Question

I have some models setup as follows:

public class Form
{
    public int FormId { get; set; }
    public virtual ICollection<SubForm> SubForms{ get; set; }
}
public class SubForm
{
    public int SubFormId { get; set; }
    public virtual Form Form {get; set;}
}

I have a list like this:

List<SubForm> subForms; <-- already populated, each subform has a form parent.

How do I get it in a format like this optimally (lambda preferred)

List<Form> formsWithChildren;

Answer 1

You can group by the FormId then assign the grouped subforms to each new Form item as follows:

var formsWithChildren = subForms.GroupBy(s => s.Form.FormId)
                                .Select(g => new Form
                                {
                                    FormId = g.Key,
                                    SubForms = g.ToList()
                                }).ToList();

EDIT: if other properties exist then you would have to map them in the object initializer, but this could become tedious, and isn’t the best option if you intend to reuse this type of query in other scenarios. I suggest creating a new Form object and mapping it using AutoMapper.

If you decide to use AutoMapper the approach would be similar to this:

var formsWithChildren = new List<Form>();
foreach (var item in subForms.GroupBy(s => s.Form))
{
    Form f = Mapper.Map<Form, Form>(item.Key);
    f.SubForms = item.Select(s => s).ToList();
    formsWithChildren.Add(f);
}

To learn more refer to the AutoMapper site.

Alternately, you could take an approach as follows, although this modifies the existing items so be aware of that.

var formsWithChildren = new List<Form>();
foreach (var item in subForms.GroupBy(s => s.Form))
{
    // re-use existing Form from grouping so it retains its assigned values
    Form f = item.Key;
    f.SubForms = item.Select(s => s).ToList();
    formsWithChildren.Add(f);
}

In both approaches shown above you will need to implement Equals on the Form class in order to GroupBy the Form instead of FormId:

public class Form
{
    public int FormId { get; set; }
    public virtual ICollection<SubForm> SubForms{ get; set; }

    public override bool Equals(object obj)
    {
        if (obj == null)
            return false;

        Form f = obj as Form;            
        return this.Equals(f);
    }

    public bool Equals(Form f)
    {
        if (f == null)
            return false;

        return this.FormId == f.FormId;
    }

    public override int GetHashCode()
    {
        return this.FormId.GetHashCode();
    }
}

You could extend the equality and GetHashCode methods to use the properties to determine uniqueness. Currently it only relies on the FormId.