Determining Field Index in QSqlRelationalTableModel when Using QSqlRelation

[Spiff]

I am using a SQLite3 database with QSqlRelationalTableModel and QSqlRecord. I have configured a small test case that demonstrates the following issue:

I have a table that has 2 columns that reference (foreign key) the same column in another table. When I add the QSqlRelation to the QSqlRelationalTableModel, Qt effectively renames the column to include the foreign key reference. And this is adequately explained in the documentation.

My problem is that when I try to insert data into the model, I don't know how to reliably determine the field index for columns that are involved in a QSqlRelation. I do not want to hard code the field indexes in my code - I need to be able to refer to them by name.

I have not found a way to *reliably* get the field index using the column name if there are multiple foreign keys to the same "master" column. (Lines 90 and 91 in the attached main.cpp).

This is the output of the attached program:
Record Field ( 0 ): "id"
Record Field ( 1 ): "some_data"
Record Field ( 2 ): "table_a_name_2"
Record Field ( 3 ): "name"
Model Field ( "id" ): 0 , Record Field Index: 0
Model Field ( "ref_data_a1" ): -1 , Record Field Index: -1
Model Field ( "ref_data_a2" ): -1 , Record Field Index: -1
Column ( 2 ) Relation: [Table: "table_a" , Index Column: "id" , Display Column: "name" ]
Model Column: "table_a_name_2"
Record Column: "table_a_name_2"
Column ( 3 ) Relation: [Table: "table_a" , Index Column: "id" , Display Column: "name" ]
Model Column: "name"
Record Column: "name"
Record insert to table_b failed: "NOT NULL constraint failed: table_b.ref_data_a1 Unable to fetch row"

I have attached a sample project that includes:
testdb.zip - Zipped SQLite3 database
testdb.png - Image showing the database structure
main.cpp - Source code to connect to the database and try to insert data
CMakeLists.txt - Build script for the project

[meredithsant]

Hello, to solve this, instead of relying on QSqlRelationalTableModel to resolve the foreign key column indices by name, you can iterate over the QSqlRecord Fields: You can iterate through the columns of the record to find the correct index for a foreign key. This is more flexible and avoids hardcoding field indices. Moreover, you can use the model's relation mappings (QSqlRelationalTableModel::relation) to determine which columns are foreign keys and handle them dynamically.

Here’s an example of how you might adapt your code to reliably handle foreign key columns without hardcoding the field index:

#include <QtSql/QSqlRelationalTableModel>
#include <QtSql/QSqlRecord>
#include <QtSql/QSqlRelation>
#include <QDebug>
 
// Function to find the field index by name
int getFieldIndexByName(QSqlRelationalTableModel* model, const QString& columnName) {
    int columnCount = model->columnCount();
    for (int i = 0; i < columnCount; ++i) {
        if (model->headerData(i, Qt::Horizontal).toString() == columnName) {
            return i;
        }
    }
    return -1;  // Return -1 if not found
}
 
// Sample code for inserting data and handling foreign key columns
void insertData(QSqlRelationalTableModel* model, const QSqlRecord& record) {
    // Iterate over each field in the QSqlRecord to insert data
    for (int i = 0; i < record.count(); ++i) {
        QString fieldName = record.fieldName(i);
        int fieldIndex = getFieldIndexByName(model, fieldName);
 
        // Check if the field is a foreign key reference
        if (fieldName.startsWith("ref_data_a")) {
            // Handle the foreign key column
            qDebug() << "Foreign key field:" << fieldName << "at index" << fieldIndex;
            // You can fetch the foreign key reference data here if needed
        }
        // Insert data into the field
        qDebug() << "Inserting value for field" << fieldName << "at index" << fieldIndex;
        model->setData(model->index(i, 0), record.value(i));
    }
 
    // Try inserting the record
    if (!model->submitAll()) {
        qDebug() << "Error inserting record:" << model->lastError().text();
    }
}
 
int main() {
    QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
    db.setDatabaseName("testdb.db");
 
    if (!db.open()) {
        qDebug() << "Database error:" << db.lastError().text();
        return 1;
    }
 
    QSqlRelationalTableModel model;
    model.setTable("table_b");
 
    // Setting relations for foreign keys
    model.setRelation(2, QSqlRelation("table_a", "id", "name")); // ref_data_a1
    model.setRelation(3, QSqlRelation("table_a", "id", "name")); // ref_data_a2
 
    if (!model.select()) {
        qDebug() << "Model select failed:" << model.lastError().text();
        return 1;
    }
 
    QSqlRecord record = model.record();
    record.setValue("some_data", "Test data");
    record.setValue("ref_data_a1", 1);  // Example foreign key reference
    record.setValue("ref_data_a2", 2);  // Example second foreign key reference
 
    insertData(&model, record);
 
    return 0;
}

#include <QtSql/QSqlRelationalTableModel>
#include <QtSql/QSqlRecord>
#include <QtSql/QSqlRelation>
#include <QDebug>

// Function to find the field index by name
int getFieldIndexByName(QSqlRelationalTableModel* model, const QString& columnName) {
    int columnCount = model->columnCount();
    for (int i = 0; i < columnCount; ++i) {
        if (model->headerData(i, Qt::Horizontal).toString() == columnName) {
            return i;
        }
    }
    return -1;  // Return -1 if not found
}

// Sample code for inserting data and handling foreign key columns
void insertData(QSqlRelationalTableModel* model, const QSqlRecord& record) {
    // Iterate over each field in the QSqlRecord to insert data
    for (int i = 0; i < record.count(); ++i) {
        QString fieldName = record.fieldName(i);
        int fieldIndex = getFieldIndexByName(model, fieldName);

        // Check if the field is a foreign key reference
        if (fieldName.startsWith("ref_data_a")) {
            // Handle the foreign key column
            qDebug() << "Foreign key field:" << fieldName << "at index" << fieldIndex;
            // You can fetch the foreign key reference data here if needed
        }
        // Insert data into the field
        qDebug() << "Inserting value for field" << fieldName << "at index" << fieldIndex;
        model->setData(model->index(i, 0), record.value(i));
    }

    // Try inserting the record
    if (!model->submitAll()) {
        qDebug() << "Error inserting record:" << model->lastError().text();
    }
}

int main() {
    QSqlDatabase db = QSqlDatabase::addDatabase("QSQLITE");
    db.setDatabaseName("testdb.db");

    if (!db.open()) {
        qDebug() << "Database error:" << db.lastError().text();
        return 1;
    }

    QSqlRelationalTableModel model;
    model.setTable("table_b");
    
    // Setting relations for foreign keys
    model.setRelation(2, QSqlRelation("table_a", "id", "name")); // ref_data_a1
    model.setRelation(3, QSqlRelation("table_a", "id", "name")); // ref_data_a2

    if (!model.select()) {
        qDebug() << "Model select failed:" << model.lastError().text();
        return 1;
    }

    QSqlRecord record = model.record();
    record.setValue("some_data", "Test data");
    record.setValue("ref_data_a1", 1);  // Example foreign key reference
    record.setValue("ref_data_a2", 2);  // Example second foreign key reference

    insertData(&model, record);

    return 0;
}

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