Appending to xml

[chawila]

I have an xml file that was generated using c++.
so i used it as a model in qml.
now as the user is interacting with the App, i want to append to that xml in c++, then reload the xmlModel in qml.

i tried these:

    QString filename= appFolder+"/db/"+xmlFile;

        if(append == true){
            qDebug() << "appending...";
            file.open(QIODevice::ReadWrite | QIODevice::Text);
            QDomDocument doc;
            doc.setContent(&file);
            QDomNode root = doc.firstChild();
            
        }else{
            qDebug() << "trancating and adding to file..";
            file.open(QIODevice::ReadWrite | QIODevice::Truncate | QIODevice::Text);
            QDomDocument doc("Products");
            QDomElement root = doc.createElement("Products");
            doc.appendChild(root);
        }

---

Then i have a loop that should iterate while appending to the roort variable.
So if i try to access the variable doc so that i can add elements to it, i get: Error: doc was not declared on the scope. And Error: root was not declared on the scope.

But the qDebug is working fine. it is printing the message that it is suppose to print, depending on the variable append.

And if i remove the whole if block and use the following, everything work fine, but it truncate the the file which i dnt want.

            file.open(QIODevice::ReadWrite | QIODevice::Truncate | QIODevice::Text);

            QDomDocument doc("Products");
            QDomElement root = doc.createElement("Products");
            doc.appendChild(root);

---

Can anybody tell me where a`im getting things wrong..

[mrjj]

Hi
Your root and doc only exists within
if (xxx)
{
} // scope ends and all is deleted

Just move them above the if(append) so they are not deleted.

If you need doc / root in more than one function, you can simply make them member of
the class.

[JonB]

@chawila said in Appending to xml:

file.open(QIODevice::ReadWrite | QIODevice::Text)

Take my observation with a pinch of salt, as I've never done any file IO in Qt! But when you want to append you have:

 file.open(QIODevice::ReadWrite | QIODevice::Text)

Don't you need:

file.open(QIODevice::ReadWrite | QIODevice::Append | QIODevice::Text)

? (Or at least some kind of "seek to end".) Else I would expect the write to be positioned initially at start of file, and you'll be writing new stuff all over existing content??

[EDIT: Hmm, you might be relying on the QDomDocument to first read to end of file before you then do any writing? Sounds a bit scary/error-prone to me if so.]

[jsulm]

@chawila said in Appending to xml:

And if i remove the whole if block and use the following, everything work fine, but it truncate the the file which i dnt want.
file.open(QIODevice::ReadWrite | QIODevice::Truncate | QIODevice::Text);

Sure it truncates the file as you tell it to do so by using QIODevice::Truncate. Actually you should write the whole file again after changing XML, for that just don't pass QIODevice::Truncate.

[JonB]

Be careful you understand what's going going on with truncate/append if you follow @jsulm 's advice.

If you open an existing file for write or read/write with content, and you do not pass in either QIODevice::Truncate or QIODevice::Append, if you start writing to it you do start overwriting the content from the start but the file's length remains as it was when you opened it. If the total number of bytes you write is then less than the current content number of bytes, the remaining content after what you have written remains at the end of file. Which is most unlikely to be what you want! Since in this situation you are never wanting to actually open the file for random access read/write (like a database would), I would expect your open for write to always have one of QIODevice::Truncate or QIODevice::Append.

Your use of QIODevice::ReadWrite concerns me, because writing will happen immediately after wherever you last read from, and if you're not careful/don't know for sure where that is you may be writing into the middle of the document. I admit I don't know just how QDomDocument attaches to the file you give it, but my inclination would not to have read/write here. Get your reading done via QIODevice::Read, then either rewrite (with truncate) the whole file with old content + new or open it for append and output just the new stuff. IMHO.

BTW, while I think of it, it's most unusual to be able to simply append to an XML file. If your XML file has one root element, like <root>...</root>, which is what most XML documents look like, then you can never just append, as any extra content will need to come before the closing </root> at end of file. The only time you can append is if the doc has no/multiple root element, so from the top-level it looks like <node>...</node><node>...</node>... with nothing enclosing them all. Is that indeed the case for your document?

[chawila]

@JNBarchan
yah. thanks, i can`t believe i forgot that.

@mrjj
Thanks for that man. yah thedoc and root variable were out of scope, so i made the class properties..

---

But it looks like it can`t read from the file( when i try to append) so it give me error: Calling appendChild() on a null node does nothing.
So i think there is something wrong with the way i read from the file..

[JonB]

@chawila said in Appending to xml:

But it looks like it can`t read from the file( when i try to append) so it give me error: Calling appendChild() on a null node does nothing.
So i think there is something wrong with the way i read from the file..

Well the appendChild() is to do with the QDomDocument state and appending a new node to it, rather than reading from file. From the error message, it is unclear to me which, but you are doing one of NULL->appendChild(something) or something->appendChild(NULL).

What about answering whether the document does or does not have a single root node? Because if it does not, the whole business of trying to "append" is a non-starter.

