Using custom scalars with gqlgen.client

[geertjanvdk]

With gqlgen you can define your own scalars, for example, I have a scalar type Date time.Time which when encoded/decoded, it is always from/to an ISO-date.

This works great, but not so with the gqlgen.client. If you use your scalars, you will get an error like: 'something.StartDate' expected a map, got 'string'. Under the hood, gqlgen uses mapstructure, and that does not know about your scalars.

That was until a new feature or fix was released with gqlgen v0.17.33. Now you can define some custom decoding configuration.
It's a bit .. hard to find this in the documentation, so I thought adding a discussion to document, and discuss maybe a better example.

I have a type with all encoding and decoding, etc..:

type Date time.Time

When I use this in my response in tests, I would get expected a map, got 'string'.

Using the new method SetCustomDecodeConfig, we can add this to the gqlgen.client like this:

        gqlClient := client.New(....)
	dc := &mapstructure.DecoderConfig{
		TagName:     "json",
		ErrorUnused: true,
		ZeroFields:  true,
		DecodeHook: func(f reflect.Type, t reflect.Type, data any) (any, error) {
			switch t.PkgPath() + "." + t.Name() {
			case "github.com/<org>/<project>/<pkg>.Date":
				return scalar.NewDate(data.(string))
			}
	
			return data, nil
		},
	}
	
	gqlClient.SetCustomDecodeConfig(dc)

Now my custom scalar works, and I can continue testing :)

Maybe it's good to add the above with the documentation?