Function tree_key 

pub fn tree_key(collection: &str, rkey: &str) -> String

The key a record sits at in the tree.

atproto keys a repository by <collection>/<rkey> and nothing else — not by the DID, which the commit already carries, and not by anything the server chooses. It is one line, and it is here rather than inlined at both call sites because a repository whose keys are built two ways is a repository with two roots.