code: def _scalar_source_digest(self, run_id, prefix):
      cur = self._db.execute(
          """
            SELECT path_digest FROM scalar_source
            WHERE run = ? AND prefix = ?
          """,
          (run_id, prefix),
      )
      row = cur.fetchone()
      if not row:
          return None
      return row[0]
  
imports: import logging
  import os
  import sqlite3
  from guild import run_util
  from guild import tfevent
  from guild import util
  from guild import var
label_desc: Use image node_id 5 for calling the ScalarReader obj's underlying member method code with example usage: obj._scalar_source_digest(run_id, prefix) and returns: row, None