unmarshal.go.tmpl

{{/* We need to generate UnmarshalJSON methods for some types that we want to
     handle specially.  Specifically, we generate an UnmarshalJSON for each
     struct with a field meeting any of these criteria:
        - a field whose type is configured with a custom unmarshaler
        - an embedded (anonymous) field; technically we only need an
          UnmarshalJSON if the embedded type needs one, but it's easier to
          generate it unconditionally
        - a field of interface type
     Additionally, since we add `json:"-"` to fields we handle specially, for
     any field which requires a MarshalJSON, we also generate an UnmarshalJSON,
     and vice versa.

     Given that, we want to specially handle the above-described fields, but
     unmarshal everything else normally.  To handle fields with custom
     unmarshalers, first we unmarshal them into a json.RawMessage, then call
     the custom unmarshaler.  Interface-typed fields are similar, except
     instead of a custom unmarshaler we call the helper we've generated
     (see unmarshal_helper.go.tmpl).  Embedded fields don't need the
     json.RawMessage; we just unmarshal our input again into the embedded
     field. */}}

func (v *{{.GoName}}) UnmarshalJSON(b []byte) error {
    {{/* Standard convention for unmarshalers is to no-op on null. */}}
    if string(b) == "null" {
        return nil
    }

    {{/* For our first pass, we ignore embedded fields, unmarshal all the
         custom-unmarshaler or abstract fields into json.RawMessage, and
         unmarshal everything else normally.  To do this, we want to call
         json.Unmarshal on the receiver (v).  But if we do that naively on a
         value of type <.GoName>, 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): Omit/simplify the first pass if all fields are
         embedded/abstract. */ -}}
    var firstPass struct{
        *{{.GoName}}
        {{range .Fields -}}
        {{if and .NeedsMarshaling (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.NeedsMarshaling -}}
    {{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.)
         
         Note that our behavior if you have two fields of the same name via
         different embeds differs from ordinary json-unmarshaling: we unmarshal
         into *all* of the fields.  See goStructType.FlattenedFields in
         types.go for more discussion of embedding and visibility. */ -}}
    err = {{$field.Unmarshaler $.Generator}}(
        b, &v.{{$field.GoType.Unwrap.Reference}})
    if err != nil {
        return err
    }
    {{else -}}
    {{/* For other fields (abstract or custom unmarshaler), first, call the
         unmarshaler (our unmarshal-helper, or the user-specified one,
         respectively).  This gets a little complicated because we may have
         a slice field.  So what we do is basically, for each field of type
         `[][]...[]MyType`:

            dst := &v.MyField      // *[][]...[]MyType
            src := firstPass.MyField  // [][]...[]json.RawMessage

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

            } // (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, `dst` is of type `**MyType`, so we have to
         pass `*dst` to the unmarshaler.  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 dst and src 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.)
    */}}
    {
        dst := &v.{{$field.GoName}}
        src := firstPass.{{$field.GoName}}
        {{range $i := intRange $field.GoType.SliceDepth -}}
        *dst = make(
            {{repeat (sub $field.GoType.SliceDepth $i) "[]"}}{{if $field.GoType.IsPointer}}*{{end}}{{$field.GoType.Unwrap.Reference}},
            len(src))
        for i, src := range src {
            dst := &(*dst)[i]
        {{end -}}
        {{/* dst now has type *<GoType>; dst is json.RawMessage */ -}}
        {{/* If the field is null in the input, skip calling unmarshaler.
             (This matches json.Unmarshal.)  If the field is missing entirely
             from the input, we will have an uninitialized json.RawMessage;
             handle that likewise. */ -}}
        if len(src) != 0 && string(src) != "null" {
            {{if $field.GoType.IsPointer -}}
            {{/* In this case, the parent for loop did `make([]*MyType, ...)`
                 and we have a pointer into that list.  But we actually still
                 need to initialize the *elements* of the list. */ -}}
            *dst = new({{$field.GoType.Unwrap.Reference}})
            {{end -}}
            err = {{$field.Unmarshaler $.Generator}}(
                src, {{if $field.GoType.IsPointer}}*{{end}}dst)
            if err != nil {
                return fmt.Errorf(
                    "Unable to unmarshal {{$.GoName}}.{{$field.GoName}}: %w", err)
            }
        }
        {{range $i := intRange $field.GoType.SliceDepth -}}
        }
        {{end -}}
    }
    {{end}}{{/* end if/else .IsEmbedded */ -}}
    {{end}}{{/* end if .NeedsMarshaling */ -}}
    {{end}}{{/* end range .Fields */ -}}

    return nil
}