List the designs of a building

Renders appear in relationships as identifiers only, without attributes: this is where render ids for an estimate or an album come from, not where render state is read. Poll GET /renders/{render} for state; has_main_render says whether the design is finished.

main_render_id is the finished render of the main view — the render an estimate, an album or a refine starts from. It is null until one has finished, which is exactly when mode: refine would refuse it as a parent.