Use AR callbacks as suggested by:

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4964#note_12744656

app/models/ci/build.rb

@@ -19,6 +19,7 @@ module Ci
     acts_as_taggable
+    before_save :update_artifacts_size, if: :artifacts_file_changed?
     before_destroy { project }
     after_create :execute_hooks
@@ -340,7 +341,6 @@ module Ci
     def erase_artifacts!
       remove_artifacts_file!
       remove_artifacts_metadata!
-      self.artifacts_size = nil
       save
     end
@@ -381,6 +381,10 @@ module Ci
     private
+    def update_artifacts_size
+      self.artifacts_size = artifacts_file.size
+    end
+
     def erase_trace!
       self.trace = nil
     end

lib/ci/api/builds.rb

@@ -147,7 +147,6 @@ module Ci
           build.artifacts_file = artifacts
           build.artifacts_metadata = metadata
           build.artifacts_expire_in = params['expire_in']
-          build.artifacts_size = artifacts.size
           if build.save
             present(build, with: Entities::BuildDetails)

spec/requests/ci/api/builds_spec.rb

@@ -476,13 +476,17 @@ describe Ci::API::API do
       describe 'DELETE /builds/:id/artifacts' do
         let(:build) { create(:ci_build, :artifacts) }
-        before { delete delete_url, token: build.token }
+
+        before do
+          delete delete_url, token: build.token
+          build.reload
+        end
         it 'should remove build artifacts' do
           expect(response).to have_http_status(200)
           expect(build.artifacts_file.exists?).to be_falsy
           expect(build.artifacts_metadata.exists?).to be_falsy
-          expect(build.artifacts_size).to be_falsy
+          expect(build.artifacts_size).to eq(0)
         end
       end