[chawila]

---

I think these might give u the answer u`re looking for

<Products>

 <Product>
  <prId>2506</prId>
  <spIcon>e19ea7d081f8782ac41269ac9268e52c.jpeg</spIcon>
  <prThumb>d044d922d380359eec7452cc751a9d2c.jpeg</prThumb>
  <prName> toilet tissue twinsaver baby soft 18s</prName>
  <saved>0</saved>
  <price>65.95</price>
 </Product>

</Products>

---

These gives me failed(when appending):

        if(append == true){
            qDebug() << "appending";
            file.open(QIODevice::ReadWrite | QIODevice::Append | QIODevice::Text);

             if(doc.setContent(&file)){
                 qDebug() << "read.";
             }else{
                 qDebug() << "failed";
             }
             root = doc.firstChildElement("Products");

        }else{
            qDebug() << "truncating..";

            file.open(QIODevice::ReadWrite | QIODevice::Truncate | QIODevice::Text);
            qDebug() << "file open..";
            root = doc.createElement("Products");
            doc.appendChild(root);

        }

---

Then

QDomElement productTag = doc.createElement("Product");
qDebug() << "appending Product tag";
root.appendChild(productTag );

[JonB]

@chawila said in Appending to xml:

firstChildElement

1. Check the return result of root = doc.firstChildElement("Products"); before root.appendChild(productTag ) ?
2. Try QDomElement root = doc.documentElement(); instead of doc.firstChildElement("Products")?
3. Do you understand that the whole principle of appending with QIODevice::Append to the document file in order to create a new Product is never going to work, because you need that new node to go inside the Products node in any case?

[chawila]

@JNBarchan

itried it still get: Calling appendChild() on a null node does nothing.
So what are u suggesting about QIODevice::Append.

[JonB]

@chawila said in Appending to xml:

@JNBarchan

itried it still get: Calling appendChild() on a null node does nothing.

So for your root.appendChild(productTag ), debug out both root & productTag?

So what are u suggesting about QIODevice::Append.

For what you want to do, you cannot use root.appendChild(productTag ) and you cannot "append to the existing XML in the file" in any shape or form. I think from what you're saying you want to:

1. Read in the whole existing XML file to create the document.
2. Do your code which alters the DOM by appending new nodes or whatever.
3. Overwrite the complete file with the serialization of the complete DOM document, i.e. no appending, just like creating it from scratch.

[chawila]

@JNBarchan
I got compilation Error:

No match for 'operator'<<(operand types are 'QDeburg' and QDomElement')
       qDebug() << root << productTag ;

---

So how can i do that?..

• Read in the whole existing XML file to create the document.

• Do your code which alters the DOM by appending new nodes or whatever.

• Overwrite the complete file with the serialization of the complete DOM document, i.e. no appending, just like creating it from scratch.

[JonB]

@chawila
Outline code:

if (append)
{
  openFileForReadOnly();
  readFileToCreateDOM();
  closeFile();
}
else
{
  createNewEmptyDom();
  possiblyCreateEmptyProductsNode();
}
doCodeToPutNewProductNodesUnderProductsNode();
saveWholeDOMDocumentToFileWithOverwrite();

No ReadWrites or Appends ; Truncate only used in saveWholeDOMDocumentToFileWithOverwrite() if that's what is required to overwrite. See e.g. https://forum.qt.io/topic/43724/solved-save-qdomdocument-to-xml or http://www.qtforum.org/article/2756/how-to-qdomdocument-to-file.html. e.g. file.open( QIODevice::WriteOnly | QIODevice::Truncate | QIODevice::Text ) when you're ready to overwrite.

[chawila]

@JNBarchan

So all these:

  openFileForReadOnly();
  readFileToCreateDOM();
//-----------------------
  createNewEmptyDom();
  possiblyCreateEmptyProductsNode();
//--------------------
doCodeToPutNewProductNodesUnderProductsNode();
saveWholeDOMDocumentToFileWithOverwrite();

---

Are methods/functions i should create

[JonB]

@chawila
Well, yes!
For:

  createNewEmptyDom();
  possiblyCreateEmptyProductsNode();

you already had:

     root = doc.createElement("Products");
     doc.appendChild(root);

(provided your DOM doc starts out empty).

BTW

QIODevice::WriteOnly 0x0002 The device is open for writing. Note that this mode implies Truncate.

[chawila]

@JNBarchan
Ok i`ll keep on trying,
but u are giving me a big task(reaserch) to do..

[JonB]

@chawila
Sorry, I don't see any "big task" or "research" for you to do?
You already seem to know how to read a file to QDomDocument, add nodes to it, and write it out, that's all we're doing.

[chawila]

@JNBarchan
Let me get into it i`ll be back, hopefully with good news..

[JonB]

@chawila
It is your use of "append to xml file" that is faulty. To add new nodes, you do not literally "append" them to the whole file, you add (append/insert) them near the end of the file but not literally at the end of the file.

P.S.
You can probably use QDomDocument::save(QTextStream) as one way of saving the dom doc back to file, if you like.

[chawila]

@JNBarchan
Thanks,
it was little bit challenging though..