title: WRITE_TRUNCATE in Bigquery
body: <p>Am trying to truncate the table in Bigquery using <code>write_truncate</code>, but it is not happening, instead it working like <code>write_append</code>. It's appending data but not truncating the table. </p>
  
  <p>Can someone please help to resolve the issue.</p>
  
  <p>My code:</p>
  
  <pre><code>        with beam.Pipeline(options=Pipeline options()) as p:
             read=(p|"Read BQ"&gt;&gt;beam.io.Read(beam.io.BigQuerySource(
      query='select empid from'\'`PRoject_Id.data_set.emp_details`',
  use_standard_sql=True))|"process"&gt;&gt;beam.Map(lambda ele:{'EMPID':ele['EMPID']})|
          "Write"&gt;&gt;beam.io.WriteToBigQuery(
      'PROJECT_ID:data_set.emp_out',
      schema='EMPID:STRING',
      write_disposition=beam.io.BigQueryDisposition.WRITE_TRUNCATE,
  create_dispositiom=beam.io.BigQueryDisposition.CREATE_IF_NEDED))
          if __name__="__main__":
            run().wait_until_finish()
  </code></pre>