQL that would be executed:
+
+```sql
+SELECT `posts.created_at` FROM `posts`
+```
+
+In case the `reselect` clause is not used,
+
+```ruby
+Post.select(:title, :body)
+```
+
+the SQL executed would be:
+
+```sql
+SELECT `posts.title`, `posts.body` FROM `posts`
+```
+
 ### `reorder`
 
 The `reorder` method overrides the default scope order. For example: