Relations built off collection associations with an unsaved owner should be null relations

For example, the following should not run any query on the database:

Post.new.comments.where(body: 'omg').to_a # => []

Fixes #5215.

2 files changed, 12 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/associations/collection_proxy.rb b/activerecord/lib/active_record/associations/collection_proxy.rb
index e73f940334..536e108f0e 100644
--- a/activerecord/lib/active_record/associations/collection_proxy.rb
+++ b/activerecord/lib/active_record/associations/collection_proxy.rb
@@ -835,8 +835,9 @@ module ActiveRecord
       # Returns a <tt>Relation</tt> object for the records in this association
       def scope
         association = @association
-
-        @association.scope.extending! do
+        scope = @association.scope
+        scope.none! if @association.owner.new_record?
+        scope.extending! do
           define_method(:proxy_association) { association }
         end
       end
diff --git a/activerecord/test/cases/associations/has_many_associations_test.rb b/activerecord/test/cases/associations/has_many_associations_test.rb
index 50c23c863f..6355094a79 100644
--- a/activerecord/test/cases/associations/has_many_associations_test.rb
+++ b/activerecord/test/cases/associations/has_many_associations_test.rb
@@ -1648,4 +1648,13 @@ class HasManyAssociationsTest < ActiveRecord::TestCase
     klass = Class.new(ActiveRecord::Base)
     assert_deprecated { klass.has_many :foo, :counter_sql => 'lol' }
   end
+
+  test "has many associations on new records use null relations" do
+    post = Post.new
+
+    assert_no_queries do
+      assert_equal [], post.comments
+      assert_equal [], post.comments.where(body: 'omg')
+    end
+  end
 end