[jira] [Created] (BEAM-2595) WriteToBigQuery does not work with nested json schema

["Andrea Pierleoni (JIRA)" <ji...@apache.org>]

Andrea Pierleoni created BEAM-2595:
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             Summary: WriteToBigQuery does not work with nested json schema
                 Key: BEAM-2595
                 URL: https://issues.apache.org/jira/browse/BEAM-2595
             Project: Beam
          Issue Type: Bug
          Components: runner-dataflow
    Affects Versions: 2.1.0
         Environment: mac os local runner, Python
            Reporter: Andrea Pierleoni
            Assignee: Thomas Groh
            Priority: Minor


I am trying to use the new `WriteToBigQuery` PTransform added to `apache_beam.io.gcp.bigquery` in version 2.1.0-RC1

I need to write to a bigquery table with nested fields.
The only way to specify nested schemas in bigquery is with teh json schema.
None of the classes in `apache_beam.io.gcp.bigquery` are able to parse the json schema, but they accept a schema as an instance of the class `apache_beam.io.gcp.internal.clients.bigquery.TableFieldSchema`

I am composing the `TableFieldSchema` as suggested [here](https://stackoverflow.com/questions/36127537/json-table-schema-to-bigquery-tableschema-for-bigquerysink/45039436#45039436), and it looks fine when passed to the PTransform `WriteToBigQuery`. 

The problem is that the base class `PTransformWithSideInputs` try to [pickle and unpickle the function](https://github.com/apache/beam/blob/master/sdks/python/apache_beam/transforms/ptransform.py#L515)  (that includes the TableFieldSchema instance) and for some reason when the class is unpickled some `FieldList` instance are converted to simple lists, and the pickling validation fails.

Would it be possible to extend the test coverage to nested json objects for bigquery?
They are also relatively easy to parse into a TableFieldSchema.




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