Category fixture not in Mage_Catalog_Model_Resource_Tree

[ScreamingDev]

Situation

Got this fixture:

eav:
  catalog_category:
    - entity_id: 21
      parent_id: 2
      path: 1/2/21
      name: Common
      level: 2
      children_count: 0
      is_active: 1
      /stores:
        - 0
        - 1

Running tests on adminhtml/catalog/category/edit

Problem

app/code/core/Mage/Adminhtml/Block/Catalog/Category/Abstract.php:109

if ($this->getCategory()) {
                $tree->loadEnsuredNodes($this->getCategory(), $tree->getNodeById($rootId));
            }

The $tree->getNodeById($rootId) is null but it shouldn't. The Varien_Data_Tree_Node_Collection is empty. Mage::app()->getStore()->getId() at that point is 0 / admin.

Solution

• what did I get wrong?
• where do these nodes come from?
• maybe mock it and provide the fixture information