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 or embedded fields, but
         unmarshal everything else normally.  To handle abstract fields,
         first we unmarshal them into a json.RawMessage, and then handle those
         further, below.  Embedded fields we just unmarshal directly into the
         embedded value.  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 which embeds both this type and NoUmnarshalJSON, which prevents
         either's UnmarshalJSON method from being promoted.  For more on why
         this is so difficult, see
         https://github.com/benjaminjkraft/notes/blob/master/go-json-interfaces.md.
         (Note there are a few different ways "hide" the method, but this one
         seems to be the best option that works if this type has embedded types
         with UnmarshalJSON methods.)
         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.)
    */}}

    {{/* TODO(benkraft): Omit/simplify the first pass if all fields are
         embedded/abstract. */ -}}
    var firstPass struct{
        *{{.GoName}}
        {{range .Fields -}}
        {{if and .IsAbstract (not .IsEmbedded) -}}
        {{.GoName}} {{repeat .GoType.SliceDepth "[]"}}{{ref "encoding/json.RawMessage"}} `json:"{{.JSONName}}"`
        {{end -}}
        {{end -}}
        {{/* TODO(benkraft): In principle you might have a field-name that
             conflicts with this one; avoid that. */ -}}
        {{ref "github.com/Khan/genqlient/graphql.NoUnmarshalJSON"}}
    }
    firstPass.{{.GoName}} = v

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

    {{/* Now, handle the fields needing special handling. */}}
    {{range $field := .Fields -}}
    {{if $field.IsEmbedded -}}
    {{/* Embedded fields are easier: we just unmarshal the same input into
         them.  (They're also easier because they can't be lists, since they
         arise from GraphQL fragment spreads.) */ -}}
    {{if $field.IsAbstract -}}
    {{/* Except if they're both abstract and embedded, in which case we need to
         call the unmarshal-helper instead.  Luckily, we don't embed
         slice-typed fields, so we don't need the full generality we handle
         below. */ -}}
    err = __unmarshal{{$field.GoType.Unwrap.Reference}}(
        b, &v.{{$field.GoType.Unwrap.Reference}})
    {{else -}}
    err = json.Unmarshal(
        b, &v.{{$field.GoType.Unwrap.Reference}})
    {{end -}}{{/* inner if .IsAbstract */ -}}
    if err != nil {
        return err
    }
    {{else if $field.IsAbstract -}}
    {{/* First, for abstract fields, call 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.)
    */}}
    {
        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 -}}{{/* end if .IsEmbedded + else if .IsAbstract */ -}}
    {{end}}{{/* end range .Fields */ -}}

    return nil
}