How can one define the Selector in ardalis.Specification library?

Question

I am trying to utilize the Ardalis.Specification library to apply the specification pattern in my asp.net 6 project.

After installing the library, I created the following specification

public class ProductByIdsSpec : Specification<Product, ProductMenuItem>
{
    public ClientRecordByIdsSpec(IEnumerable<int> ids)
    {
        if (ids == null || !ids.Any())
        {
            return;
        }

        Query.Where(x => ids.Contains(x.Id));


        // some how I need to map Product to ProductMenuItem so only the needed columns are pulled from the database.
    }

}

Instead of pulling every value in Product from the database, I want to only pull the needed data by projecting the data to ProductMenuItem. The above specification is returning the following error

SelectorNotFoundException Ardalis.Specification.SelectorNotFoundException: The specification must have Selector defined

How can I define the map between entity (i.e., Product) and and the result object (i.e., ProductMenuItem)?

I tried to add Select() definition but is giving me the same error

public class ProductByIdsSpec : Specification<Product, ProductMenuItem>
{
    public ClientRecordByIdsSpec(IEnumerable<int> ids)
    {
        if (ids == null || !ids.Any())
        {
            return;
        }

        Query.Where(x => ids.Contains(x.Id));


        Query.Select(x => new ProductMenuItem() { Name = x.Name, x.Id = x.Id });
    }

}

Answer 1

As of today, this works for me. In the current implementation of the lib the Selector property is set via ISpecificationBuilder which is not at all obvious. I'd rather check unit tests and code than docs.

public class ProductByIdsSpec : Specification<Product, ProductMenuItem>
{
    public ClientRecordByIdsSpec(IEnumerable<int> ids)
    {
        if (ids == null || !ids.Any())
        {
            return;
        }

        Query
            .Select(x => new ProductMenuItem() { Name = x.Name, x.Id = x.Id });
            .Where(x => ids.Contains(x.Id));
    }

}

Answer 2

public class ProductByIdsSpec : Specification<Product, ProductMenuItem>
{
    public ClientRecordByIdsSpec(IEnumerable<int> ids)
    {
        ...
    }

    public override Expression<Func<Product, ProductMenuItem>> Selector { get; }
        = (product) => new ProductMenuItem();
}

You can override the Selector property in your Specification class and implement your projection there

https://github.com/ardalis/Specification/blob/main/Specification/src/Ardalis.Specification/Specification.cs#L29