Queries on schema and JSON data conversion

Question

We already have flatbuffer library embedded in our software code for simple schemas with JSON output data generation.

More update: We are generating the header files using flatc compiler against the schema and integrate these files inside of our code along with FB library for further serialization/deserialization.

Now we also need to have the following schema tree to be supported.

namespace SampleNS;

/// user defined key value pairs to add custom metadata
/// key namespacing is the responsibility of the user
table KeyValue {
    key:string (key, required); 
    value:string (required);
}

enum SchemaVersion:byte {
    V1,
    V2
}

table Sometable {
    value1:ubyte;
    value2:ushort (key);
}

table ComponentData {
    inputs: [Sometable];
    outputs: [Sometable];
}

table Node {
    name:string (key);
    
    /// IO definition
    data:ComponentData;
            
    /// nested child
    child:[Components];
}

table Components {
    type:ubyte;
    index:ubyte;
    nodes:[Node];
}

table GroupMasterData {

    schemaversion:SchemaVersion = sampleNS::SchemaVersion::V1;

    metainfo:[KeyValue];

    /// List of expected components in the system
    components:[Components];
}

root_type GroupMasterData;

As from above, table Components is nested recursively. The intention is components may have childs that have the same fields.

I have few queries:

• Flatc didnt gave me any error during schema compilation for such recursive nested tables. But is this supported during the field access for such tables?
• I tried to generate a sample json data file based on above data but I could not see the field for schemaversion. I learned FB doesn't serialize the default values. so, I removed the default value that I assigned in the schema. But, it still doesnt write into the json data file. On this I also learned we can forcefully write into the file using force_defaults option. I don't know where is this is to be put: in the attribute or elsewhere?
• Can I create a struct of enum field?

Is their any API to set Flatbuffer options that we otherwise pass to the compiler arguments? or if not, may be I think we have to tinker with the FB library code. Please suggest.

Method 1: In our serialization method, we do this:

flatbuffers::Parser* parser = new flatbuffers::Parser();
parser->opts.output_default_scalars_in_json = true;

Is this the right method or should I use any other API?

Answer 1

• Yes, trees (and even DAG) structures are fully supported. The type definition is recursive, but the data will eventually have leaf nodes with an empty vector of children, presumably.
• The integer value for V1 is 0, and that is also the default value for all fields with no explicit default assigned. Use --defaults-json to see this field when converting. Note that explicit versions in a schema is an anti-pattern, since schemas are naturally evolvable without breaking backwards compatibility.
• You can put enum fields in structs, yes. Is that what you mean?