[api] Use better way to set container object for attributes

Before we had this logic in our controller. This code should
belong to the model and cleans up the controller.

src/api/app/controllers/attribute_controller.rb

@@ -243,12 +243,7 @@ def cmd_attribute
       attrib_type = AttribType.find_by_namespace_and_name!(attr.value("namespace"), attr.value("name"))
       attrib = Attrib.new(attrib_type: attrib_type)
 
-      if @attribute_container.is_a?(Project)
-        attrib.project = @attribute_container
-      else
-        attrib.package = @attribute_container
-      end
-
+      attrib.container = @attribute_container
       authorize attrib, :create?
     end
 

src/api/app/models/attrib.rb

@@ -56,6 +56,14 @@ def container
     end
   end
 
+  def container=(container_object)
+    if container_object.is_a?(Project)
+      self.project = container_object
+    else
+      self.package = container_object
+    end
+  end
+
   def project
     if package
       package.project

src/api/spec/models/attrib_spec.rb

@@ -0,0 +1,19 @@
+require 'rails_helper'
+
+RSpec.describe Attrib do
+  let(:attribute) { create(:attrib, project: create(:project)) }
+  let(:project) { create(:project) }
+  let(:package) { create(:package) }
+
+  context "#container=" do
+    it "sets the project" do
+      attribute.container = project
+      expect(attribute.container).to be(project)
+    end
+
+    it "sets the package" do
+      attribute.container = package
+      expect(attribute.container).to be(package)
+    end
+  end
+end