Polars Rust: Read json file like Data Fusion or Spark?

How can I read a JSON file with polars, with the following format:

{<json object>},
{<json object>}

I can read the same file in DataFusion as follows:

#[tokio::main]
async fn main() -> datafusion::error::Result<()> {
    env_logger::init();
    let start = Instant::now();
    let file_path = "datalayers/landing/Toys_and_Games_5.json";
    // let file_path = "datalayers/landing/test_file.json";
    let df = read_data(file_path.to_string()).await?;
    let duration = start.elapsed();
    info!{"Pipeline executed successfully!"}
    info!("Pipeline Execution time: {:?}", duration);
    Ok(())
}

async fn read_data(path: String) -> datafusion::error::Result<Arc<DataFrame>> {
    let mut ctx = SessionContext::new();
    let selected_columns = vec![
        "asin",
        "vote",
        "verified",
        "unixReviewTime",
        "reviewTime",
        "reviewText",
    ];
    let df_ = ctx.read_json(path, NdJsonReadOptions::default()).await?;
    let df_ = df_.select_columns(&selected_columns)?;
    info!("Data loading plan created successfully!");
    Ok(df_)
}

The spark code is quite similar. The only reference I found for polars in old API documentation with JsonReader and Cursor. But it does not show how to read data from file. The File in the example can be downloaded with wget as follows:

wget -P datalayers/landing http://deepyeti.ucsd.edu/jianmo/amazon/categoryFilesSmall/Toys_and_Games_5.json.gz

Solution

Using the latest polars as of 2022 You have to make sure that "json" is added as to the feature in [dependencies] in Cargo for polars. For example,

[dependencies]
polars = { version="0.24.2", features = ["lazy", "json"] }
tokio = { version = "1.21.1", features = ["full"] }

Now for reading line-separated json (I know there is an official name but it escapes me right now) into a dataframe:

use polars::prelude::*;

fn main() -> PolarsResult<()> {
    let schema = Schema::from(vec![
        Field::new("reviwerID", DataType::Utf8),
        Field::new("asin", DataType::Utf8),
        Field::new("reviewerName", DataType::Utf8),
        Field::new("helpful", DataType::List(Box::new(DataType::Int32))),
        Field::new("reviewText", DataType::Utf8),
        Field::new("overall", DataType::Float64),
        Field::new("summary", DataType::Utf8),
        Field::new("unixReviewTime", DataType::Int64),
        Field::new("reviewTime", DataType::Utf8),
        Field::new("style", DataType::Utf8),
    ]);
    let df = match LazyJsonLineReader::new("Toys_and_Games_5.ndjson".into())
        .with_schema(schema)
        .finish() {
        Ok(lf) => lf,
        Err(e) => panic!("Error: {}", e),
    }
    .collect();
    println!("{:?}", df);
    Ok(())
}

The output is:

Ok(shape: (3695, 10)
┌───────────┬────────────┬──────────────────────────┬─────────┬─────┬──────────────────────────────────┬────────────────┬─────────────┬───────┐
│ reviwerID ┆ asin       ┆ reviewerName             ┆ helpful ┆ ... ┆ summary                          ┆ unixReviewTime ┆ reviewTime  ┆ style │
│ ---       ┆ ---        ┆ ---                      ┆ ---     ┆     ┆ ---                              ┆ ---            ┆ ---         ┆ ---   │
│ str       ┆ str        ┆ str                      ┆ i32     ┆     ┆ str                              ┆ i64            ┆ str         ┆ str   │
╞═══════════╪════════════╪══════════════════════════╪═════════╪═════╪══════════════════════════════════╪════════════════╪═════════════╪═══════╡
│ null      ┆ 0486427706 ┆ Ginger                   ┆ null    ┆ ... ┆ Nice book                        ┆ 1381017600     ┆ 10 6, 2013  ┆ null  │
├╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┤
│ null      ┆ 0486427706 ┆ Dragonflies &amp; Autumn ┆ null    ┆ ... ┆ Great pictures                   ┆ 1376006400     ┆ 08 9, 2013  ┆ null  │
│           ┆            ┆ Leaves                   ┆         ┆     ┆                                  ┆                ┆             ┆       │
├╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┤
│ null      ┆ 0486427706 ┆ barbara ann              ┆ null    ┆ ... ┆ The pictures are great, I've     ┆ 1459814400     ┆ 04 5, 2016  ┆ null  │
│           ┆            ┆                          ┆         ┆     ┆ don...                           ┆                ┆             ┆       │
├╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┤
│ null      ┆ 0486427706 ┆ Samantha                 ┆ null    ┆ ... ┆ So beautiful!                    ┆ 1455321600     ┆ 02 13, 2016 ┆ null  │
├╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┤
│ ...       ┆ ...        ┆ ...                      ┆ ...     ┆ ... ┆ ...                              ┆ ...            ┆ ...         ┆ ...   │
├╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┤
│ null      ┆ B00IEOH8KO ┆ A. Red                   ┆ null    ┆ ... ┆ Made party decorating easy and   ┆ 1453939200     ┆ 01 28, 2016 ┆ null  │
│           ┆            ┆                          ┆         ┆     ┆ a...                             ┆                ┆             ┆       │
├╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┤
│ null      ┆ B00IEOH8KO ┆ nilda morales            ┆ null    ┆ ... ┆ Four Stars                       ┆ 1452988800     ┆ 01 17, 2016 ┆ null  │
├╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┤
│ null      ┆ B00IEOH8KO ┆ W. Ross                  ┆ null    ┆ ... ┆ Five Stars                       ┆ 1449014400     ┆ 12 2, 2015  ┆ null  │
├╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌╌╌╌╌╌╌┼╌╌╌╌╌╌╌┤
│ null      ┆ B00IEOH8KO ┆ Liz89                    ┆ null    ┆ ... ┆ Great Deal                       ┆ 1431129600     ┆ 05 9, 2015  ┆ null  │
└───────────┴────────────┴──────────────────────────┴─────────┴─────┴──────────────────────────────────┴────────────────┴─────────────┴───────┘)

#IMPORTANT NOTE

Notice that I had to explicitly provide the schema because the "style" column in your data-set is completely weird, which means that polars has difficulty inferring it. If you fill out the complete schema it should work, so I didn't bother creating a Struct for it, however you can go ahead and do that if you like :).