How to relationship with mysql table in QT QSqlRelationalTableModel? - c++

Question

I am trying to work with QSqlRelationalTableModel of QT. I am new to MySQL table relationship but still I tried and can't make it work properly in QT.
I can get the result from MySQL:
create table stu(idd int auto_increment primary key,stu_name varchar(60),stu_age int);
create table stuInfo(idd int auto_increment primary key,stu_city varchar(60),stu_sub varchar(100), foreign key(id) references stu(id));
select stu.stu_name,stuInfo.stu_city from stu inner join stuInfo on stu.id=stuInfo.id;
To retrieve data from MySQL :
select stu.stu_name,stuInfo.stu_city from stu inner join stuInfo on stu.id=stuInfo.id;
In QT I can't make it work. I am getting confused with setRelation() and QSqlRelation() . I am not exactly understanding that how I can execute the same query in QT, I tried it in various way but sometime I get blank data, ugly header, errors etc.
Here is my learning code:
model = new QSqlRelationalTableModel();
model->setTable("stu");
model->setRelation(0,QSqlRelation("stu","id","stu_name","stu_age"));
model->setRelation(0,QSqlRelation("stuInfo","id","stu_city","stu_sub"));
model->select();
ui->tableView->setModel(model);

Answer 1

A QSqlRelation replaces the value of a field by the value of the other field in the relation, the replaced field won't appear in the query anymore, so you can't have 2 relations assigned to the same column, and you can't assign a relation to the primary key (as stated in the documentation of setRelation).
Basically the structure for which QSqlRelationalTableModel should be used would be a main table which would have 1 or more foreign index fields, and each of these fields could be replaced by the value of a chosen field in the tables from which the foreign indexes comes from (e.g.: to replace a "city_id" numerical field in the main table by the name of the city coming from another table for which that "city_id" is the primary key).
For what you want to do, you should use QSqlQueryModel with a manually constructed query instead of QSqlRelationalTableModel.

Answer 2

The problem is that your code does not really express the model you described.
You have a primary table called stuInfo, which references another table called stu.
To do this in Qt, you should create a table based on "stuInfo" (and not "stu"!):
model=new QSqlRelationalTableModel();
model->setTable("stuInfo");
Then you can implement your foreign key, as a relation:
model->setRelation(3,QSqlRelation("stu","id","stu_name"));
You need to point to index "3", which is the position of the reference field "id", on stuInfo table (0 will point to the primary key, which is not what you want!). The parameters of the QsqlRelation are the reference table name ("stu") the primary field name ("id") and the reference table field to which you want to point: in this case I am pointing to "stu_name"; if I wanted to point to the age, I could do something like this instead:
model->setRelation(3,QSqlRelation("stu","id","stu_age"));
After this code:
model->select();
ui->tableView->setModel(model);
you should have a view that shows you all the fields on stuInfo, and whose last field ("id") is mapped to the name (or age) on the "stu" table;