ItemApi-Aggregation.cs

public class Item
{
    public string Name { get; }
    public string Id { get; }
    public string ColourId { get; }

    public Item(string name, string id, string colourId)
    {
        Name = name ?? throw new ArgumentNullException(nameof(name));
        Id = id ?? throw new ArgumentNullException(nameof(id));
        ColourId = colourId ?? throw new ArgumentNullException(nameof(colourId));
    }
}


public class ItemController : Controller
{
    public static string RedId = "abc123";
    public static string YellowId = "def456";
    public static string BlueId = "ghi789";

    [HttpGet("/items/{colourId}")]
    public IActionResult Get(string colourId)
    {
        if (string.Equals(colourId, RedId, StringComparison.OrdinalIgnoreCase))
        {
            return Ok(
                new {
                    Items = new [] {
                        new Item("Rose", "1", RedId)
                    }});
        }

        if (string.Equals(colourId, BlueId, StringComparison.OrdinalIgnoreCase))
        {
            return Ok(
                new {
                    Items = new [] {
                        new Item("Violet", "2", BlueId)
                    }});
        }

        return Ok(new { Items = new Item[0]});
    }
}