The items above each item at the source, for the item's :ancestors calculation: its parent's item (source_parent, of its kind and source host on its site), that one's parent, and so on, the top one first, as the Items page draws an item's place (Where it sits) and the Map view places an item under its parents. nil for an item with no parent.

A parent the migration keeps no item of (the source does not list it, such as a page in the trash) ends the chain as an item with no id, its source_id the parent's, which the page reads as Not in the source. An item in a loop of parents has each of them once.

The chains of every item asked for are read in one query for each site, whatever their depth: a recursive query from the items up through their parents, by the identity that makes an item one per site, source host, kind and source id, read as the reader under the item's site. An item with no parent costs nothing, so a page with none reads nothing.