QSqlQueryModel, trees and more

[napajejenunedk0]

I will examplify the problem using the following scenario:

1. A database that contains:
   • manufacturer: id, description
   • model: id, manufacturerId, description
   • car: id, modelId, description

So we have a tree-like structure in the db:

@

• manufacturer
  • model
    • car
      @

My target is to make every path in the tree (here the paths are guaranteed to be three) to be listed in a QComboBox.
So the final UI will be:

@
"Manufacturer:" QComboBox
"Model:" QComboBox
"Car:" QComboBox
@

So what I want is a tree view but displayed as a form with three rows each of which contains a combo box that contains path from the tree.

How I am currently trying to solve this:

1. Subclassed QSqlQueryModel and set its query to:
   @
   "SELECT "
   "manufacturers.id, "
   "manufacturers.description, "
   "models.id, "
   "models.description, "
   "cars.id, "
   "cars.description "
   "FROM "
   "cars "
   "INNER JOIN models "
   "ON cars.modelId = models.id "
   "INNER JOIN manufacturers "
   "ON models.manufacturerId = manufacturers.id "
   "ORDER BY "
   "manufacturers.description, "
   "models.description, "
   "cars.description";
   @

In the model I override the data() member function and the columnCount() one, so that I have three model columns:
Manufacturers
Models
Cars

and for each column I have two roles:
-> Qt::DisplayRole - returns the description,
-> RoleId = Qt::UserRole + 1 - the id of the given entity

So, at this point I have a tabular model of my tree-like data.

2. Convert the QSqlQueryModel subclass to tree model:
   "This thread":http://www.qtcentre.org/threads/23698-how-to-display-QSqlQueryModel-in-QTreeView recommends using QAbstractProxyModel and implement the:
   -> mapToSource
   -> mapFromSource
   member functions.

3. Make each QComboBox use the same instance of the tree model, but making it use different column from it.
   @
   TreeModel* const treeModel = new TreeModel( this ); // the QAbstractProxyModel subclass

manufacturerComboBox.setModel( treeModel );
manufacturerComboBox.setModelColumn( 0 );

modelComboBox.setModel( treeModel );
modelComboBox.setModelColumn( 1 );

carComboBox.setModel( treeModel );
carComboBox.setModelColumn( 2 );
@

I am also thinking of using the QDataWidgetMapper class if using a given column for a tree-view doesn't work.

So my question is: is this the right direction e.g. create a QAbstractProxyModel, and if so how should I implement the mapToSource and mapFromSource? Do I need to implement the parent() member function?