How to build object hierarchy from SQL query? (for WPF TreeView)

Question

thanks for taking the time out to read this post.

I'm having trouble trying to build a hierarchial object when getting data from my SQL database. Please note that I am a little bit of a newbie programmer.

How do you build a hierarchial object that has unknown levels? When I say unknown levels I mean, each node may have varying numbers of child nodes, which in turn may have varying numbers of its own child nodes, so on and so on.

The idea is that I need to create a hierarchial object using my SQL data to bind to WPF TreeView control.

Below I have included the code I have so far. The first bit of code is my Class made up of Properties. Note that the "Products" class has an ObservableCollection referencing itself. I think this is how you construct the nested nodes. i.e. a list inside a list.

The second piece of code is my Get Method to download the data from the SQL database. Here is where I need to some how sort the downloaded data into a hierarchy.

Products Class (properties)

public class Products : INotifyPropertyChanged, IDataErrorInfo
{
    private Int64 m_ID;
    private SqlHierarchyId m_Hierarchy;
    private string m_Name;
    private ObservableCollection<Products> m_ChildProducts;

    // Default Constructor
    public Products()
    {
        ChildProducts = new ObservableCollection<Products>();
    }

    //Properties

    public Int64 ID
    {
        get
        {
            return m_ID;
        }
        set
        {
            m_ID = value;
            OnPropertyChanged(new PropertyChangedEventArgs("ID"));
        }
    }

    public SqlHierarchyId Hierarchy
    {
        get
        {
            return m_Hierarchy;
        }
        set
        {
            m_Hierarchy = value;
            OnPropertyChanged(new PropertyChangedEventArgs("Hierarchy"));
        }
    }

    public String Name
    {
        get
        {
            return m_Name;
        }
        set
        {
            m_Name = value;
            OnPropertyChanged(new PropertyChangedEventArgs("Name"));
        }
    }

    public Int16 Level
    {
        get
        {
            return m_Level;
        }
        set
        {
            m_Level = value;
            OnPropertyChanged(new PropertyChangedEventArgs("Level"));
        }
    }

    public Int64 ParentID
    {
        get
        {
            return m_ParentID;
        }
        set
        {
            m_ParentID = value;
            OnPropertyChanged(new PropertyChangedEventArgs("ParentID"));
        }
    }

    public ObservableCollection<Products> ChildProducts
    {
        get
        {
            return m_ChildProducts;
        }
        set
        {
            m_ChildProducts = value;
            OnPropertyChanged(new PropertyChangedEventArgs("ChildProducts"));
        }
    }

    //INotifyPropertyChanged Event
    public event PropertyChangedEventHandler PropertyChanged;
    public void OnPropertyChanged(PropertyChangedEventArgs e)
    {
        if (PropertyChanged != null)
            PropertyChanged(this, e);
    }
}

Method which gets data from SQL DB:

public static ObservableCollection<Products> GetProductsHierarchy()
    {

        ObservableCollection<Products> productsHierarchy = new ObservableCollection<Products>();

        SqlConnection connection = new SqlConnection(DBConnection.GetConnection().ConnectionString);

        string selectStatement = "SELECT ID, Hierarchy, Name, Hierarchy.GetLevel() AS Level, Hierarchy.GetAncestor(1) AS ParentHierarchy, " +
                                                 "(SELECT ID " +
                                                 "FROM SpecProducts " +
                                                 "WHERE (Hierarchy = SpecProducts_1.Hierarchy.GetAncestor(1))) AS ParentID " +
                                 "FROM  SpecProducts AS SpecProducts_1 " +
                                 "WHERE (EnableDisable IS NULL) " +
                                 "ORDER BY Hierarchy";

        SqlCommand selectCommand = new SqlCommand(selectStatement, connection);

        try
        {
            connection.Open();
            SqlDataReader reader = selectCommand.ExecuteReader();

            while (reader.Read())
            {

                Products product = new Products();
                product.ID = (Int64)reader["ID"];
                product.Name = reader["Name"].ToString();
                product.Hierarchy = (SqlHierarchyId)reader["Hierarchy"];
                product.Level = (Int16)reader["Level"];
                if (reader["ParentID"] != DBNull.Value)
                {
                    product.ParentID = (Int64)reader["ParentID"];
                }
                else
                {
                    product.ParentID = 0;
                }

                productsHierarchy.Add(product);

                // *** HOW TO BUILD HIERARCHY OBJECT WITH UNKNOWN & VARYING LEVELS?
                // *** ADD PRODUCT TO CHILDPRODUCT
            }

            return productsHierarchy;
        }
        catch (SqlException ex)
        {
            throw ex;
        }
        finally
        {
            connection.Close();
        }
    }

Below I have attached an image showing the structure of my SQL Query Data. Please note that the hierarchy level may go deeper when more products are added in the future. The Hierarchy object I need to create should be flexible enough to expand no matter what the number of node levels are.

Thank you very much for your time, all help is greatly appreciated.

********* EDIT 26/04/2012 14:37 *******************

Please find below a link to download my project code (this only contains treeview code). Can someone please take a look at it to see why I cannot create nodes beyond 2 levels?

The code was given to me by user HB MAAM. Thank you "HB MAAM" for your help so far!

