Google Dataflow: PCollection<String> to PCollection<TableRow> for BigQuery insertion

Question

I'm fairly new to the Google Cloud Platform and I'm trying Google Dataflow for the first time for a project for my postgraduate programme. What I want to do is write an automated load job that loads files from a certain bucket on my Cloud Storage and inserts the data from it into a BigQuery table.

I get the data as a PCollection<String> type, but for insertion in BigQuery I apparently need to transform it to a PCollection<TableRow> type. So far I haven't found a solid answer to do this.

Here's my code:

public static void main(String[] args) {
    //Defining the schema of the BigQuery table
    List<TableFieldSchema> fields = new ArrayList<>();
    fields.add(new TableFieldSchema().setName("Datetime").setType("TIMESTAMP"));
    fields.add(new TableFieldSchema().setName("Consumption").setType("FLOAT"));
    fields.add(new TableFieldSchema().setName("MeterID").setType("STRING"));
    TableSchema schema = new TableSchema().setFields(fields);

    //Creating the pipeline
    PipelineOptions options = PipelineOptionsFactory.fromArgs(args).withValidation().create();
    Pipeline p = Pipeline.create(options);

    //Getting the data from cloud storage
    PCollection<String> lines = p.apply(TextIO.Read.named("ReadCSVFromCloudStorage").from("gs://mybucket/myfolder/certainCSVfile.csv"));

    //Probably need to do some transform here ...

    //Inserting data into BigQuery
    lines.apply(BigQueryIO.Write
            .named("WriteToBigQuery")
            .to("projectID:datasetID:tableID")
            .withSchema(schema)
            .withWriteDisposition(BigQueryIO.Write.WriteDisposition.WRITE_APPEND)
            .withCreateDisposition(BigQueryIO.Write.CreateDisposition.CREATE_IF_NEEDED));
}

I'm probably just forgetting something basic, so I hope you guys can help me with this ...

Answer 1

BigQueryIO.Write operates on PCollection<TableRow>, as outlined in Writing to BigQuery. You'll need to apply a transform to convert PCollection<TableRow>into PCollection<String>. For an example, take a look at StringToRowConverter:

  static class StringToRowConverter extends DoFn<String, TableRow> {
    /**
     * In this example, put the whole string into single BigQuery field.
     */
    @Override
    public void processElement(ProcessContext c) {
      c.output(new TableRow().set("string_field", c.element()));
    }
  ...
}