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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"ValueByDiscriminator() generates incorrect method call when union type is referenced via external ref in allOf","articleBody":"##### Description\n\nWhen a schema is defined as `allOf: [ExternalUnion, LocalObject]` where `ExternalUnion` is a discriminated `oneOf` from another file, `ValueByDiscriminator()` is generated with incorrect syntax.\n\n##### Reproduction\n\n**Spec structure (two files):**\n\n`diff.yaml`:\n\n```yaml\nDiffFile:\n  oneOf:\n    - $ref: '#/components/schemas/GitDiffFile'\n    - $ref: '#/components/schemas/AddedImageDiffFile'\n  discriminator:\n    propertyName: diff_type\n```\n\n`pulls.yaml` (references diff.yaml as external):\n\n```yaml\nPRFile:\n  allOf:\n    - $ref: './diff.yaml#/components/schemas/DiffFile'\n    - $ref: '#/components/schemas/ViewedStatus'\n```\n\nWith `import-mapping: {\"./diff.yaml\": \"mypkg/diff\"}` in oapi-codegen config.\n\n##### Generated code (broken)\n\n```go\nfunc (t PRFile) ValueByDiscriminator() (interface{}, error) {\n    discriminator, err := t.Discriminator()\n    switch discriminator {\n    case \"GitDiffFile\":\n        return t.AsexternalRef0.GitDiffFile()  // ← compile error: undefined\n    }\n}\n```\n\n##### Expected\n\n```go\ncase \"GitDiffFile\":\n    return t.AsExternalRef0GitDiffFile()  // matches the actually generated As* method\n```\n\n##### Root cause\n\nIn `union.tmpl`, `ValueByDiscriminator()` uses `$type` from `$discriminator.Mapping` directly:\n\n```\nreturn t.As{{$type}}()\n```\n\nFor external refs, `$type` = `\"externalRef0.GitDiffFile\"` which produces invalid Go (`t.AsexternalRef0.GitDiffFile()`).\n\nThe `As*` / `From*` / `Merge*` methods correctly use `{{ .Method }}` from the union element, which produces `ExternalRef0GitDiffFile`. `ValueByDiscriminator()` should do the same.\n\n##### Patch\n\n```\n--- a/pkg/codegen/templates/union.tmpl\n+++ b/pkg/codegen/templates/union.tmpl\n@@ -82,7 +82,9 @@ func (t {{.TypeName}}) ValueByDiscriminator() (interface{}, error) {\n             switch discriminator{\n                 {{range $value, $type := $discriminator.Mapping -}}\n                     case \"{{$value}}\":\n-                        return t.As{{$type}}()\n+                        {{- range $el := .Schema.UnionElements -}}\n+                        {{- if eq (print $el) $type}}return t.As{{$el.Method}}(){{end -}}\n+                        {{- end}}\n                 {{end -}}\n                 default:\n                     return nil, errors.New(\"unknown discriminator value: \"+discriminator)\n```\n\n##### Version\n\noapi-codegen v2.7.0","author":{"url":"https://github.com/alekgit","@type":"Person","name":"alekgit"},"datePublished":"2026-07-16T14:40:08Z","url":"https://github.com/oapi-codegen/oapi-codegen/issues/2470"}