Click this link to download code

Answer 1

I will create an example for you,
1- first i will create a class that holds the data that comes from the DB

public class SqlDataDto
{
    public int? Id { get; set; }
    public int? ParentId { get; set; }

    public String Name { get; set; }
    public String OtherDataRelatedToTheNode { get; set; }
}

2- that data will be converted to hierarchal data and we will use this class to hold it:

public class LocalData : INotifyPropertyChanged
{
    private int? _id;
    public int? Id
    {
        get { return _id; }
        set { _id = value; OnPropertyChanged("Id"); }
    }

    private int? _parentId;
    public int? ParentId
    {
        get { return _parentId; }
        set { _parentId = value; OnPropertyChanged("ParentId"); }
    }

    private string _name;
    public String Name
    {
        get { return _name; }
        set { _name = value; OnPropertyChanged("Name"); }
    }

    private string _otherDataRelatedToTheNode;
    public String OtherDataRelatedToTheNode
    {
        get { return _otherDataRelatedToTheNode; }
        set { _otherDataRelatedToTheNode = value; OnPropertyChanged("OtherDataRelatedToTheNode"); }
    }

    private LocalData _parent;
    public LocalData Parent
    {
        get { return _parent; }
        set { _parent = value; OnPropertyChanged("Parent"); }
    }

    private ObservableCollection<LocalData> _children;
    public ObservableCollection<LocalData> Children
    {
        get { return _children; }
        set { _children = value; OnPropertyChanged("Children"); }
    }

    public event PropertyChangedEventHandler PropertyChanged;
    public void OnPropertyChanged(String propertyName)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this,new PropertyChangedEventArgs(propertyName));
        }
    }
}     

3- finally we need to change the sql data to hierarchical one:

public List<LocalData> GetHerachy(List<SqlDataDto> sqlData)
{
    var sqlParents = sqlData.Where(q => q.ParentId == null).ToList();
    var parents = sqlParents.Select(q => new LocalData {Id = q.Id, Name = q.Name}).ToList();
    foreach (var parent in parents)
    {
        var childs = sqlData.Where(q => q.ParentId == parent.Id).Select(q => new LocalData { Id = q.Id, Name = q.Name , Parent = parent});
        parent.Children = new ObservableCollection<LocalData>(childs);
    }
    return parents;
}

4- then you can create a dummy data and convert it and show it in the tree:

var sqlData = new List<SqlDataDto>
                  {
                      new SqlDataDto {Id = 1, ParentId = null, Name = "F1"}
                      , new SqlDataDto {Id = 2, ParentId = null, Name = "F2"}
                      , new SqlDataDto {Id = 3, ParentId = 1, Name = "S1"}
                      , new SqlDataDto {Id = 4, ParentId = 2, Name = "S21"}
                      , new SqlDataDto {Id = 5, ParentId = 2, Name = "S22"}
                  };
treeView.ItemsSource = GetHerachy(sqlData);

5- the tree should be like:

<TreeView Name="treeView">
    <TreeView.ItemTemplate>
        <HierarchicalDataTemplate ItemsSource="{Binding Children}">
            <TextBlock Text="{Binding Name}" />
        </HierarchicalDataTemplate>
    </TreeView.ItemTemplate>
</TreeView>

Answer 2

You need to use recursion to fill the Child-List of every object. This is necessary for the WPF HierarchicalDataTemplate to work. Otherwise you only get the first level. There is an alternative using the Linq method ForEach() and passing an Action Argument. The following solution is very straight forward and easy to understand:

public List<Product> Products { get; set; }

public MainViewModel()
{
    Products = new List<Product>();

    Products.Add(new Product() { Id = 1, Name = "Main Product 1", ParentId = 0 });
    Products.Add(new Product() { Id = 3, Name = "Sub Product 1", ParentId = 1 });
    Products.Add(new Product() { Id = 4, Name = "Sub Product 2", ParentId = 1 });
    Products.Add(new Product() { Id = 5, Name = "Sub Product 3", ParentId = 1 });
    Products.Add(new Product() { Id = 6, Name = "Sub Product 3.1", ParentId = 5 });


    this.ProcessRootNodes();
}

private void ProcessRootNodes()
{
    var rootNodes = Products.Where(x => x.ParentId == 0).ToList();

    for (int i = 0; i < rootNodes.Count; i++)
    {
rootNodes[i].Children = this.AddChildren(rootNodes[i]);
    }
}

private List<Product> AddChildren(Product entry)
{
    var children = Products.Where(x => x.ParentId == entry.Id).ToList();

    for(int i=0;i<children.Count;i++)
    {
children[i].Children = this.AddChildren(children[i]);
    }

    return children;
}

Answer 3

// *** HOW TO BUILD HIERARCHY OBJECT WITH UNKNOWN & VARYING LEVELS?

Instead of
ObservableCollection<Products> productsHierarchy = new ObservableCollection<Products>();
use Dictionary<Int64, Products> IdToProduct = new ...

As you loop your products; do a IdToProduct[product.ID] = product;

Then, loop the completed IdToProduct collection and do;

if(product.ParentID != 0)
{
    IdToProduct[product.ParentID].ChildProducts.Add(product);
}

Now, your Product --> ChildProducts relation is mapped out.

Optionally, add properties to the Products class:
public bool IsCategory { get { return (ChildProducts.Count >= 1); } } // e.g. Oven
public bool IsProduct { get { return !(IsCategory); } } // e.g. Electric (Oven)

Now, you have most of the model for your view defined.

This article is the de facto starting point for using the WPF TreeView.

Hint: a starting point for your HierarchicalDataTemplate

<TreeView.ItemTemplate>
    <HierarchicalDataTemplate DataType="{x:Type local:Products}" 
                              ItemsSource="{Binding ChildProducts}">
      <TextBlock Text="{Binding Name}" />
    </HierarchicalDataTemplate>
</TreeView.ItemTemplate>

You should create a MainViewModel class which has:
public Products RootProduct { get; set; } (notify property changed property)

after you do your SQL parsing and what not; do:
RootProduct = IdToProduct.FirstOrDefault(product => (product.Level == 0));

<TreeView ItemsSource="{Binding RootProduct.ChildProducts}">