class_docstr: Primary DataFrame-based loading data from elastic search interface,
  defining API to read data from ES to DataFrame.
code: def flatten_df(df):
      fields = elastic_search.flatten(df.schema)
      flatten_df = df.select(fields)
      return flatten_df
  
file_path: BigDL/python/orca/src/bigdl/orca/data/elastic_search.py
imports: from bigdl.orca import OrcaContext
  from bigdl.dllib.nncontext import init_nncontext
label_desc: Use image node_id 3 for calling the elastic_search obj's underlying member method code with example usage: obj.flatten_df(df) and returns: flatten_df