Improvement: parse query results with user-supplied function

[paul-avisseau]

First and foremost, thanks for the great job done here!

I am running a website with ExpressJs that executes a bunch of queries that return arrays of data. I was wondering if there is a possibility to turn the raw data from the database into meaningful objects in a simple manner. I usually use queries and then walk through their results with a for loop in order to turn the raw data into objects that can be serialized with JSON.

let query = {
    text: 'SELECT ...',
    values: [ ... ],
    rowMode: 'array'
};
dbClient.query(query, (error, results) => {
    let data = new Array(results.rows.length);
    let i = 0;
    for (let row of results.rows) {
        data[i++] = MyClass.fromDatabase(row);
    }
    response.status(200).json(data);
});

This actually creates two arrays: the one that comes from PostgreSQL, and the one I create from the results. Not very good for memory!
Instead, it would be much simpler I think to specify a transformation function in the query object, in order to directly turn the raw data into objects:

let query = {
    text: 'SELECT ...',
    values: [ ... ],
    rowMapper: MyClass.fromDatabase
};
dbClient.query(query, (error, results) => {
    response.status(200).json(results.rows);
});

What I propose is to add a function to the query object, that will allow to convert a single row to an object. The outcome of the function will be added to the final results array. No more memory overhead.

The template for the function would be:

function convert(row, rowNumber);

In the Java world, Spring JDBC has similar concepts (RowMapper), and it is very efficient.