invalid polymorphic type %v for %v on field %s, missing fiel

Error message

invalid polymorphic type %v for %v on field %s, missing field %s

What it means

A field tagged `gorm:"polymorphic:..."` requires the related (FieldSchema) struct to contain a `<polymorphic>Type` string column. GORM looks up `FieldsByName[typeName]` and, when the field does not exist, records this error. The polymorphic ID column (`<polymorphic>ID`) is checked separately.

Source

Thrown at schema/relationship.go:223

	)

	if value, ok := field.TagSettings["POLYMORPHICTYPE"]; ok {
		typeName = strings.TrimSpace(value)
	}

	if value, ok := field.TagSettings["POLYMORPHICID"]; ok {
		typeId = strings.TrimSpace(value)
	}

	relation.Polymorphic.PolymorphicType = relation.FieldSchema.FieldsByName[typeName]
	relation.Polymorphic.PolymorphicID = relation.FieldSchema.FieldsByName[typeId]

	if value, ok := field.TagSettings["POLYMORPHICVALUE"]; ok {
		relation.Polymorphic.Value = strings.TrimSpace(value)
	}

	if relation.Polymorphic.PolymorphicType == nil {
		schema.err = fmt.Errorf("invalid polymorphic type %v for %v on field %s, missing field %s",
			relation.FieldSchema, schema, field.Name, polymorphic+"Type")
	}

	if relation.Polymorphic.PolymorphicID == nil {
		schema.err = fmt.Errorf("invalid polymorphic type %v for %v on field %s, missing field %s",
			relation.FieldSchema, schema, field.Name, polymorphic+"ID")
	}

	if schema.err == nil {
		relation.References = append(relation.References, &Reference{
			PrimaryValue: relation.Polymorphic.Value,
			ForeignKey:   relation.Polymorphic.PolymorphicType,
		})

		primaryKeyField := schema.PrioritizedPrimaryField
		if len(relation.foreignKeys) > 0 {
			if primaryKeyField = schema.LookUpField(relation.foreignKeys[0]); primaryKeyField == nil || len(relation.foreignKeys) > 1 {
				schema.err = fmt.Errorf("invalid polymorphic foreign keys %+v for %v on field %s", relation.foreignKeys,

View on GitHub (pinned to 1d6ce99528)

Solutions

Add the missing `<Poly>Type string` field to the related struct (for `polymorphic:Owner` that is `OwnerType string`).
If the column exists under a different name, point GORM at it with the POLYMORPHICTYPE tag setting.
Verify the tag value spelling matches the prefix used for the Type and ID columns.
Check the error message tail: it names the exact missing field (`...Type` here).

Example fix

// before
type User struct {
    ID       uint
    Children []Cat `gorm:"polymorphic:Owner"`
}
type Cat struct {
    ID       uint
    OwnerID  int
    // OwnerType missing
}

// after
type Cat struct {
    ID         uint
    OwnerID    int
    OwnerType  string
}

Defensive patterns

Strategy: validation

Validate before calling

// Verify polymorphic type column exists on the related model
_, err := schema.Parse(&Cat{}, &sync.Map{}, schema.NamingStrategy{})
// or check directly:
if !hasField(&Cat{}, "OwnerType") { // for polymorphic:Owner
    panic("Cat must declare OwnerType string for polymorphic:Owner")
}

Try / catch

if err := db.AutoMigrate(models...); err != nil {
    if strings.Contains(err.Error(), "missing field") {
        return fmt.Errorf("polymorphic model missing Type/ID column: %w", err)
    }
    return err
}

Prevention

Always create the pair `<Poly>Type string` + `<Poly>ID <keytype>` together on the related model.
Write a model-convention test asserting polymorphic tag values have matching columns.
Prefer letting GORM's convention names over custom POLYMORPHICTYPE overrides unless necessary.

When it happens

Trigger: Declaring `Children []Cat `gorm:"polymorphic:Owner"`` on a User model while the Cat struct lacks an `OwnerType string` field; or misspelling the POLYMORPHIC tag value so the derived name (e.g. `petType`) matches nothing in the related struct.

Common situations: Renaming the polymorphic tag value without adding matching Type/ID fields to the target model; overriding the type field name via POLYMORPHICTYPE tag with a typo; adding polymorphic relations to a shared/embedded base struct where the type column lives elsewhere.

Related errors

AI-assisted analysis of go-gorm/gorm@1d6ce99528 (2026-08-15). Data as JSON: /api/errors/307083096d333835. Report an issue: GitHub.