genqlient/generate/unmarshal.go.tmpl

{{/* (the blank lines at the start are intentional, to separate
     UnmarshalJSON from the function it follows) */}}

func (v *{{.GoName}}) UnmarshalJSON(b []byte) error {
    {{/* We want to specially handle the abstract fields (.AbstractFields),
         but unmarshal everything else normally.  To handle abstract fields,
         first we unmarshal them into a json.RawMessage, and then handle those
         further, below.  For the rest, we just want to call json.Unmarshal.
         But if we do that naively on a value of type `.Type`, it will call
         this function again, and recurse infinitely.  So we make a wrapper
         type -- with a different name, thus different methods, but the same
         fields, and unmarshal into that.  For more on why this is so
         difficult, see
         https://github.com/benjaminjkraft/notes/blob/master/go-json-interfaces.md
         TODO(benkraft)): Ensure `{{.Type}}Wrapper` won't collide with any
         other type we need.  (For the most part it being locally-scoped saves
         us; it's not clear if this can be a problem in practice.)
    */}}
    type {{.GoName}}Wrapper {{.GoName}}

    var firstPass struct{
        *{{.GoName}}Wrapper
        {{range .AbstractFields -}}
        {{.GoName}} {{repeat .GoType.SliceDepth "[]"}}{{ref "encoding/json.RawMessage"}} `json:"{{.JSONName}}"`
        {{end}}
    }
    firstPass.{{.GoName}}Wrapper = (*{{.GoName}}Wrapper)(v)

    err := {{ref "encoding/json.Unmarshal"}}(b, &firstPass)
    if err != nil {
        return err
    }

    {{/* Now, for each field, call out to the unmarshal-helper.
         This gets a little complicated because we may have a slice field.
         So what we do is basically, for each field of type `[][]...[]MyType`:

            target := &v.MyField      // *[][]...[]MyType
            raw := firstPass.MyField  // [][]...[]json.RawMessage

            // repeat the following three lines n times; each time, inside
            // the loop we have one less layer of slice on raw and target
            *target = make([][]...[]MyType, len(raw))
            for i, raw := range raw {
                // We need the &(*target)[i] because at each stage we want to
                // keep target as a pointer.  (It only really has to be a
                // pointer at the innermost level, but it's easiest to be
                // consistent.)
                target := &(*target)[i]
            
                // (now we have `target *MyType` and `raw json.RawMessage`)
                __unmarshalMyType(target, raw)

            } // (also n times)

         Note that if the field also uses a pointer (`[][]...[]*MyType`), we
         now pass around `*[][]...[]*MyType`; again in principle
         `[][]...[]*MyType` would work but require more special-casing.  Thus
         in the innermost loop, `target` is of type `**MyType`, so we have to
         pass `*target` to the unmarshal-helper.  Of course, since MyType is an
         interface, I'm not sure why you'd any of that anyway.

         One additional trick is we wrap everything above in a block ({ ... }),
         so that the variables target and raw may take on different types for
         each field we are handling, which would otherwise conflict.  (We could
         instead suffix the names, but that makes things much harder to read.)
    */}}
    {{range $field := .AbstractFields -}}
    {
        target := &v.{{$field.GoName}}
        raw := firstPass.{{$field.GoName}}
        {{range $i := intRange $field.GoType.SliceDepth -}}
        *target = make(
            {{repeat (sub $field.GoType.SliceDepth $i) "[]"}}{{if $field.GoType.IsPointer}}*{{end}}{{$field.GoType.Unwrap.Reference}},
            len(raw))
        for i, raw := range raw {
            target := &(*target)[i]
        {{end -}}
        {{if $field.GoType.IsPointer -}}
        {{/* In this case, the parent for loop did `make([]*MyType, ...)` and
             we have a pointer into that list.  But we actually still need to
             initialize the *elements* of the list. */ -}}
        *target = new({{$field.GoType.Unwrap.Reference}})
        {{end -}}
        err = __unmarshal{{$field.GoType.Unwrap.Reference}}(
            {{if $field.GoType.IsPointer}}*{{end}}target, raw)
        if err != nil {
            return fmt.Errorf(
                "Unable to unmarshal {{$.GoName}}.{{$field.GoName}}: %w", err)
        }
        {{range $i := intRange $field.GoType.SliceDepth -}}
        }
        {{end -}}
    }
    {{end -}}

    return nil
}