Field Selection

In addition to computing aggregates, we can also return fields selected directly from the rows themselves.

This is done by mapping over the computed rows, and using the eval_field function to evaluate each selected field in turn:

    let rows = query
        .fields
        .as_ref()
        .map(|fields| {
            let mut rows: Vec<IndexMap<models::FieldName, models::RowFieldValue>> = vec![];
            for item in &paginated {
                let row = eval_row(fields, collection_relationships, variables, state, item)?;
                rows.push(row);
            }
            Ok(rows)
        })
        .transpose()?;

The eval_field function works by pattern matching on the field type:

• A column is selected using the eval_column function (or eval_nested_field if there are nested fields to fetch)
• A relationship field is selected by evaluating the related collection using eval_path_element (we will cover this in the next section), and then recursively executing a query using execute_query:

fn eval_field(
    collection_relationships: &BTreeMap<models::RelationshipName, models::Relationship>,
    variables: &BTreeMap<models::VariableName, serde_json::Value>,
    state: &AppState,
    field: &models::Field,
    item: &Row,
) -> Result<models::RowFieldValue> {
    match field {
        models::Field::Column {
            column,
            fields,
            arguments,
        } => {
            let col_val = eval_column(variables, item, column, arguments)?;
            match fields {
                None => Ok(models::RowFieldValue(col_val)),
                Some(nested_field) => eval_nested_field(
                    collection_relationships,
                    variables,
                    state,
                    col_val,
                    nested_field,
                ),
            }
        }
        models::Field::Relationship {
            relationship,
            arguments,
            query,
        } => {
            let relationship = collection_relationships.get(relationship).ok_or((
                StatusCode::BAD_REQUEST,
                Json(models::ErrorResponse {
                    message: "relationship is undefined".into(),
                    details: serde_json::Value::Null,
                }),
            ))?;
            let source = vec![item.clone()];
            let collection = eval_path_element(
                collection_relationships,
                variables,
                state,
                relationship,
                arguments,
                &source,
                None,
                &None,
            )?;
            let row_set = execute_query(
                collection_relationships,
                variables,
                state,
                query,
                Root::Reset,
                collection,
            )?;
            let row_set_json = serde_json::to_value(row_set).map_err(|_| {
                (
                    StatusCode::INTERNAL_SERVER_ERROR,
                    Json(models::ErrorResponse {
                        message: "cannot encode rowset".into(),
                        details: serde_json::Value::Null,
                    }),
                )
            })?;
            Ok(models::RowFieldValue(row_set_json))
        }
    }
}