Proxy Model for Combining Table Models

[Flick]

Hello. I have two SQLite tables being used as QAbstractTableModel objects in C++ code. I want to use these two models as sources of truth for a new TableView that will display a combination of the two table models joined together.

How can I use a proxy model to combine data from two or more other models, and keep the new model in sync with both parent models? Are there good tutorials or examples anywhere?

Model 1

id	data1	data2	data3	other_id

Model 2

id (other_id)	data3	data4

Model 3 (goal)

id	other_id	data1	data2	data3	data3	data4

When I filter either model 1 or model 2, I want model 3 to update.

[SGaist]

Hi,

How do you want to assemble these data ?
Is it some sort of join ? What are the join points ?

[Flick]

@SGaist Yes, I'm essentially joining the two tables where Model1.other_id = Model2.id and wanting my combined view to update based on changes to either base model.

[SGaist]

How are the base models updated ?

[Flick]

@SGaist Each model (model1, model2) is its own QAbstractTableModel made to read data w/ QSQLQuery commands. These models handle

1. SQL role name extraction
2. SQL filtering / sorting
3. item selection (QItemSelectionModel)

We get data through the last ran QSQLQuery* _query, and re-query whenever sorting, filtering, or CRUD operations are applied with the same _query object. My issue is, I can only setModel once for QML views like TableView and I'm not sure how to base one view on two models.

[JonB]

@Flick
There is nothing out-of-the-box for JOINing Qt models once data read into them. Let alone that I have no idea what restrictions QML might impose. You would have to write code to do that.

Since you seem to already re-query on any change in view (e.g. filtering, sorting), would you consider putting the JOIN at the SQL side (QSqlQuery), which is easy to do, and re-reading that too? It will be simpler than what you must add to do the join yourself on the QSql models in memory.

[Flick]

I'm still lost. How would a sample TableView render two SQL tables as one model? Does anyone have an example?

[SGaist]

@Flick taking the join part aside for a second, you would need a proxy model that announces a number of columns matching the sum of columns of your two tables and the number of row of the biggest of the two tables. Then in the data function you would need to grab the data of the correct model when requested. Qt has such a model but just to concatenate rows of models.

Taking the join back in, you should really consider what @JonB wrote: create the model out of the query from the database. You database will be way better at doing the join than a model in code.

[Flick]

If anyone could provide examples of 2+ joined SQL tables in a single QML model, I'd appreciate it. I can't use QSqlTableModel because it functions on just one table, but I would like all the functionality QSqlTableModel provides (hiding columns, sorting/filtering).

[GrecKo]

Use QSqlQueryModel and add the sorting/filtering there or with a QSortFilterProxyModel on top.

[Flick]

@GrecKo This is what I'm working on implementing right now. Would there be a reason to use QAbstractTableModel over QSqlQueryModel?