This code snippet flattens multiple lists within a list, joins the resulting list to a related list where the two share an ID, and lastly, creates a comma delimited string by joining a list of strings.

ListWithinList.cs

    /// <summary>
    /// Using https://msdn.microsoft.com/en-us/library/bb534336(v=vs.110).aspx as a guide,
    /// here's how I flattend out a list within a list.
    /// </summary>
    public class ListWithinList
    {
        public static void Main(string[] args)
        {
            var widgets = new List<Widget>
            {
                new Widget { WidgetID = 1 },
                new Widget { WidgetID = 2 },
            };

            var factories = new List<Factory>
            {
                new Factory
                {
                    Parts = new List<Part>
                    {
                        new Part { PartID = 1, PartDescription = "Hammer" },
                        new Part { PartID = 1, PartDescription = "Screwdriver" },
                        new Part { PartID = 1, PartDescription = "Sharpener" },

                        new Part { PartID = 2, PartDescription = "Saw" },
                        new Part { PartID = 2, PartDescription = "Rake" },
                    }
                }
            };

            // Flatten
            IEnumerable<Part> parts = factories.SelectMany(f => f.Parts);

            // With a little help from John Skeet
            // http://stackoverflow.com/questions/7325278/group-by-in-linq
            IEnumerable<Widget> updatedWidgets = from widget in widgets
                                                 join part in parts on widget.WidgetID equals part.PartID
                                                 group part.PartDescription by part.PartID into partsGrouping
                                                 select new Widget
                                                 {
                                                     WidgetID = partsGrouping.Key,
                                                     WidgetDescription = string.Join(", ", partsGrouping.ToList())
                                                 };
            // Outputs
            // [0]{ WidgetID = 1, Description = "Hammer, Screwdriver, Sharpener" }
            // [1]{ WidgetID = 2, Description = "Saw, Rake" }
        }
    }

    public class Widget
    {
        public int WidgetID { get; set; }
        public string WidgetDescription { get; set; }
    }

    public class Factory
    {
        public List<Part> Parts { get; set; }
    }

    public class Part
    {
        public int PartID { get; set; }
        public string PartDescription { get; set; }
    }