If multiple relations are set on a QSqlRelationalTableModel, the relations can crash when updating the relation model values.

QSqlRelationalTableModel model;
model.setTable("table");
model.setRelation(0, QSqlRelation("table2", "id", "Name"));
model.setRelation(1, QSqlRelation("table3", "id", "Name"));

// Use model

// Foreign keys changed, update models
QSqlTableModel* relationModel = model.relationModel(0);
if(relationModel)
{
  relationModel->select(); // Crash here
}

Looking at the internal code:
https://github.com/qt/qtbase/blob/dev/src/sql/models/qsqlrelationaltablemodel.cpp
QSqlRelationalTableModel::setRelation()

Resizes the d->relations vector.
However, each QRelatedTableModel stores an internal pointer into this array. This pointer becomes invalid when a new relationship is added and the array is resized.

A potential fix is to either update all existing pointers on array resize, or store a parent class pointer and index in QRelatedTableModel instead of a pointer to the array element.

A workaround is to set a dummy relation on a column that exceeds the data column size on creation.
QSqlRelationalTableModel::setRelation(ModelColumnCount + 1, QSqlRelation())
This pre-sizes the internal array, so a later resize does not need to occur.