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Get nested json values

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  • A avmg
    31 Jul 2020, 09:02

    @Christian-Ehrlicher sorry but I don't see it...

    I have a mix of things that I don't understand, I just see different codes on each answer...

    In order to use json it must be declared previously, so I don't understand what are you doing. My target is to change the value of a nested json, I tried like:

    QJsonObject json;
    json["Food"].toObject()["Fruits"]toObject()["Apples"] = 10;

    I found a lot of info and hundred of ways to do the things with json files and I think it's just creating more confusion to me...

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    JonB
    wrote on 31 Jul 2020, 09:20 last edited by JonB
    #10

    @avmg
    If you do that, your empty QJsonObject json won't even have a Food sub-object in it. You need to put stuff into your object.

    Untested, but you will want something like:

    QJsonObject json, food, fruits;
    json["Food"] = food;
    food["Fruits"] = fruits;
    fruits["Apples"] = 10;
    

    EDIT Wrong, see my next post for correction.

    A 1 Reply Last reply 31 Jul 2020, 09:41
    0
    • J JonB
      31 Jul 2020, 09:20

      @avmg
      If you do that, your empty QJsonObject json won't even have a Food sub-object in it. You need to put stuff into your object.

      Untested, but you will want something like:

      QJsonObject json, food, fruits;
      json["Food"] = food;
      food["Fruits"] = fruits;
      fruits["Apples"] = 10;
      

      EDIT Wrong, see my next post for correction.

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      avmg
      wrote on 31 Jul 2020, 09:41 last edited by
      #11

      @JonB and is it possible to nest again?

      The target of all of this is to save the values from a numeric struct into a json file, and show it in a treeview, I like the way how the json is organized because when you load it into the treeview it results very clean and organized for the user. Actually I can load the json and update the struct with its values, but when I modify the struct and I want to save it back into the json file is when I struggle... that's why i'm asking how to modify one specific value from a nested json, the json already exists, but I can't find the way to change it.

      J 1 Reply Last reply 31 Jul 2020, 09:58
      0
      • A avmg
        31 Jul 2020, 09:41

        @JonB and is it possible to nest again?

        The target of all of this is to save the values from a numeric struct into a json file, and show it in a treeview, I like the way how the json is organized because when you load it into the treeview it results very clean and organized for the user. Actually I can load the json and update the struct with its values, but when I modify the struct and I want to save it back into the json file is when I struggle... that's why i'm asking how to modify one specific value from a nested json, the json already exists, but I can't find the way to change it.

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        JonB
        wrote on 31 Jul 2020, 09:58 last edited by
        #12

        @avmg
        I'm sorry, I don't have time right now to complete looking into this.

        I can say that my example for setting the values needs the order of its assignments reversed:

        QJsonObject json, food, fruits;
        fruits["Apples"] = 10;
        food["Fruits"] = fruits;
        json["Food"] = food;
        
        // next line works right
        qDebug() << json["Food"].toObject()["Fruits"].toObject()["Apples"];
        
        // assignment not right here, it must be into into a copy of objects
        json["Food"].toObject()["Fruits"].toObject()["Apples"] = 999;
        // next line wrong, still prints `10`
        qDebug() << json["Food"].toObject()["Fruits"].toObject()["Apples"];
        

        Somewhere the assignment at the end needs to use QJsonValueRefs instead of those toObject()s which I think are doing a copy. I may have time to look at it later, but have a play....

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        • A Offline
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          avmg
          wrote on 31 Jul 2020, 10:31 last edited by
          #13

          Hi JonB,

          That's exactly what I was trying and the point where I got stacked, there's no error but the value isn't assigned... thanks anyway!

          J 1 Reply Last reply 31 Jul 2020, 12:47
          0
          • A avmg
            31 Jul 2020, 10:31

            Hi JonB,

            That's exactly what I was trying and the point where I got stacked, there's no error but the value isn't assigned... thanks anyway!

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            JonB
            wrote on 31 Jul 2020, 12:47 last edited by JonB
            #14

            @avmg
            I too have had little joy. In the end I searched, and you should read through (all of) https://stackoverflow.com/questions/17034336/how-to-change-qjsonobject-value-in-a-qjson-hierarchy-without-using-copies carefully.

            The gist being, you can't do it :)

            According to information from Qt developer who actually wrote QJson in Qt5 -

            What's currently included in Qt is a 'read-only' implementation to provide parsing facilities. He has an intention to extend design with 'references' support in future, but it's not yet done.

            After wasting three hours of my life I can confirm that as of today this is still impossible with Qt 5.4. You can modify JSON objects, but not nested JSON objects.

            You can pursue any of the suggestions there to achieve what you want.

            Alternatively, which I would be tempted to do: convert the whole of the JSON object hierarchy into actual C++ structures/QVariantLists/whatever. Do your updates/manipulations on that, as C++/Qt variables instead of on the JSON object tree. And then regenerate the whole JSON document tree from your C++ structures and write that back to file.

            A 1 Reply Last reply 31 Jul 2020, 15:05
            0
            • J JonB
              31 Jul 2020, 12:47

              @avmg
              I too have had little joy. In the end I searched, and you should read through (all of) https://stackoverflow.com/questions/17034336/how-to-change-qjsonobject-value-in-a-qjson-hierarchy-without-using-copies carefully.

