Appending to xml

[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..

[JonB]

@chawila
Well, I'm sorry but "appending to an XML file" is "a bit challenging", because as you've seen you don't quite want to do that... :)