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Editorial Team
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Asked: 2026-06-18T03:43:57+00:00

I am trying to create a QTreeView to display data from a SQL database.

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I am trying to create a QTreeView to display data from a SQL database. This is a large database, so simply loading the data into a QStandardItemModel seems prohibitive.

None of Qt’s pre-built SQL model classes are sufficient for the task. Therefore it seems necessary to subclass QAbstractItemModel.

In the first place, I can find no examples where this is done, so I am wondering whether it is the correct approach.

Implementing QAbstractItemModel::data is pretty straightforward. I am uncertain how to implement QAbstractItemModel::parent.

Qt’s “Simple Tree Model Example” example would be informative, but in that example the tree structure is represented in memory with the TreeItem class. I could copy that, but if I am going to duplicate the database structure, it would be just as easy to use QStandardItemModel. If I need to maintain a separate data structure (in addition to the database and the QAbstractItemModel subclass) to represent the tree structure, is there any advantage to subclassing QAbstractItemModel over just using a QStandardItemModel?

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1 Answer

  1. Editorial Team

    The challenge in the tree structure is to always be able to identify a model index’s parent (i.e., overloading the parent() method). In the Simple Tree example, this is done by storing the three structure in a separate data structure. For large SQL queries this is impractical. For the right database structure, you might be able to calculate the proper parent node given the child, but that is not a guarantee. The only alternative I can imagine is passing a quint32 to QAbstractItemModel::createIndex which encodes the item’s parent.

    One performance consideration that might be useful. After giving up on sublcassing QAbstractItemModel, I tried populating a QStandardItemModel from the database. I loaded about 1200 items into the model, and four child items to each item with two separate database calls. This took about 3 seconds on a 2009 laptop. That is faster than I had been expecting. (And there would be performance gains if I used a single query instead of repeated queries.)

    In the end I went another route: having several QTableViews in a the GUI, with signals and slots to show different aspects of the data. My code is much simpler, and the proper functionality is in place, so this feels like the “right” solution.

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