Make project#namespace_id not null

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

db/post_migrate/20180301084653_change_project_namespace_id_not_null.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class ChangeProjectNamespaceIdNotNull < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  class Project < ActiveRecord::Base
+    self.table_name = 'projects'
+    include EachBatch
+  end
+
+  BATCH_SIZE = 1000
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    Project.where(namespace_id: nil).each_batch(of: BATCH_SIZE) do |batch|
+      batch.delete_all
+    end
+
+    change_column_null :projects, :namespace_id, false
+  end
+
+  def down
+    change_column_null :projects, :namespace_id, true
+  end
+end