title: Airflow - BigQuery Invalid value at schema fields
body: <p>I have a GCP cloud composer to load the data into BQ from GCS. Im using <code>schema_fields</code> option to pass the source schema. Im passing the source schema in a variable. Im getting this from <code>xcom pull</code>. See the <code>print(schema)</code> below.</p>
  <pre><code>[{&quot;mode&quot;: &quot;NULLABLE&quot;, &quot;name&quot;: &quot;id&quot;, &quot;type&quot;: &quot;INTEGER&quot;}, {&quot;mode&quot;: &quot;NULLABLE&quot;, &quot;name&quot;: &quot;c1&quot;, &quot;type&quot;: &quot;DATE&quot;}, {&quot;mode&quot;: &quot;NULLABLE&quot;, &quot;name&quot;: &quot;c2&quot;, &quot;type&quot;: &quot;TIME&quot;}, {&quot;mode&quot;: &quot;NULLABLE&quot;, &quot;name&quot;: &quot;c3&quot;, &quot;type&quot;: &quot;DATETIME&quot;}, {&quot;mode&quot;: &quot;NULLABLE&quot;, &quot;name&quot;: &quot;c4&quot;, &quot;type&quot;: &quot;TIMESTAMP&quot;}]
  </code></pre>
  <p>And in BQ operator I use <code>schema_fields=schema</code>.</p>
  <p>But when I run the dag, its throwing error.</p>
  <pre class="lang-py prettyprint-override"><code>ERROR - &lt;HttpError 400 when requesting https://bigquery.googleapis.com/bigquery/v2/projects/xxx-xxx/jobs?
  alt=json returned &quot;Invalid value at 'job.configuration.load.schema.fields' (type.googleapis.com/google.cloud.bigquery.v2.TableFieldSchema), 
  &quot;[{&quot;mode&quot;: &quot;NULLABLE&quot;, &quot;name&quot;: &quot;id&quot;, &quot;type&quot;: &quot;INTEGER&quot;}, {&quot;mode&quot;: &quot;NULLABLE&quot;, &quot;name&quot;: &quot;c1&quot;, &quot;type&quot;: &quot;DATE&quot;}, {&quot;mode&quot;: &quot;NULLABLE&quot;, &quot;name&quot;: &quot;c2&quot;, &quot;type&quot;: &quot;TIME&quot;}, {&quot;mode&quot;: &quot;NULLABLE&quot;, &quot;name&quot;: &quot;c3&quot;, &quot;type&quot;: &quot;DATETIME&quot;}, {&quot;mode&quot;: &quot;NULLABLE&quot;, &quot;name&quot;: &quot;c4&quot;, &quot;type&quot;: &quot;TIMESTAMP&quot;}]&quot;&quot;&gt;
  </code></pre>
  <p>But when I save this schema as a file in GCS and tried with <code>schema_object</code> then it worked.  But the same thing via variable didn't work.</p>