title: Write to BQ one field of the rows of a PColl - Need the entire row for table selection
body: <p>I have a problem:</p>
  <p>My Pcoll is made of rows with this format</p>
  <p>{'word':'string','table':'string'}</p>
  <p>I want to write into BigQuery only the words, however I need the table field to be able to select the right table in BigQuery.</p>
  <p>This is how my pipeline looks:</p>
  <pre><code>tobq = (input
          | 'write names to BigQuery '&gt;&gt; beam.io.gcp.bigquery.WriteToBigQuery(
            table=compute_table_name, schema=compute_schema,
            insert_retry_strategy='RETRY_ON_TRANSIENT_ERROR',
            create_disposition=beam.io.gcp.bigquery.BigQueryDisposition.CREATE_IF_NEEDED,
            write_disposition=beam.io.gcp.bigquery.BigQueryDisposition.WRITE_APPEND)
         )
  </code></pre>
  <p>The function compute_table_name accesses an element and returns the table field. Is there a way to write into BQ just the words while still having this table selection mechanism based on rows?</p>
  <p>Many thanks!</p>