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 {
    {{/* Standard convention for unmarshalers is to no-op on null. */}}
    if string(b) == "null" {
        return nil
    }

    {{/* We want to specially handle certain fields, but unmarshal everything
         else normally.  To handle abstract fields and fields with custom
         unmarshalers, first we unmarshal them into a json.RawMessage, and then
         handle those further, below.  Embedded fields don't need the
         json.RawMessage; we just use our input again.  Either way, we first
         want to call json.Unmarshal on the receiver (v).  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): Omit/simplify the first pass if all fields are
         embedded/abstract. */ -}}
    var firstPass struct{
        *{{.GoName}}
        {{range .Fields -}}
        {{if and .NeedsUnmarshaler (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.NeedsUnmarshaler -}}
    {{if $field.IsEmbedded -}}
    {{/* Embedded fields are easier: we just unmarshal the same input into
         them.  (They're also easier because they can't be lists, since they
         arise from GraphQL fragment spreads.) */ -}}
    err = {{$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 .NeedsUnmarshaler */ -}}
    {{end}}{{/* end range .Fields */ -}}

    return nil
}