Modify nested QJsonValue

[Donner]

Hello,

please consider a JSON-structure like the following one:

@{
"TestObject": {
"Name": "foo",
"TestObject2": {
"FooAttribute" : "bar"
}
}
}@

Now with Qt 5, using the JSON-classes, I read this structure using some code like

@QJsonDocument tmpDoc = QJsonDocument::fromJson(text_from_file);
QJsonObject tmpObject = tmpDoc.object();@

Now I want to store the tmpObject and work with it - and reading it is no problem. But what if I actually want to manipulate the "FooAttribute" of "TestObject2"?

I tried something like this:
@tmpObject["TestObject"].toObject()["TestObject2"].toObject()["FooAttribute"] = QJsonValue(QString("test"))@

but since QJsonValue and QJsonObject are implicitly shared and creating a deep copy when being modified, that's exactly what happens here, so the original tmpObject doesn't get manipulated.

Using the []-Operator I can get write access to the QJsonObject, but as soon as I convert a QJsonValue I create an (implicitly shared) copy.

Do I really have to create a deep copy and then reassign it to tmpObject, or is there a simple and (most importantly) efficient way to manipulate "FooAttribute" within tmpObjet?

Thank you very much!
D.

[b1gsnak3]

Apparently the Qt Dev Team is working on this according to "this":https://bugreports.qt-project.org/browse/QTBUG-29095

[sl.sy.ifm]

I don't think this is the same issue ... I would say it is worth to crate an additional issue entry for that
It's definitely not intuitive ... e.g. the Qt-Xml-Dom-API behaves in a different way ...

I think putting it in one code-block makes it more clear:
@
QByteArray jsonStr =
" {"
" "TestObject": {"
" "Name": "foo","
" "TestObject2": {"
" "FooAttribute" : "bar""
" }"
" }"
" }";

QJsonDocument tmpDoc = QJsonDocument::fromJson(jsonStr);
QJsonObject tmpObject = tmpDoc.object();

tmpObject["TestObject"].toObject()["TestObject2"].toObject()["FooAttribute"] = QJsonValue(QString("test"));

// this will print "bar", because changing inner objects in the same document doesn't work!
qDebug() << tmpObject["TestObject"].toObject()["TestObject2"].toObject()["FooAttribute"].toString();
@

... when modifying inner object it doesn't change the document

[sl.sy.ifm]

As I also planned to use the Json-utilities in my next project, I tried to find some way to do this ... and it really seems that it is only possible to modify inner-data, by reassigning the outer objects after the modification ...

here is some hacky utility function which does the magic:

@
void modifyJsonValue(QJsonObject& obj, const QString& path, const QJsonValue& newValue) {
const int indexOfDot = path.indexOf('.');
const QString propertyName = path.left(indexOfDot);
const QString subPath = indexOfDot>0 ? path.mid(indexOfDot+1) : QString();

QJsonValue subValue = obj[propertyName];

if(subPath.isEmpty()) {
subValue = newValue;
}
else {
QJsonObject obj = subValue.toObject();
modifyJsonValue(obj,subPath,newValue);
subValue = obj;
}

obj[propertyName] = subValue;
}
void modifyJsonValue(QJsonDocument& doc, const QString& path, const QJsonValue& newValue) {
QJsonObject obj = doc.object();
modifyJsonValue(obj,path,newValue);
doc = QJsonDocument(obj);
}
@

Usage example ("tmpDoc" initialized like in my previous post):
@modifyJsonValue(tmpDoc,"TestObject.TestObject2.FooAttribute",QString("test"));@

... this will modifier the Json so that the "test" is really set inside the Json-document

[andresantos]

Hey man,

Thank you for this snippet!

I was just getting frustrated trying to work with QJson and you just saved my day!

[rrozek]

Hello!

