Apply default scope when joining associations.

For example:

    class Post < ActiveRecord::Base
      default_scope -> { where published: true }
    end

    class Comment
      belongs_to :post
    end

When calling `Comment.join(:post)`, we expect to receive only
comments on published posts, since that is the default scope for
posts.

Before this change, the default scope from `Post` was not applied,
so we'd get comments on unpublished posts.

activerecord/CHANGELOG.md

@@ -1,3 +1,22 @@
+*   Apply default scope when joining associations. For example:
+
+        class Post < ActiveRecord::Base
+          default_scope -> { where published: true }
+        end
+
+        class Comment
+          belongs_to :post
+        end
+
+    When calling `Comment.joins(:post)`, we expect to receive only
+    comments on published posts, since that is the default scope for
+    posts.
+
+    Before this change, the default scope from `Post` was not applied,
+    so we'd get comments on unpublished posts.
+
+    *Jon Leighton*
+
 *   Remove `activerecord-deprecated_finders` as a dependency
 
     *Łukasz Strzałkowski*

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

@@ -106,6 +106,8 @@ module ActiveRecord
               ]
             end
 
+            scope_chain_items += [reflection.klass.send(:build_default_scope)].compact
+
             constraint = scope_chain_items.inject(constraint) do |chain, item|
               unless item.is_a?(Relation)
                 item = ActiveRecord::Relation.new(reflection.klass, table).instance_exec(self, &item)

activerecord/test/cases/associations/inner_join_association_test.rb

@@ -104,4 +104,12 @@ class InnerJoinAssociationTest < ActiveRecord::TestCase
     assert !posts(:welcome).tags.empty?
     assert Post.joins(:misc_tags).where(:id => posts(:welcome).id).empty?
   end
+
+  test "the default scope of the target is applied when joining associations" do
+    author = Author.create! name: "Jon"
+    author.categorizations.create!
+    author.categorizations.create! special: true
+
+    assert_equal [author], Author.where(id: author).joins(:special_categorizations)
+  end
 end