Add ActiveRecord::Relation#extract_associated for extracting associated record

activerecord/CHANGELOG.md

@@ -1,3 +1,12 @@
+*   Add `ActiveRecord::Relation#extract_associated` for extracting associated records from a relation. 
+
+    ```
+    account.memberships.extract_associated(:user)
+    # => Returns collection of User records
+    ```
+
+    *DHH*
+
 *   Add `ActiveRecord::Relation#annotate` for adding SQL comments to its queries.
 
     For example:

activerecord/lib/active_record/relation/query_methods.rb

@@ -154,6 +154,19 @@ def preload!(*args) # :nodoc:
       self
     end
 
+    # Extracts a named +association+ from the relation. The named association is first preloaded, 
+    # then the individual association records are collected from the relation. Like so:
+    #
+    #   account.memberships.extract_associated(:user) 
+    #   # => Returns collection of User records
+    #
+    # This is short-hand for:
+    #
+    #   account.memberships.preload(:user).collect(&:user)
+    def extract_associated(association)
+      preload(association).collect(&association)
+    end
+
     # Use to indicate that the given +table_names+ are referenced by an SQL string,
     # and should therefore be JOINed in any query rather than loaded separately.
     # This method only works in conjunction with #includes.

activerecord/test/cases/relations_test.rb

@@ -602,6 +602,11 @@ def test_preload_applies_to_all_chained_preloaded_scopes
     end
   end
 
+  def test_extracted_association
+    authors = Post.all.extract_associated(:author)
+    assert_equal Post.all.collect(&:author), authors
+  end
+
   def test_find_with_included_associations
     assert_queries(2) do
       posts = Post.includes(:comments).order("posts.id")