I know this topic is really old, but i've seen it more then a year ago for the first time, and then G redirected me to it several times when i was struggling with read/write json in Qt.
Code snippet provided by @sl-sy-ifm works really good on objects, but i needed to extend it to arrays as well, i've uploaded my code to github, feel free to use it anywhere ;)
now modifyJsonValue takes QJsonValue as param, not QJsonObject so you can put there anything (on github, there are examples provided)
https://github.com/rrozek/QtJsonModifyValue/blob/master/main.cpp

oh, and maybe a snippet here as well:

usage:
    modifyJsonValue(doc, "firstName", QJsonValue("Natalia"));
    modifyJsonValue(doc, "age", 22);
    modifyJsonValue(doc, "address.state", "None");
    modifyJsonValue(doc, "phoneNumber[0].number", "333 543-3210");
    modifyJsonValue(doc, "family[0][2]", "Bill");
    modifyJsonValue(doc, "family[1][1]", "Winston");
    modifyJsonValue(doc, "family[2].father.age", 56);

void modifyJsonValue(QJsonValue& destValue, const QString& path, const QJsonValue& newValue)
{
    const int indexOfDot = path.indexOf('.');
    const QString dotPropertyName = path.left(indexOfDot);
    const QString dotSubPath = indexOfDot > 0 ? path.mid(indexOfDot + 1) : QString();

    const int indexOfSquareBracketOpen = path.indexOf('[');
    const int indexOfSquareBracketClose = path.indexOf(']');

    const int arrayIndex = path.mid(indexOfSquareBracketOpen + 1, indexOfSquareBracketClose - indexOfSquareBracketOpen - 1).toInt();

    const QString squareBracketPropertyName = path.left(indexOfSquareBracketOpen);
    const QString squareBracketSubPath = indexOfSquareBracketClose > 0 ? (path.mid(indexOfSquareBracketClose + 1)[0] == '.' ? path.mid(indexOfSquareBracketClose + 2) : path.mid(indexOfSquareBracketClose + 1)) : QString();

    // determine what is first in path. dot or bracket
    bool useDot = true;
    if (indexOfDot >= 0) // there is a dot in path
    {
        if (indexOfSquareBracketOpen >= 0) // there is squarebracket in path
        {
            if (indexOfDot > indexOfSquareBracketOpen)
                useDot = false;
            else
                useDot = true;
        }
        else
            useDot = true;
    }
    else
    {
        if (indexOfSquareBracketOpen >= 0)
            useDot = false;
        else
            useDot = true; // acutally, id doesn't matter, both dot and square bracket don't exist
    }

    QString usedPropertyName = useDot ? dotPropertyName : squareBracketPropertyName;
    QString usedSubPath = useDot ? dotSubPath : squareBracketSubPath;

    QJsonValue subValue;
    if (destValue.isArray())
        subValue = destValue.toArray()[usedPropertyName.toInt()];
    else if (destValue.isObject())
        subValue = destValue.toObject()[usedPropertyName];
    else
        qDebug() << "oh, what should i do now with the following value?! " << destValue;

    if(usedSubPath.isEmpty())
    {
        subValue = newValue;
    }
    else
    {
        if (subValue.isArray())
        {
            QJsonArray arr = subValue.toArray();
            QJsonValue arrEntry = arr[arrayIndex];
            modifyJsonValue(arrEntry,usedSubPath,newValue);
            arr[arrayIndex] = arrEntry;
            subValue = arr;
        }
        else if (subValue.isObject())
            modifyJsonValue(subValue,usedSubPath,newValue);
        else
            subValue = newValue;
    }

    if (destValue.isArray())
    {
        QJsonArray arr = destValue.toArray();
        arr[arrayIndex] = subValue;
        destValue = arr;
    }
    else if (destValue.isObject())
    {
        QJsonObject obj = destValue.toObject();
        obj[usedPropertyName] = subValue;
        destValue = obj;
    }
    else
        destValue = newValue;
}

void modifyJsonValue(QJsonDocument& doc, const QString& path, const QJsonValue& newValue)
{
    QJsonValue val;
    if (doc.isArray())
        val = doc.array();
    else
        val = doc.object();

    modifyJsonValue(val,path,newValue);

    if (val.isArray())
        doc = QJsonDocument(val.toArray());
    else
        doc = QJsonDocument(val.toObject());
}

[heksbeks]

Here is an improved implementation:
https://github.com/heksbeks/qjsonpath

[Christian Ehrlicher]

This post is deleted!