Many-to-many collection of same entity, with two-way relationship

Question

Suppose I have a widget entity, and I want to track other widgets that are adjacent to each. If the first widget is adjacent to the second widget, then the inverse is also t

Answer 1

You need to introduce another collection inside the widget, something like.

public virtual ICollection<Widget> AdjacentFrom { get; set; }
public virtual ICollection<Widget> AdjacentTo { get; set; }

By default with no fluent-api configuration, this code only will create a container table of WidgetWidgets in the database that contains two columns Widget_Id and Widget_Id1.

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But you need to be consistent to only use one of the collection to make an adjacent relationship. If you use AdjacentTo collection to make an adjacent relationship.

widget1.AdjacentTo.Add(widget2);

After being saved widget1.AdjacentTo will have widget2 and widget2.AdjacentFrom will have widget1.

Widget_Id   Widget_Id1
    2           1

But if you input again with AdjacentFrom collection to make an adjacent relationship.

widget1.AdjacentFrom.Add(widget2);

After being saved widget1.AdjacentFrom and widget1.AdjacentTo will have widget2. Same thing happens with widget2.

Widget_Id   Widget_Id1
    2           1
    1           2

Composite unique key can't prevent second record to be inserted, because the second record is not considered as duplicate row. But there is a workaround by adding a check constraint, you can add this constraint in the migration.

Sql("alter table WidgetWidgets add constraint CK_Duplicate_Widget check (Widget_Id > Widget_Id1)");

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To select all adjacent you can add another collection, something like.

[NotMapped]
public ICollection<Widget> Adjacent
{
   get { return (AdjacentFrom ?? new Widget[0]).Union((AdjacentTo ?? new Widget[0])).Distinct().ToArray(); }
}

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After adding check constraint, then you can use this extension to add or remove adjacent.

public static class WidgetDbExtension
{
    public static void AddAdjacent(this Widget widget1, Widget widget2)
    {
        if (widget1.Id < widget2.Id)
        {
            widget1.AdjacentTo.Add(widget2);
        }
        else
        {
            widget2.AdjacentTo.Add(widget1);
        }
    }
    public static void RemoveAdjacent(this Widget widget1, Widget widget2)
    {
        if (widget1.Id < widget2.Id)
        {
            widget1.AdjacentTo.Remove(widget2);
        }
        else
        {
            widget2.AdjacentTo.Remove(widget1);
        }
    }
}