Problems with NULL values from mySQL

[Johnny78]

Hello I would appreciate your help with a small problem.
I am using python 3.4.6 with PyQt5 version 5.5.1 and Qt 5.6.2.
This might be a trivial problem but I could not find any solution when I searched for it. I am also connecting to a mariaDB.

I have derived a class from QSqlTableModel.

class HSqlTableModel(QtSql.QSqlTableModel):
    def __init__(self, parent, db):
        super().__init__(parent, db)
        self.translate  = False
        self.dirtyData  = False
        self.rLib       = dict()  # this dict contains the replace info
        self.db         = db

#in current table and column ind, replace from str_table, column str_fieldA with str_fieldB
    def setReplacement(self, ind, str_table, str_fieldA, str_fieldB, active=False):
        slist          = SqlList(self.db,str_table, active, colA=str_fieldA, colB=str_fieldB)
        self.rLib[ind] = slist
        self.replace   = True
    
    # a shorter version for helper tables,
    def setReplacement2(self, ind, str_table, active=False):
        slist          = SqlList(self.db,str_table, active)
        self.rLib[ind] = slist
        self.replace   = True

    # return Data
    def data(self, modind, role = QtCore.Qt.DisplayRole ):
        
        col = modind.column()
        var = super().data(modind, role)
        if var == None:
            var = QtCore.QVariant(0)
            var.clear()
            return var
        
        if (not col in self.rLib):
            return var
        
        
        if role == QtCore.Qt.EditRole:
            #print("data:  editRole Var = ", var)
            if self.translate:
                trValue = self.rLib[col].getBatAval(var)
                return trValue
            else :
                return var
            
        if   role == QtCore.Qt.DisplayRole:
            return self.rLib[col].getBatAval(var)
        elif role == QtCore.Qt.ForegroundRole:
            if self.rLib[col].isActive():
                blueText = QtGui.QBrush(QtCore.Qt.darkBlue)
                return blueText
        else : return var
        
    
    
    #setData ( const QModelIndex & index, const QVariant & value, int role = Qt::EditRole )
    def setData(self, modind, value, role ):
        #print("setData: role = ", role, "  value = ", value)
    
        # if value is None, nothing is done
        if value == None or value =="":
            return False
        
        if not role == QtCore.Qt.EditRole:
            return super().setData(modind, value, role)
        
        col = modind.column()
        if  col in self.rLib:
            if isinstance(value,int):
                return super().setData(modind, value, role)
            
            trValue = self.rLib[col].getAatBval(value)
            #print("     transformed Value: ", trValue)
            if trValue == None:
                return False
            else:
                ans = super().setData(modind, trValue, role)
                if ans == True:
                    self.dirtyData = True
                return ans
        else:
            ans = super().setData(modind, value, role)
            if ans == True:
                self.dirtyData = True
            return ans

I display the table with a QTableView. However I have a problem to update rows that contain NULL values (from the mariaDB server, NOT NULL pointers). While I can change them in the QTableView, I can't save the changes to the mariaDB server. When I try no error is displayed, but the row reverts back to the values before the change.
However I can add new rows, that contain NULL values. This behaviour is mystifying to me. Any suggestions?
Thanks for your help.

[Johnny78]

Thank you Denni for your answer. I think I do understand roughly what you are saying. This sound like a good design concept. How would I implement this in QT. Is there are class that I can use as a controller?
There is also a model-view concept in Qt, where QSqlTalbeModel is the model and QTableView is the view. But I guess you mean that the controller would be between Qt (the view) and mariaDB (the model). How can I stick something between them? Do you have any suggestions about that?
Thanks again for your suggestion.

[Johnny78]

Thank you Denni for your answer. I can see why this is a good idea, but it is a bit above my knowledge and requirements. The database server is on my own computer and the program is written purely for my use. I have decided to rename NULL values in the database. I have set new default values. Of course this is not portable at all but it is a quick fix and solves my problems.
Maybe once I have more time and more knowledge I will try to implement your idea. Thanks for taking the time for your answer. I will mark this thread as solved.