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ARROW-5191: [Rust] Expose CSV and JSON reader schemas #4181

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ARROW-5191: [Rust] Expose CSV and JSON reader schemas #4181

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36 changes: 33 additions & 3 deletions rust/arrow/src/csv/reader.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -217,6 +217,21 @@ impl<R: Read> Reader<R> {
)
}

/// Returns the schema of the reader, useful for getting the schema without reading
/// record batches
pub fn schema(&self) -> Arc<Schema> {
match &self.projection {
Some(projection) => {
let fields = self.schema.fields();
let projected_fields: Vec<Field> =
projection.iter().map(|i| fields[*i].clone()).collect();

Arc::new(Schema::new(projected_fields))
}
None => self.schema.clone(),
}
}

/// Create a new CsvReader from a `BufReader<R: Read>
///
/// This constructor allows you more flexibility in what records are processed by the
Expand Down Expand Up @@ -536,7 +551,8 @@ mod tests {

let file = File::open("test/data/uk_cities.csv").unwrap();

let mut csv = Reader::new(file, Arc::new(schema), false, 1024, None);
let mut csv = Reader::new(file, Arc::new(schema.clone()), false, 1024, None);
assert_eq!(Arc::new(schema), csv.schema());
let batch = csv.next().unwrap().unwrap();
assert_eq!(37, batch.num_rows());
assert_eq!(3, batch.num_columns());
Expand Down Expand Up @@ -594,6 +610,12 @@ mod tests {
let builder = ReaderBuilder::new().has_headers(true).infer_schema(None);

let mut csv = builder.build(file).unwrap();
let expected_schema = Schema::new(vec![
Field::new("city", DataType::Utf8, false),
Field::new("lat", DataType::Float64, false),
Field::new("lng", DataType::Float64, false),
]);
assert_eq!(Arc::new(expected_schema), csv.schema());
let batch = csv.next().unwrap().unwrap();
assert_eq!(37, batch.num_rows());
assert_eq!(3, batch.num_columns());
Expand Down Expand Up @@ -625,14 +647,16 @@ mod tests {
let builder = ReaderBuilder::new().infer_schema(None);

let mut csv = builder.build(file).unwrap();
let batch = csv.next().unwrap().unwrap();

// csv field names should be 'column_{number}'
let schema = batch.schema();
let schema = csv.schema();
assert_eq!("column_1", schema.field(0).name());
assert_eq!("column_2", schema.field(1).name());
assert_eq!("column_3", schema.field(2).name());
let batch = csv.next().unwrap().unwrap();
let batch_schema = batch.schema();

assert_eq!(&schema, batch_schema);
assert_eq!(37, batch.num_rows());
assert_eq!(3, batch.num_columns());

Expand Down Expand Up @@ -667,7 +691,13 @@ mod tests {
let file = File::open("test/data/uk_cities.csv").unwrap();

let mut csv = Reader::new(file, Arc::new(schema), false, 1024, Some(vec![0, 1]));
let projected_schema = Arc::new(Schema::new(vec![
Field::new("city", DataType::Utf8, false),
Field::new("lat", DataType::Float64, false),
]));
assert_eq!(projected_schema.clone(), csv.schema());
let batch = csv.next().unwrap().unwrap();
assert_eq!(&projected_schema, batch.schema());
assert_eq!(37, batch.num_rows());
assert_eq!(2, batch.num_columns());
}
Expand Down
43 changes: 40 additions & 3 deletions rust/arrow/src/json/reader.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -345,6 +345,29 @@ impl<R: Read> Reader<R> {
}
}

/// Returns the schema of the reader, useful for getting the schema without reading
/// record batches
pub fn schema(&self) -> Arc<Schema> {
match &self.projection {
Some(projection) => {
let fields = self.schema.fields();
let projected_fields: Vec<Field> = fields
.iter()
.filter_map(|field| {
if projection.contains(field.name()) {
Some(field.clone())
} else {
None
}
})
.collect();

Arc::new(Schema::new(projected_fields))
}
None => self.schema.clone(),
}
}

/// Read the next batch of records
pub fn next(&mut self) -> Result<Option<RecordBatch>> {
let mut rows: Vec<Value> = Vec::with_capacity(self.batch_size);
Expand Down Expand Up @@ -742,7 +765,9 @@ mod tests {
assert_eq!(4, batch.num_columns());
assert_eq!(12, batch.num_rows());

let schema = batch.schema();
let schema = reader.schema();
let batch_schema = batch.schema();
assert_eq!(&schema, batch_schema);

let a = schema.column_with_name("a").unwrap();
assert_eq!(0, a.0);
Expand Down Expand Up @@ -798,7 +823,9 @@ mod tests {
assert_eq!(4, batch.num_columns());
assert_eq!(12, batch.num_rows());

let schema = batch.schema();
let schema = reader.schema();
let batch_schema = batch.schema();
assert_eq!(&schema, batch_schema);

let a = schema.column_with_name("a").unwrap();
assert_eq!(&DataType::Int64, a.1.data_type());
Expand Down Expand Up @@ -855,10 +882,12 @@ mod tests {

let mut reader: Reader<File> = Reader::new(
BufReader::new(File::open("test/data/basic.json").unwrap()),
Arc::new(schema),
Arc::new(schema.clone()),
1024,
None,
);
let reader_schema = reader.schema();
assert_eq!(reader_schema, Arc::new(schema));
let batch = reader.next().unwrap().unwrap();

assert_eq!(4, batch.num_columns());
Expand Down Expand Up @@ -909,13 +938,21 @@ mod tests {
1024,
Some(vec!["a".to_string(), "c".to_string()]),
);
let reader_schema = reader.schema();
let expected_schema = Arc::new(Schema::new(vec![
Field::new("a", DataType::Int32, false),
Field::new("c", DataType::Boolean, false),
]));
assert_eq!(reader_schema.clone(), expected_schema);

let batch = reader.next().unwrap().unwrap();

assert_eq!(2, batch.num_columns());
assert_eq!(2, batch.schema().fields().len());
assert_eq!(12, batch.num_rows());

let schema = batch.schema();
assert_eq!(&reader_schema, schema);

let a = schema.column_with_name("a").unwrap();
assert_eq!(0, a.0);
Expand Down