How can I use a projection instead of raw columns

I have a dataset with columns for

number of items (an integer value)
weight of the items (a fractional value)
A product category
A start time
An end time

and from this dataset I want to train a data model that given a number of items, weight and category can predict a duration (end time - start time).

How can I transform or set my label column to the duration, so that I get an EstimatorChain that I can call Fit on with an IDataView that I've loaded from CSV?

Solution

You can use the ML.NET CustomMapping functionality, to calculate the duration, and call that Label. This contains an example of how to use it.

Action<Data, Data> mapping =
            (input, output) => output.Label= input.End - Input.Start;

where Data would be your data model that contains a Label property, in addition to the other properties.