Accessing fields of a ScopedJson returned by helper

[ilya-zlobintsev]

Maybe I'm misunderstanding how objects returned by helpers are supposed to be used, but I was unable to access the fields of an object that my custom handler returns.

I have a helper that looks like this:

impl HelperDef for MyHelper {
    fn call_inner<'reg: 'rc, 'rc>(
        &self,
        _: &Helper<'reg, 'rc>,
        _: &'reg Handlebars,
        _: &'rc Context,
        _: &mut RenderContext<'reg, 'rc>,
    ) -> Result<Option<ScopedJson<'reg, 'rc>>, RenderError> {
        Ok(Some(ScopedJson::Derived(json!({
            "a": 1,
            "b": 2,
        })))
}

I'm not sure what would be the proper way to access "a" and "b" from it. I have tried using {{ myhelper["a"] }}, but it seems to always return [object], regardless of accessing the entire helper object or one of it's fields. How do I access the value in it as a string?

[sunng87]

@ilyazzz I'm afraid {{myhelper["a"]}} is not valid handlebars expression. You can test it on the playground from javascript version: http://tryhandlebarsjs.com/

To implement your scenario, you can try lookup with subexpression, like:

{{ lookup (myhelper) "a" }}

[ilya-zlobintsev]

I have tried using lookup, but it doesn't seem to work.

Here's a minimal example. It doesn't print out anything, and I'm not sure why:

use handlebars::{Context, Handlebars, Helper, HelperDef, RenderContext, RenderError, ScopedJson};
use serde_json::json;

fn main() {
    let mut registry = Handlebars::new();

    registry.register_helper("myhelper", Box::new(MyHelper {}));

    let result = registry.render_template("{{ lookup (myhelper) \"a\" }}", &TemplateContext {}).unwrap();
    
    println!("{}", result) ;
}

struct MyHelper;

impl HelperDef for MyHelper {
    fn call_inner<'reg: 'rc, 'rc>(
        &self,
        _: &Helper<'reg, 'rc>,
        _: &'reg Handlebars,
        _: &'rc Context,
        _: &mut RenderContext<'reg, 'rc>,
    ) -> Result<Option<ScopedJson<'reg, 'rc>>, RenderError> {
        Ok(Some(ScopedJson::Derived(json!({
            "a": 1,
            "b": 2,
        }))))
    }
}

#[derive(serde::Serialize)]
struct TemplateContext;

This is a working basic helper in javascript, where the same template syntax does work:

Handlebars.registerHelper('myhelper', function (value) {
  return { "a": 1, "b": 2 }
});

[sunng87]

Let me look into this. It seems to be a bug with lookup helper

[sunng87]

Confirmed there is a bug with parser that it cannot recognize subexpression with no parameters. For now, you can workaround this with {{lookup (myhelper 0) "a" }}

[ilya-zlobintsev]

if also seems to be affected, where {{ #if (myhelper) }} doesn't work and {{ #if (myhelper 0) }} does

[sunng87]

@ilyazzz could you please verify this behaviour with our latest master?

[ilya-zlobintsev]

@sunng87 yes, it seems to work properly within my with expression now. Thank you.