              The gist being, you can't do it :)

              According to information from Qt developer who actually wrote QJson in Qt5 -

              What's currently included in Qt is a 'read-only' implementation to provide parsing facilities. He has an intention to extend design with 'references' support in future, but it's not yet done.

              After wasting three hours of my life I can confirm that as of today this is still impossible with Qt 5.4. You can modify JSON objects, but not nested JSON objects.

              You can pursue any of the suggestions there to achieve what you want.

              Alternatively, which I would be tempted to do: convert the whole of the JSON object hierarchy into actual C++ structures/QVariantLists/whatever. Do your updates/manipulations on that, as C++/Qt variables instead of on the JSON object tree. And then regenerate the whole JSON document tree from your C++ structures and write that back to file.

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              avmg
              wrote on 31 Jul 2020, 15:05 last edited by
              #15

              @JonB
              Ok, so in the same way I'm actually loading the json into the treeview, I should make modifications there and do the opposite operation saving the treeview data into a json file, right?

              Thanks for your time!

              J 1 Reply Last reply 31 Jul 2020, 16:08
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              • A avmg
                31 Jul 2020, 15:05

                @JonB
                Ok, so in the same way I'm actually loading the json into the treeview, I should make modifications there and do the opposite operation saving the treeview data into a json file, right?

                Thanks for your time!

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                JonB
                wrote on 31 Jul 2020, 16:08 last edited by
                #16

                @avmg
                Yes, exactly that. You produced the treeview internally and output that as JSON. Now, read in the JSON , convert to treeview/internal format, do adds/deletes/updates where you know what you're doing in C++, finally ask for the tree to once again produce the whole of the JSON to save back externally.

                1 Reply Last reply
                1
                • A Offline
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                  avmg
                  wrote on 29 Aug 2020, 18:35 last edited by avmg
                  #17

                  Hi again!

                  I'm trying to do what we were talking about but I'm fighting with the same thing but opposite...

                  How can I nest a value inside another in a json? I've tried things like:

                  json["Food"].toObject()["Fruits"].toObject()["Apples"].toDouble(5);
                  

                  and

                  json["Food"].toObject()["Fruits"].toObject()["Apples"] = 5;
                  

                  It doesn't generate errors but the file shows equal to null after Food... I imagine that firstly I should create another QJsonObject and insert it in another, but I've tried many things without success...

                  Any suggestion?
                  Thanks in advance!

                  1 Reply Last reply
                  0
                  • Christian EhrlicherC Offline
                    Christian EhrlicherC Offline
                    Christian Ehrlicher
                    Lifetime Qt Champion
                    wrote on 29 Aug 2020, 19:10 last edited by Christian Ehrlicher
                    #18

                    JonB already showed you how to do it: https://forum.qt.io/topic/116865/get-nested-json-values/13

                    Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
                    Visit the Qt Academy at https://academy.qt.io/catalog

                    A 1 Reply Last reply 30 Aug 2020, 10:35
                    1
                    • Christian EhrlicherC Christian Ehrlicher
                      29 Aug 2020, 19:10

                      JonB already showed you how to do it: https://forum.qt.io/topic/116865/get-nested-json-values/13

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                      avmg
                      wrote on 30 Aug 2020, 10:35 last edited by
                      #19

                      @Christian-Ehrlicher Hi Christian,

                      You are right, I was looking the line:

                      json["Food"].toObject()["Fruits"].toObject()["Apples"] = 999;
                      

                      ...but the solution was at the beginning just in front of me:

                      QJsonObject json, food, fruits;
                      fruits["Apples"] = 10;
                      food["Fruits"] = fruits;
                      json["Food"] = food;
                      
                      // next line works right
                      qDebug() << json["Food"].toObject()["Fruits"].toObject()["Apples"];
                      

                      Thanks again and sorry, I didn't pay attention.

                      Cheers

                      J 1 Reply Last reply 31 Aug 2020, 07:23
                      0
                      • A avmg
                        30 Aug 2020, 10:35

                        @Christian-Ehrlicher Hi Christian,

                        You are right, I was looking the line:

                        json["Food"].toObject()["Fruits"].toObject()["Apples"] = 999;
                        

                        ...but the solution was at the beginning just in front of me:

                        QJsonObject json, food, fruits;
                        fruits["Apples"] = 10;
                        food["Fruits"] = fruits;
                        json["Food"] = food;
                        
                        // next line works right
                        qDebug() << json["Food"].toObject()["Fruits"].toObject()["Apples"];
                        

                        Thanks again and sorry, I didn't pay attention.

                        Cheers

                        J Offline
                        J Offline
                        JonB
                        wrote on 31 Aug 2020, 07:23 last edited by
                        #20

                        @avmg
                        Yes.

                        Note that we are saying you can read nested JSON objects via

                        qDebug() << json["Food"].toObject()["Fruits"].toObject()["Apples"];
                        

                        but you cannot write the value of a nested object via

                        json["Food"].toObject()["Fruits"].toObject()["Apples"] = 10;
                        

                        That's just how it is, as the code above copies the JSON objects, so assigning does not change what is in json[].... If you want to achieve the second example you have to build it up bit by bit as per the 4 lines

                        QJsonObject json, food, fruits;
                        fruits["Apples"] = 10;
                        food["Fruits"] = fruits;
                        json["Food"] = food;
                        
                        1 Reply Last reply
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